OpenMolcas: From Source Code to Insight

A computational study of the aqueous pertechnetate anion: Elucidation of the hydration structure and spectroscopic properties

A computational study of the TcO4- anion in water has been carried out, the study includes an analysis of the structure of the solvation shells, the hydration energy and the electronic transitions that affect the shape of the UV-Vis absorption spectrum. The solvated system was characterized by combined QM/MM molecular dynamics simulations. The second-order perturbation theory restricted active space method and the novel self-consistent field density matrix renormalization group approach were employed to describe the static correlation. Two distinct solvation shells were found with 24 and 65 water molecules, respectively. The water molecules surrounding the anion have a general orientation in which one hydrogen is oriented away from the oxyanion while the remaining atoms are at a similar distance. However, there are specific water molecules that form hydrogen bonds with the oxygens of the oxyanion, while others are oriented with their oxygen atom towards the anion. The observed excitations in the UV-Vis spectrum are of a T2 character, with the main source of the observed behavior being the charge transfer from oxygen atoms to the central technetium atom. The calculations show that the most intense band of the spectrum is broader and has a blue tail with respect to the gas phase spectrum. This difference is due to the lower symmetry caused by the aqueous environment, which allows different states to mix and leads to broadening of the band.

Analytical Gradient Theory for Density-Fitted Exact Two-Component Hartree-Fock, State-Specific Complete Active Space Self-Consistent Field, and Second-Order Møller-Plesset Perturbation Theories

The exact two-component (X2C) relativistic quantum chemistry calculations can be used to describe scalar relativistic effects and spin-orbit couplings at reasonable computational cost. However, they have limited applicability to wave function-based quantum chemistry methods, particularly geometric optimizations and dynamics simulations, owing to the high computational demands of these methods in sizable molecular systems. In this work, we report our implementation of an analytical gradient algorithm with a density-fitting approximation for Hartree-Fock, state-specific complete active space self-consistent field (CASSCF), and second-order Møller-Plesset perturbation theory (MP2) calculations with the X2C one-electron Hamiltonian. This implementation uses a second-order orbital optimization scheme to facilitate convergence in X2C-CASSCF calculations, as well as a response (Z-vector) equation for evaluation of the X2C-MP2 nuclear gradient. We demonstrate the applicability of the algorithm for optimization of the geometry of Ir(ppy)2(bpy)+ and evaluate its computational cost and parallelization (multithreading) efficiency.

Coupling Dy3 toroics in macrocycles

A Dy6 complex with centrosymmetric edge-to-edge assembly of Dy3 triangles showing toroidal arrangement of the magnetic moments was encapsulated in a macrocycle.

Photoisomerization and Ultrafast Dynamics of Phenylazothiazoles: Theoretical Perspective

Phenylazothiazole (PAT) is a novel heteroaryl azo photoswitch that undergoes trans (E) to cis (Z) photoisomerization under visible light, making it promising for biological applications. However, the quantum yield of the E-to-Z isomerization is significantly lower than that of the Z-to-E process, limiting its practical utility. In this study, we employ nonadiabatic dynamics simulations to investigate the ultrafast dynamics of the E-to-Z photoisomerization. Through a systematic analysis of electronic structure methods, we demonstrate that spin-flip time-dependent density functional theory (SF-TDDFT) provides a reliable description of the electronic excited states, particularly at conical intersections, yielding results consistent with multireference methods. Based on two-dimensional potential energy surfaces, we reveal that E-PAT initially relaxes along the torsional coordinate to reach the minimum of the S2 state, followed by two distinct pathways returning to the ground state. One pathway involves a planar minimum in the S1 state, predominantly leading back to the E isomer rather than the Z isomer, which explains the lower E-to-Z quantum yield. Additionally, we explore the substituent effects on the optical properties and thermal isomerization of PAT derivatives, showing that substituents not only induce a redshift in the absorption spectrum but also modulate the activation barrier of ground-state isomerization. These findings provide valuable theoretical insights into the photoisomerization mechanism of PATs and offer guidance for designing optoelectronic materials with tunable optical properties.

Capturing Ring Opening in Photoexcited Enolic Acetylacetone upon Hydrogen Bond Dissociation by Ultrafast Electron Diffraction

Photoinduced biological and chemical reactions are often based on key structural transformations of a molecule driven across multiple electronic states. Acetylacetone (AcAc) is a prototypical system for complex chemical pathways involving several conical intersections (CI) and singlet-triplet intersystem crossings (ISC) characterized by distinct geometries. In the gas phase, AcAc is predominantly in a planar ring-like enolic form stabilized by a strong intramolecular O-H···O hydrogen bond. Following excitation into the S2 (ππ*) state at 266 nm, acetylacetone undergoes rapid internal conversion followed by intersystem crossing. Such relaxation pathways are associated with structural changes including ring opening, deplanarization, and bond elongation. In this work, ultrafast electron diffraction (UED) at the SLAC MeV-UED setup is employed as a direct structural probe with a time resolution of 160 fs. Together with trajectory surface hopping simulations, analysis of the UED data provides a new perspective on the early time nuclear dynamics in acetylacetone. Specifically, AcAc is observed to undergo ring opening, deplanarization, and bond elongation all within the first 700 fs after photoexcitation. The monitored dynamics is associated mainly with the nuclear motion on the S1 potential energy surface, formed after very rapid transfer from S2 to S1, allowing AcAc to reach the conical intersection to intersystem crossing. Such time scales of nuclear motion are contrasted with the time scales of electronic transitions in AcAc that were previously characterized with spectroscopic methods, specifically internal conversion (<100 fs) and intersystem crossing (∼1.5 ps).

UV-photoelectron spectroscopy and MS-CASPT2/CASSCF study of the thermolysis of azidoethyl-methyl sulfide: Characterization and mechanism of the formation of S-methyl-N-sulfenylethanimine

The thermal decomposition of azidoethyl methyl sulfide was studied by real-time UV-photoelectron spectroscopy (UV-PES) at temperatures ranging from 773 to 1023 K. Different ionization energies were obtained using density functional theory calculations to assign UV-PES spectra. The complete active space self-consistent field and multistate second-order perturbation methods were used to predict the formation of different species present in the thermal decomposition process. N2 and S-methyl-N-sulfenylethanimine are generated at 773 K. The first step of the reaction is the dissociation of the molecule into nitrene and nitrogen. The spin state (singlet or triplet) of nitrene formed in the first step of the reaction is temperature-dependent. At low temperatures (T ≤ 650 K), both states are formed with almost the same probability; in contrast, at high temperatures (T ≥ 1000 K), singlet nitrene is the majority intermediate. From this singlet nitrene, three stable reaction products were detected in the experiments: an imine derivative, a four-member cyclic derivative, and a sulfenyl derivative.

Elucidating the effects of isoelectronic atomic substitution on the excited-state dynamics and reactivity of aromatic compounds

Photochemistry of aromatic compounds has attracted significant interest for their distinctive excited-state behavior and potential applications as energy materials. Isoelectronic atomic substitution, which involves replacing ring atoms while maintaining the same number of electrons in aromatic compounds (e.g., from C-C to B-N), offers a promising strategy for modulating these properties by tuning electronic structures. However, a fundamental understanding of its effect on the excited-state dynamics and reactivity remains elusive. In this study, we investigate the photochemical properties of oxaborine, using benzene and azaborine as references, to elucidate the effects of isoelectronic atomic substitution. Potential energy surface (PES) calculations reveal that oxaborine undergoes barrierless ring distortion toward S1/S0 conical intersection (CI), followed by barrierless Dewar isomerization directly from the S1/S0 CI. Using nonadiabatic molecular dynamics simulations, we observe a rapid S1 decay time (125 fs) and a notably high Dewar isomerization yield (23.3%), consistent with the results of PES calculations, highlighting the unique photochemical properties of oxaborine. Orbital-level analysis reveals that atomic substitution breaks the π orbital degeneracy and destabilizes the S1 state, explaining the much faster S1 decay for azaborine and oxaborine. The p orbital energy modulations from O-B or N-B substitution weaken the meta-bridge bond while enhancing para-bridge C-C interactions, favoring Dewar isomerization. These findings offer a comprehensive understanding of the effect of isoelectronic atomic substitution in aromatic compounds and demonstrate its potential as a design strategy for controlling photochemical properties.

Photoinduced Dissociation of Halobenzenes: Nonadiabatic Molecular Dynamics Simulations Reveal Key Pathways

Aryl halides are prototypical molecules for studying photodissociation, yet the role of spin-orbit coupling (SOC) in their dynamics remains incompletely understood. Using state-of-the-art ab initio calculations and excited-state dynamics simulations, we explore the photodissociation pathways of iodobenzene (PhI) and bromobenzene (PhBr). For PhI, two dissociation pathways, direct and indirect modes, are identified, consistent with gas-phase experiments. In contrast, photodissociation of PhBr occurs only after overcoming the energy barrier between bound and repulsive states, which requires activation of specific vibrational modes, particularly those associated with boat-like out-of-plane motion. While previous studies have suggested that SOC primarily accelerates intersystem crossing and photodissociation in heavier halogens, our results show that, in addition to SOC, the activation of specific vibrational modes also plays a crucial role in the dissociation process. These findings enhance our understanding of how SOC influences excited-state dynamics, providing insight into controlling photochemical reactivity in halogenated organic compounds.

Mechanism of Chemiluminescence in the Air Afterglow Reaction

Chemiluminescent emission resulting from the thermal reaction between nitric oxide and atomic oxygen to yield NO2 is known as an air afterglow. The product forms in both the ground (2A1) and excited (2B2) states, with the latter being responsible for the chemiluminescence. Despite the reaction being recognized for several decades, the underlying mechanism, particularly the formation of the 2B2 state of NO2, remains unclear. The ground- and excited-state PESs describing the NO-O reaction have been explored using multireference electronic structure methods (CASSCF combined with XMS-CASPT2 energy corrections). Our results suggest that the formation of NO2 in two distinct electronic states involves a ridge-mediated bifurcation of the ground-state pathway. Additionally, a thermally accessible excited-state channel has been identified on the excited-state PES. Molecular orbital analysis along the bifurcated pathways has been performed to elucidate how a single reactant transforms into a product with two distinct electronic natures. Surface-hopping-based nonadiabatic dynamics simulations reveal that the excited-state pathway plays a significant role in generating the NO2 in the 2B2 state. Energetic analysis of computed MEPs indicates that the NO2 in the excited state forms in the vibronic states associated with the 2A1 and 2B2 states, which undergo dipole-allowed radiative emission. The resulting frequencies are in good agreement with the observed broad chemiluminescent spectrum. The mechanism of the air afterglow reaction is strikingly different from that of cycloperoxides, the sole class of chemiluminescent systems studied in depth to date. This study provides new fundamental insights into the chemiluminescence phenomenon.

Electrical Control of the Nuclear Spin States of Rare-Earth Adatoms

Rare-earth adatoms on surfaces have been studied for potential atomic-scale magnetic storage, quantum sensing, and quantum computing applications. Despite accumulating experimental efforts, a comprehensive description of the electronic configurations of the adatoms remains elusive. Here, we investigate two charge states and several electronic configurations, including 5d and 6s valence shells, for a Sm adatom on a MgO substrate using multiconfigurational ab initio methods, for the possibility of using the Sm nuclear spin levels as qubits. For the configurations in a neutral charge state, we find that the electronic ground state is a singlet, and thus the hyperfine interaction associated with the 147Sm nucleus is absent, which may greatly enhance nuclear spin coherence time. The degeneracy of the nuclear levels is lifted by the nuclear quadrupole interaction. We show that the splitting of the nuclear levels can be controlled by a static electric field, and that Rabi oscillations between the nuclear levels can be induced by a time-dependent electric field. For the configurations in a singly charged state, electronic Kramers doublets are formed. The electronic configurations including an unpaired 6s orbital exhibit a strong hyperfine Stark effect due to a large Fermi contact contribution to the hyperfine interaction. In these configurations, electric-field-induced Rabi oscillations between the electronic-nuclear levels can occur at frequencies up to 3 orders of magnitude higher than those for the neutral charge state. The proposed system may be experimentally observed within scanning tunneling microscopy.

Chiral Pseudo-D6h Dy(III) Single-Molecule Magnet Based on a Hexaaza Macrocycle

A mononuclear complex [Dy(phenN6)(HL')2]PF6·CH2Cl2 (H2L' = R/S-1,1'-binaphthyl-2,2'-diphenol) with local D6h symmetry was synthesized. Structural determination shows that Dy3+ was encapsulated within the coordination cavity of the neutral hexaaza macrocyclic ligand phenN6, forming a non-planar coordination environment. The axial positions are occupied by two phenoxy groups of binaphthol in the trans form. The local geometry of Dy3+ closely resembles a regular hexagonal bipyramid D6h configuration. The axial Dy-Ophenoxy distances are 2.189(5) and 2.145(5) Å, respectively, while the Dy-N bond lengths in the equatorial plane are in the range of 2.524(7)-2.717(5) Å. The axial Ophthalmoxy-Dy-Ophthalmoxy bond angle is 162.91(17)°, which deviates from the ideal linearity. Under the excitation at 320 nm, the complex exhibits a characteristic emission peak at 360 nm, corresponding to the naphthalene ring. The AC susceptibility measurements under an applied DC field of 1800 Oe show distinct temperature-dependent and frequency-dependent AC magnetic susceptibility, typical of single-molecule magnetic behavior. The Cole-Cole plot in the temperature range of 6.0-28.0 K was fitted using a model incorporating Orbach and Raman relaxation mechanisms, giving an effective energy barrier of Ueff = 300.2 K. Theoretical calculations on complex 1 reveal that the magnetization relaxation proceeds through the first excited Kramers doublets with a calculated magnetization blocking barrier of 404.1 cm-1 (581.4 K).

Quantum Mechanics/Molecular Mechanics Studies on the Excited-State Relaxation Mechanisms of Cytidine Analogues: 2'-Deoxy-5-Methylcytidine and 2'-Deoxy-5-Hydroxymethylcytidine in Aqueous Solution

We have used the high-level QM(CASPT2//CASSCF)/MM method to investigate the excited-state properties and decay pathways of two important cytidine analogues, i.e., 2'-deoxy-5-methylcytidine (5mdCyd) and 2'-deoxy-5-hydroxymethylcytidine (5hmdCyd), in aqueous solution. In view of the computed minimum-energy structures, conical intersections, and crossing points, and the relevant excited-state decay paths including the different internal conversion (IC) and intersystem crossing (ISC) routes in and between the S1, T1, T2, and S0 states, we finally provided the feasible excited-state relaxation mechanisms of these two important epigenetic DNA nucleosides. Upon 285 nm photoexcitation, the lowest spectroscopically bright S1(ππ*) state is initially populated in the Franck-Condon (FC) region in both solvated systems and then mainly occurs direct IC to the ground state through the nearby accessible S1/S0 conical intersection, with the QM(CASPT2)/MM computed energy barriers of 9.5 and 1.6 kcal/mol for 5mdCyd and 5hmdCyd, respectively. In addition, the S1(ππ*) state can partially hop to the T1(ππ*) state directly or is mediated by the T2(ππ*) state. In comparison to the favorable singlet-mediated IC channel, the minor S1→T1 and S1→T2→T1 ISCs would take place slowly. Subsequently, the T1 state will further approach the nearby T1/S0 crossing point to slowly deactivate to the S0 state. Due to the T1/S0 crossing point above the T1-MIN as well with the small T1/S0 SOC, i.e., 9.8 kcal/mol and 0.3 cm-1 in 5mdCyd and 8.7 kcal/mol and 1.9 cm-1 in 5hmdCyd, the slow ISC would trap the system in the T1 state for a long time. The present work rationalizes the excited-state dynamics of 5mdCyd and 5hmdCyd in aqueous solution and could provide mechanistic insights into understanding the photophysics and photochemistry of similar epigenetic DNA nucleosides and their derivatives.

Breakdown of broken-symmetry approach to exchange interaction

Broken-symmetry (BS) approaches are widely employed to evaluate Heisenberg exchange parameters, primarily in combination with DFT calculations. For many magnetic materials, BS-DFT calculations give reasonable estimations of exchange parameters, although systematic failures have also been reported. While the latter were attributed to deficiencies of approximate exchange-correlation functional, we prove here by treating a simple model system that the broken-symmetry methodology has serious problems. Detailed analysis clarifies the intrinsic issue with the broken-symmetry treatment of low-spin states. It shows, in particular, that the error in the BS calculation of exchange parameter scales with the degree of covalency between the magnetic and the bridging orbitals. This is due to the constraint on the form of multiconfigurational state imposed by the BS determinant, a feature common to other single-reference methods too. As a possible tool to overcome this intrinsic drawback of single-determinant BS approaches, we propose their extension to a minimal multiconfigurational version.

Lanthanide-nickel molecular intermetallic complexes featuring a ligand-free Ni2- anion in endohedral fullerenes

Transition metals (TMs) typically exhibit rich redox chemistry and can be found in various oxidation states. In most cases, TMs are positively charged. Strong π-accepting ligands have been shown to stabilize molecular complexes with TMs in formal negative oxidation states. By contrast, organic-ligand-free TM anions remain rare, limited to intermetallic compounds based on third-row TMs such as gold or platinum. Here we report the synthesis of air-stable lanthanide-nickel molecular intermetallic complexes featuring a ligand-free Ni2- confined within fullerenes, namely, Tb2Ni@C82. The charged Tb2Ni lanthanide nickelide cluster forms metal-only Lewis pairs, featuring strongly polarized Tb-Ni covalent bonds with short bond lengths in the range of 2.50-2.57 Å. X-ray absorption spectroscopy supports the -2 oxidation state of Ni with 3d104s2 electron count, in line with the spectroscopic and magnetic measurements, and theoretical study. This finding opens up an efficient way to stabilize intermetallic clusters with elusive nucleophilic TM anions by confining them inside molecular carbon cages.

Non-adiabatic photodissociation dynamics of vinyl iodide from nσ* and nπ* transitions

The photodissociation dynamics of vinyl iodide upon photoexcitation at 199.2 and 200 nm are investigated in a joint theoretical and experimental study. The gas-phase absorption spectrum measured by Fourier transform spectroscopy along with the use of synchrotron radiation is reported and a reassignment of the excited electronic states responsible for the absorption at the energy range of interest is proposed. Femtosecond time-resolved velocity map imaging in conjunction with resonance enhanced multiphoton ionization detection of the I(2P3/2) and I*(2P1/2) photofragments have been carried out. The experimental results are discussed in view of high-level ab initio calculations including potential energy curves and semiclassical dynamics. Three conical intersections (CIs) governing the dynamics are identified in a search for stationary points using spin-orbit gradients. Based on these results, a complete picture of the photodissociation dynamics of vinyl iodide is obtained. Photoexcitation at 200 nm, associated with a nI(⊥)σ* transition, leads to a fast dissociation occurring in a repulsive potential energy surface, which is mediated by a CI with a low-lying excited electronic state. This mechanism resembles the typical dissociation of alkyl iodides in the first absorption A-band. In contrast, one-photon excitation at 199.2 nm into a well-defined vibronic structure of the absorption spectrum is assigned to a nI(‖)π* transition. The subsequent dissociation dynamics from that state features an ultrafast electronic predissociation with sub-200 femtosecond reaction time. State-switching at a first CI with a low-lying electronic state governing the mechanism involves states of completely different character, occurring in less than 20 fs. This remarkably fast process takes place through an initial stretch of the CC bond, followed by a C-I elongation with subsequent vibrational activity in the CC stretch mode.

The Photophysical Path to the Triplet State in Light-Oxygen-Voltage (LOV) Domains

Upon blue-light absorption, LOV domains efficiently undergo intersystem crossing (ISC) to the triplet state. Several factors potentially contribute to this efficiency. One often proposed in the literature is the heavy atom effect of the nearby (and eventually adduct-forming) cysteine. However, some LOV domain derivatives that lack the cysteine residue also undergo ISC efficiently. Using hybrid multireference quantum mechanical/molecular mechanical (QM / MM) models, we investigated the effect of the electrostatic environment in a prototypal LOV domain, Arabidopsis thaliana Phototropin 1 LOV2 (AtLOV2), compared to the effect of the dielectric of an aqueous solution. We find that the electrostatic environment of AtLOV2 is especially well tuned to stabilize a triplet( n N , π * ) ${(n_{\rm{N}}, \pi ^{\ast} )}$ state, which we posit is the state involved in the ISC step. Other low-lying triplet states that have( π , π * ) ${(\pi, \pi ^{\ast} )}$ and( n O , π * ) ${(n_{\rm{O}}, \pi ^{\ast} )}$ character are ruled out on the basis of energetics and/or their orbital character. The mechanistic picture that emerges from the calculations is one that involves the ISC of photoexcited flavin to a triplet( n N , π * ) ${(n_{\rm{N}}, \pi ^{\ast} )}$ state followed by rapid internal conversion to a triplet( π , π * ) ${(\pi, \pi ^{\ast} )}$ state, which is the state detected spectroscopically. This insight into the ISC mechanism can provide guidelines for tuning flavin's photophysics through mutations that alter the protein electrostatic environment and potentially helps to explain why ISC (and subsequent flavin photochemistry) does not occur readily in many classes of flavin-binding enzymes.

Relaxation and Photochemistry of Nitroaromatic Compounds: Intersystem Crossing through 1ππ* to Higher 3ππ* States, and NO• Dissociation in 9-Nitroanthracene─A Theoretical Study

Determination of the photodegradation pathways of nitroaromatic compounds, known for their mutagenic properties and toxicity, is a relevant topic in atmospheric chemistry. In the present theoretical study, mechanisms for the photophysical relaxation and NO• dissociation of 9-nitroanthracene (9-NA) are proposed that challenge the commonly assumed pathways based on the El-Sayed rules. The analysis of the stationary points on the potential energy surfaces obtained with multiconfigurational methods indicates that after light absorption and subsequent relaxation of the S1 state, the system undergoes ultrafast intersystem crossing to T2, which serves as a gate-state to the triplet manifold due to favorable energetic couplings. This occurs despite the nature of the singlet and triplet states being 1ππ* and 3ππ*, where the receiver triplet involves NO2 orbitals that are tilted from the polyaromatic plane, with no involvement of the 3nπ state in the process. After the singlet to triplet manifold crossing, the system evolves along two possible trajectories. One leads to the global minimum of T1 (phosphorescent end state) and the other involves the dissociation into antryloxy and NO• radicals. Overall, the information obtained is in agreement with steady-state and time-resolved spectroscopic data reported for 9-NA. Furthermore, it suggests that the deactivation mechanism of nitroaromatic compounds can take place between 1ππ* and 3ππ* states, which opens a new landscape for the rationalization of the photophysics of these and other systems.

Quantum mechanics/molecular mechanics studies on mechanistic photophysics of epigenetic C5-halogenated DNA nucleosides: 2'-deoxy-5-chlorocytidine and 2'-deoxy-5-bromocytidine in aqueous solution

In this work, we have employed the high-level QM(CASPT2//CASSCF)/MM method to study the photophysical mechanisms of two important metabolized DNA/RNA nucleoside byproducts, i.e., 2'-deoxy-5-chlorocytidine (5CldCyd) and 2'-deoxy-5-bromocytidine (5BrdCyd), in aqueous solution. On the basis of our optimized minimum-energy structures, conical intersections, and crossing points, as well as the computed associated excited-state relaxation pathways involving the different internal conversion (IC) and intersystem crossing (ISC) processes in and between the S1, T1, T2, and S0 states, we have suggested the feasible excited-state relaxation mechanisms of these two important epigenetic halogenated DNA nucleosides. The initially populated spectroscopic bright 1ππ* state in the Franck-Condon (FC) region is the S1 state both for 5CldCyd and 5BrdCyd under 295 nm irradiation. The excited S1 state first evolves into its minimum S1-MIN and rapidly undergoes efficient IC to the S0 state via the nearby low-lying S1/S0 conical intersection. The corresponding energy barrier of the S1 → S0 IC path in 5CldCyd is estimated to be 4.6 kcal mol-1 at the QM(CASPT2)/MM level, while it is found to be an almost barrierless process in 5BrdCyd. In addition to this very efficient IC, the S1 state can partially slowly undergo ISC to transfer to the T1 state. Because the small spin-orbit couplings (SOCs) of S1/T1 and S1/T2 are estimated to be less than 5.0 cm-1 at the QM(CASPT2)/MM level, the ISC involved T1 formation is not so efficient. The resulting T1 state from the minor S1 → T1 and S1 → T2 → T1 ISCs will first relax to its minimum T1-MIN and continue to approach the nearby accessible T1/S0 crossing point, followed by further T1 → S0 ISC to the S0 state. Relatively, the T1 → S0 ISC of 5BrdCyd is significantly enhanced by a large T1/S0 SOC of 32.9 cm-1 at the T1/S0 crossing point. The present work rationalizes the excited-state dynamics of 5CldCyd and 5BrdCyd in aqueous solution and could provide mechanistic insights into understanding the photophysics of similar halogenated DNA nucleosides and their derivatives.

Semiquantitative studies on the correlations between the electrostatic potential of a single DyIII ion and its energy barrier in the containing-DyIII single molecule magnets

Although the design of containing-DyIII single molecule magnets (SMMs) based on electrostatic potential has been employed for several years, rational studies on the correlations between the electrostatic potential of a single DyIII ion and its energy barriers have not been reported previously. In this work, a novel "Y"-shaped MnIII5DyIII3 complex, [MnIII5DyIII3(NO3)(OCH3)(L)4(L')2(tea)(teaH)4]OH·3CH3OH·2H2O (1, H2L = 2-methoxy-6-[(Z)-(1H-1,2,4-triazol-3-ylimino)methyl]phenol, H3tea = triethanolamine and HL' = 2-hydroxy-3-methoxybenzaldehyde), was obtained through the reaction of the H2L ligand, teaH3, Dy(NO3)3·6H2O and MnCl2·4H2O in CH3OH and DMF. Structural analysis revealed that the coordination geometry of Dy1 in 1 is muffin-like (MFF-9, Cs), while the geometries of Dy2 and Dy3 fall among spherical capped square antiprism-like (CSAPR-9, C4v), spherical tricapped trigonal prism-like (TCTPR-9, D3h) and muffin-like (MFF-9, Cs). Magnetic studies indicated that 1 exhibits slow magnetic relaxation behavior under zero dc field. More importantly, for the first time, we proposed the two parameters Req and Rax to describe the relationship. The two parameters were employed for three DyIII ions in 1, one Zn2Dy complex and one mononuclear Dy complex reported by others. Results indicate that the calculated energy barrier (Ucal) continuously increases with decreasing Rax absolute values. However, the higher the Rax absolute values, the lower the Ucal values. This indicates that when the electrostatic potentials from two sides of the equatorial plane of DyIII are completely equivalent to each other, Ucal is the highest. Moreover, POLY_ANISO and DFT calculations for 1 indicated that except for J4 showing ferromagnetic coupling, all magnetic couplings are antiferromagnetic, which were interpreted from magneto-structural correlation.

Redox, spectroscopic and magnetic properties of C3-symmetric rare earth complexes featuring atypical ortho-dioxolene binding

The molecular symmetry in rare earth (RE) coordination chemistry is critically important for controlling the electronic structure of the RE ion and the resulting magnetic and photophysical properties. Here, we report a family of complexes with unusual C3-point symmetry: [REIII(Br4catH)3(tpa)] (Br4catH- = tetrabromocatecholate, tpa = tris(2-pyridylmethyl)amine). The synthesis and solid-state characterisation of eleven analogues (RE = Y, Sm to Lu) were performed, enabling a systematic investigation of the effect of symmetry on various physical properties across the RE series. The crystal structures reveal a unique cooperative coordination motif, featuring a cyclic hydrogen-bonding network between the atypical monodentate monoprotonated Br4catH- ligands. Electrochemical analysis reveals a single oxidation process that suggests a concerted three-electron oxidation of all tetrabromocatecholate ligands to semiquinonate. Furthermore, single-molecule magnet (SMM) behaviour was investigated, revealing unexpected in-field slow magnetic relaxation for both Dy and Yb analogues, which can be rationalised by the effect of C3-symmetry. Finally, luminescence measurements were performed to probe the CF splitting of the Yb analogue and quantify the error in the overall CF splitting predicted by ab initio calculations. The governing effects of C3-symmetry are consistent observations in all RE3+ metals studied in this work, manifesting in the concerted three-electron oxidation, SMM behaviour, ground state composition, and luminescence properties.

Theoretical Study of Spectroscopy and Photodynamics of Decatetraene as a Representative Dimethylated Polyene

The photodynamics and UV spectroscopy of decatetraene following excitation to the bright 1Bu state are studied theoretically, based on ab initio computations of the underlying potential energy (PE) surfaces. Both photophysical and photochemical aspects are investigated. The former involves smaller amplitude displacements, and - in addition to determining multidimensional PE surfaces - also a quantal treatment of the ensuing nuclear dynamics. The inclusion of the 1Bu-2Ag vibronic interaction allows to compute the vibrational structure of the 1Ag-1Bu UV spectral band and the femtosecond 1Bu-2Ag internal conversion (population transfer). The results are compared with analogous features of octatetraene and octatriene. The photochemical aspects involving larger-amplitude displacements are investigated from a quantum-chemical point of view, focusing on the stationary points and the seams of conical intersections that are involved. A comparison of decatetraene with octatetraene reveals the contrasting features of their 2Ag and 1Bu state minima, where the latter is more stable in the dimethylated system. The small barrier connecting these two states lies between 0.06 and 0.11 eV. The nonradiative decay channels originating from these minima are characterized by comparatively higher barriers in decatetraene and influence the outcome of the radiative processes in a different manner in comparison to that of octatetraene.

Role of Electron Correlation beyond the Active Space in Achieving Quantitative Predictions of Spin-Phonon Relaxation

Single-molecule magnets (SMMs) are promising candidates for molecular-scale data storage and processing due to their strong magnetic anisotropy and long spin relaxation times. However, as the temperature rises, interactions between electronic states and lattice vibrations accelerate spin relaxation, significantly limiting their practical applications. Recently, ab initio simulations have made it possible to advance our understanding of phonon-induced magnetic relaxation, but significant deviations from the experiments have often been observed. The description of molecules' electronic structure has been mostly based on complete active space self-consistent field (CASSCF) calculations, and the impact of electron correlation beyond the active space remains largely unexplored. In this study, we provide the first systematic investigation of spin-phonon relaxation in SMMs with post-CASSCF multiconfigurational methods, specifically CAS, followed by second-order perturbation theory and multiconfiguration pair-density functional theory. Taking Co(II)- and Dy(III)-based SMMs as case studies, we analyze how electron correlation influences spin-phonon relaxation rates across a range of temperatures by comparing theoretical predictions with experimental observations. Our findings demonstrate that post-CASSCF treatments make it possible to achieve quantitative predictions for Co(II)-based SMMs. For Dy(III)-based systems, however, accurate predictions require the consideration of additional effects, underscoring the urgent necessity of further advancing the study of the effects of electronic correlation in these complex systems.

Legion: A Platform for Gaussian Wavepacket Nonadiabatic Dynamics

Nonadiabatic molecular dynamics is crucial in investigating the time evolution of excited states in molecular systems. Among the various methods for performing such dynamics, those employing frozen Gaussian wavepacket propagation, particularly the multiple spawning approach, offer a favorable balance between computational cost and reliability. It propagates on-the-fly trajectories used to build and propagate the nuclear wavepacket. Despite its potential, efficient, flexible, and easily accessible software for Gaussian wavepacket propagation is less common compared to other methods, such as surface hopping. To address this, we present Legion, a software that facilitates the development and application of classical-trajectory-guided quantum wavepacket methods. The version presented here already contains a highly flexible and fully functional ab initio multiple spawning implementation, with different strategies to improve efficiency. Legion is written in Python for data management and NumPy/Fortran for numerical operations. It is created under the umbrella of the Newton-X platform and inherits all of its electronic structure interfaces beyond other direct interfaces. It also contains new approximations that allow it to circumvent the computation of the nonadiabatic coupling, extending the electronic structure methods that can be used for multiple spawning dynamics. We test, validate, and demonstrate Legion's functionalities through multiple spawning dynamics of fulvene (CASSCF and CASPT2) and DMABN (TDDFT).

Homochiral Dy2 zero-field single-molecule magnets derived from axial chiral ligands (R)/(S)-octahydro-1,1'-bi-2-naphthyl phosphate

The construction of homochiral zero-field single-molecule magnets (SMMs), especially with axial chiral ligands, remains a great challenge. Herein the first examples of homochiral Dy(III) Schiff base complexes based on axial chiral ligands (R)/(S)-5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthyl phosphate (R-HL/S-HL), [Dy2(R-L/S-L)2(LSchiff)2(H2O)(MeOH)]·2MeOH·CH2Cl2·2MeCN·2H2O (R-1/S-1) [H2LSchiff = (E)-N'-(5-fluoro-2-hydroxybenzylidene)pyrazine-2-carbohydrazide], were synthesized at room temperature, which show ferromagnetic coupling and act as zero-field SMMs, with a Ueff/k value of 61 K at 0 Oe. Ab initio calculations were used to explain their magnetic interaction and magnetic relaxation properties. Furthermore, the magnetic circular dichroism (MCD) spectra of R-1/S-1 revealed that they display obvious magneto-optical effects.

Diatomic C2/NC Ligand Induced Conformation Variation of Trimetallic Clusters within a Carbon Cage

Understanding the interplay between conformation and electronic properties of metal complexes at the atomic level is the key for rational design of new functional molecules. Trimetallic clusterfullerenes (TMCFs) encapsulating quinary M3C2 carbide or M3NC carbonitride clusters offer an ideal model system for elucidating conformation-electronic property correlation due to its unusual conformational versatility. Herein, we synthesize and isolate two novel lanthanide-transition metal heteronuclear TMCFs, namely, CeTi2C2@Ih(7)-C80 and CeTi2NC@Ih(7)-C80. Crystallographic studies reveal singly bonded C2/NC ligands coordinating vertically to the CeTi2 trimetallic plane within the cluster, drastically different from the bat-ray conformations reported for all homonuclear TMCFs. Such bonding characters give rise to variable electronic structures of [Ce4+(Ti2)8+(C2)6-]6+@C806- and [Ce3+(Ti2)8+(NC)5-]6+@C806-, featuring a fixed tetravalent oxidation state of Ti and tunable oxidation states of Ce (Ce4+/Ce3+), as confirmed by X-ray absorption spectroscopy and magnetic measurements in combination with theoretical studies. Furthermore, scrutinizing all reported TMCFs bearing M3C2/M3NC clusters, we find that the conformations of trimetallic carbide/carbonitride clusters are precisely dictated by the bonding nature of the diatomic C2/NC ligand so as to suffice strong coordination interactions with the encapsulated metals. Following such a general rule governing conformational variation, a very strong ligand field is achieved on Ce3+ in the presence of a singly bonded NC ligand in CeTi2NC@C80, giving rise to the first Ce-based single-molecule magnet that shows an open magnetic hysteresis at 2 K.

Coupled Nuclear and Electron Dynamics in Chlorophyll Unraveled by XMS-CASPT2 X-ray Absorption Spectra

Attosecond spectroscopy, especially time-resolved X-ray absorption spectra (XAS), enables direct observation of ultrafast molecular dynamics. The complementary and even preceding development of theoretical simulations can offer the necessary guidance and stimulate new experiments. In this work, we simulated high-level XAS for the magnesium and nitrogen K-edge of chlorophyll a. In our previous work on the ultrafast relaxation process in the Q-band, our quantum dynamics simulations found the Qx and Qy states to be energetically close and therefore strongly coupled. Here, we analyze the strong coupling between Qx and Qy via XAS, indicating promising possibilities for experimental observation. The excited-state energies, potential energy surfaces, and XAS are computed at the XMS-CASPT2 level of theory to capture the complex multireference character of chlorophyll excitations. In our simulated spectra, we could follow the ultrafast population transfer between Qx and Qy and thus draw conclusions about the strong vibrational coupling between them.

Ultrafast structural dynamics of carbon-carbon single-bond rotation in transient radical species at non-equilibrium

Bond rotation is an important phenomenon governing the fate of reactions. In particular, heterogeneously substituted ethane derivatives provide distinct structural conformations around the bond, empowering them as ideal systems for studying the rotation along carbon-containing single bonds. However, structural dynamics of ultrafast single-bond rotation, especially along C-C• bonds, have remained elusive as tracking the detailed changes in structural parameters during the rotational isomerization is challenging with conventional spectroscopic tools. Here, we employ femtosecond time-resolved X-ray liquidography to visualize the rotational isomerization between anti and gauche conformers of tetrafluoroiodoethyl radical (C2F4I•) and 1,2-tetrafluorodiiodoethane (C2F4I2), simultaneously. The TRXL data captures perturbations in conformer ratios and structures of each reacting species, revealing that the rotational isomerization of C2F4I• and C2F4I2 follows anti-to-gauche and gauche-to-anti paths with time constants of 1.2 ps and 26 ps, respectively. These findings also align with the computational predictions. This work offers an atomic-level insight into the kinetics and structural dynamics of single-bond rotation.

Ab Initio Description of Vibronic Emission Bands in Noncentrosymmetric Lanthanide Complexes

Understanding the effect of coupling between the electronic and vibrational states is important for interpretation of the emission spectra of lanthanide-based materials. We present an approach for predicting vibronic bands in the luminescent noncentrosymmetric lanthanide complexes based on ab initio electronic structure calculations. We apply this approach to the complex vibronic structure of the 4S3/2 → 4I15/2 green emission in the erbium trensal complex, analyze the positions and intensities of vibronic peaks, and identify the key molecular vibrations contributing to the vibronic bands. The introduced approach is highly complementary to the experimental studies of lanthanide luminescence and can be used to gain valuable insight into electron-vibrational coupling in lanthanide-based materials.

Neodymium(III) Aqua Ion as a Model System in Ab Initio Crystal Field Analysis Beyond Point Charges and Crystal Field Theory

Correlating the molecular structure with the electronic structure of lanthanide(III) solvates is a challenging task. Neodymium(III) in aqueous solution serves as an appealing and straightforward model to address this issue. Herein, the experimentally determined electronic structure of neodymium(III) in water is compared with ab initio calculated electronic structures based on various models of its molecular structure. This comparison enables the determination of the most reliable molecular structure. The findings reveal that the molecular structure of the neodymium(III) aqua ion that best aligns with its electronic structure corresponds to a nine coordinated neodymium(III) complex, surrounded by 17 water molecules in the second coordination sphere. The role of second-sphere water molecules was investigated by calculating the crystal field splitting of the five Kramers doublets within the 4I9/2 low-energy multiplet for several calculated molecular structures with coordination numbers of eight, nine, and ten. The results demonstrated that the shape of the donor molecular orbitals plays a critical role in the crystal field splitting of the neodymium(III) ion. Furthermore, the findings confirmed that the orientation of the donating orbitals, specifically the orientation of the O-H bonds in water, is essential for accurately describing the electronic structure. Finally, manual alteration of the Nd-O bond lengths revealed that CAS(3,7)CF calculations tend to underestimate the crystal field strength.

Approximate Hamiltonians from a Linear Vibronic Coupling Model for Solution-Phase Spin Dynamics

The linear vibronic coupling (LVC) model is an approach for approximating how a molecular Hamiltonian changes in response to small changes in molecular geometry. The LVC framework thus has the ability to approximate molecular Hamiltonians at low computational expense but with quality approaching multiconfigurational ab initio calculations, when the change in geometry compared to the reference calculation used to parametrize it is small. Here, we show how the LVC approach can be used to project approximate spin Hamiltonians of a solvated lanthanide complex along a room-temperature molecular dynamics trajectory. As expected, the LVC approximation is less accurate as the geometry diverges from that at which the model was parametrized. We examine the accuracy of the predicted Hamiltonians by performing time-dependent quantum simulations of the spin dynamics of the molecule, with reference to the dynamics obtained using spin Hamiltonians projected from ab initio calculations at each step. We find that quantitatively accurate behavior is obtained when LVC parametrizations are performed at least every 10 fs during the trajectory.

DNA Triplet Energies by Free Energy Perturbation Theory

Determining the energetics of triplet electronic states of nucleobases in the biological macromolecular environment of nucleic acids is essential for an accurate description of the mechanism of photosensitization and the design of drugs for cancer treatment. In this work, we aim at developing a methodological approach to obtain accurate free energies of triplets in DNA beyond the state of the art, able to reproduce the decrease of triplet energies measured experimentally for T in DNA (270 kJ/mol) vs in the isolated nucleotide in aqueous solution (310 kJ/mol). For such purposes, we adapt the free energy perturbation method to compute the free energy related to the transformation of a pure singlet state into a pure triplet state via "alchemical" intermediates with mixed singlet-triplet nature. By this means, standard deviation errors are only a few kJ/mol, contrary to the large errors of tenths of kJ/mol obtained by averaging the singlet and triplet energies derived from molecular dynamics simulations. The reduced statistical errors obtained by the free energy perturbation approach allow us to rationalize with confidence the triplet stabilization observed experimentally when comparing the thymine nucleotide and thymine in DNA. Spin polarization rather than excimer interactions between the π-stacked nucleobases originates the lower values of the triplet energies in DNA. The developed approach implemented in QM3 shall be useful for determining free energies of triplets and other states like ionic or charge separation states in any other macromolecular system with impact in biomedicine and materials science.

Excited state relaxation mechanisms and tautomerism effects in 2,6-Diamino-8-Azapurine

The photochemistry of 9H-2,6-diamino-8-azapurine (9H-8AZADAP), a promising fluorescent probe, was investigated using the Multi-State Complete-Active-Space Second-Order Perturbation Theory (MS-CASPT2) quantum chemical method, along with the Average Solvent Electrostatic Configuration and Free Energy Gradient (ASEC-FEG) and Polarizable Continuum Model (PCM) to take into account water solvation effects. For both isolated and solvated species, the main photochemical event is initiated by the absorption of light from ground-state to the bright 1(ππ* La) state, which undergoes barrierless evolution to its minimum energy region (1(ππ* La)min) without crossing any other potential energy surface (PES). Subsequently, the excess of energy is released through fluorescence. From the 1(ππ* La)min region, two radiationless decay pathways back to the initial ground state, mediated by two distinct conical intersections between the ground and 1(ππ* La) states, are found to be unlikely due to the presence of high energy barriers in both environments. Our results also indicate that the solvation effects are more pronounced when using the ASEC-FEG method, which predicts larger structural and energy changes, especially concerning energetic barriers. Based on the free energy perturbation theory (FEP), a hypothetical thermodynamic cycle was devised, from which we infer that in an aqueous environment the N3 site is the most favorable for protonation. We also conclude that the 8H-8AZADAP tautomer is responsible for the fluorescent band observed experimentally at 410 nm and elucidates the mechanism of phototautomerism.

Rare earth stibolyl and bismolyl sandwich complexes

The design of molecular rare earth complexes to achieve unique magnetic and bonding properties is a growing area of research with possible applications in advanced materials and molecular magnetics. Recent efforts focus on developing ligand frameworks that can enhance magnetic characteristics. Here we show the synthesis and characterization of a class of rare earth complexes, [(η5-C4R4Sb)Ln(η8-C8H8)] and [(η5-C4R4Bi)Ln(η8-C8H8)], featuring η5-coordinated stibolyl and bismolyl ligands. The ligand aromaticity and bonding situation within these complexes are investigated by quantum chemical calculations. Magnetic studies of the ErIII analogues reveal large barriers and intriguing properties, including waist-restricted hysteresis and slow relaxation of the magnetization, making them single-molecule magnets. Comparison between the experimental barrier and CASSCF-SO calculations indicates that relaxation in all systems occurs through high-energy excited states. These findings suggest that stibolyl and bismolyl ligands can be promising candidates for achieving high-energy barriers in Er-based SMMs, offering a pathway to molecular designs with enhanced magnetic properties.

Automated search of minimum-energy conical intersections with projected metadynamics

We present a new method for the automated search of minimum-energy conical intersections (MECIs) based on metadynamics. In this method, two independent forces are constructed and projected into the minimization subspace and the constraint subspace, respectively. One force is directed toward the minimum-energy point, while the other is directed toward the conical intersection seam. The root-mean-square deviation based bias potential is added to the potential energy surface to force the structure escape from the already explored regions. The additional constraint function is used to enable the structure reach different intersection seams. This method can be used for systematically and automatically searching MECIs or exploring conical intersection seams. Compared to the penalty function-based metadynamics method, this new method is more effective and stable in searching MECIs. Furthermore, this method can be combined with any kind of constraint, whether geometric or non-geometric, making it a generalized tool for the automated search of constrained minimum.

NO Oxidation States in Nonheme Iron Nitrosyls: A DMRG-CASSCF Study of {FeNO}6-10 Complexes

Building upon an earlier study of heme-nitrosyl complexes (Inorg. Chem. 2023, 62, 20496-20505), we examined a wide range of nonheme {FeNO}6-10 complexes (the superscript represents the Enemark-Feltham count) and two dinitrosyl iron complexes using DMRG-CASSCF calculations. Analysis of the wave functions in terms of resonance forms with different [π*(NO)]i occupancies (where i = 0-4 for mononitrosyl complexes) identified the dominant electronic configurations of {FeNO}6 and {FeNO}7 complexes as FeIII-NO0 and FeII-NO0, respectively, mirroring our previous findings on heme-nitrosyl complexes. A trigonal-bipyramidal S = 1 {FeNO}8 complex with an equatorial triscarbene ligand set appears best described as a resonance hybrid of FeI-NO0 and FeII-NO-. Reduction to the corresponding S = 1/2 {FeNO}9 state was found to involve both the metal and the NO, leading to an essentially FeI-NO- complex. Further reduction to the {FeNO}10 state was found to be primarily metal-centered, leading to a predominantly Fe0-NO- configuration. Based on the weights wi of the [π*(NO)]i resonance forms, an overall DMRG-CASSCF-based π*(NO) occupation number could be derived, which was found to exhibit a linear correlation with both the NO bond distance and NO stretching frequency, allowing a readout of the NO oxidation state from the NO bond distance.

Unveiling the Werner-Type Cluster Chemistry of Heterometallic 4f/Post-Transition Metals: A {Dy3Bi8} Complex Exhibiting Quantum Tunneling Steps in the Hysteresis Loops and its 1-D Congener

A new [Dy3Bi8O6Cl3(saph)9] (1) Werner-type cluster has been prepared, which is the first DyIII/BiIII polynuclear compound with no metal-metal bond and one of the very few LnIII-BiIII (Ln = lanthanide) heterometallic complexes reported to date. The molecular compound 1 has been deliberately transformed to its 1-D analogue [Dy3Bi8O6(N3)3(saph)9]n (2) via the replacement of the terminal Cl- ions by end-to-end bridging N3- groups. The overall metallic skeleton of 1 (and 2) can be described as consisting of a diamagnetic {Bi8} unit with an elongated trigonal bipyramidal topology, surrounded by a magnetic {Dy3} equilateral triangle, which does not contain μ3-oxo/hydroxo/alkoxo groups. Detailed magnetic studies in a microcrystalline sample and a single crystal of 1 revealed a rare two-step hysteresis loop at various low temperatures and field-sweep rates, with the steps located at zero and ±0.26 T fields providing a measure of intermolecular interactions. Extended ab initio calculations unravel the dominant pathways of magnetization relaxation, as well as the type and magnitude of the magnetic exchange interactions between the DyIII centers and the orientation of their anisotropy axes, thus rendering the {Dy3} unit of 1 as a rare triangle among its congeners with a nontoroidal magnetic state. The combined results demonstrate the potential of heterometallic lanthanide/post-transition metal chemistry to provide molecule-based materials with unprecedented structures and compelling methods to rationalize the obtained magnetic properties.

Non-Resonant Magnetic X-ray Scattering as a Probe of Ultrafast Molecular Spin-State Dynamics: An Ab Initio Theory

With the advancement of high harmonic generation and X-ray free-electron lasers (XFELs) to the attosecond domain, the studies of the ultrafast electron and spin dynamics became possible. Yet, the methods for efficient control and measurement of the quantum state are to be further developed. In this publication, we propose using magnetic X-ray scattering (MXS) for resolving the molecular spin-state dynamics and establish a complete protocol to simulate MXS diffraction patterns in molecules with ab initio quantum chemistry based on the multiconfigurational method. The performance of the method is demonstrated for the simulation of the spin-flip dynamics in the TiCl4 molecule, initiated by an ultrashort X-ray pulse. The consistent variation of the electron population and the circular dichroic patterns show the capability of MXS to quantitatively detect the spin-state dynamics in real time quantitatively. We also conclude that the spatial shape and extent of the spin density can also be inferred by analyzing the diffraction patterns for randomly oriented and aligned molecules.

Analytic First-Order Derivatives of CASPT2 Combined with the Polarizable Continuum Model

The complete active space second-order perturbation theory (CASPT2) is valuable for accurately predicting electronic structures and transition energies. However, optimizing molecular geometries in the solution phase has proven challenging. In this study, we develop analytic first-order derivatives of CASPT2 using an implicit solvation model, specifically the polarizable continuum model, within the open-source package OpenMolcas. Analytic gradients and nonadiabatic coupling vectors are computed by solving a modified Z-vector equation. Comparisons with existing theoretical and experimental results demonstrate that the solvent effects can be qualitatively captured using the developed method.

Photoluminescence of a Uranium(IV) Alkoxide Complex

In this report, we describe the photoluminescence of a homoleptic uranium(IV) alkoxide complex. Excitation of [Li(THF)]2[UIV(O t Bu)6] leads to the first example of photoluminescence from a well-defined actinide complex originating from an f-f excitation, supported by second order multiconfigurational electronic structure calculations including spin-orbit coupling. These calculations show strong spin-orbit coupling between the excited triplet and singlet states for the 5f-orbital manifold, which leads to a long-lived excited state lifetime of 0.85 s at low temperature. The photophysical properties of homoleptic uranium(V) and uranium(VI) tertbutoxide complexes are also presented; we find that oxidation of the uranium(IV) alkoxide results in quenching of luminescence in [Li(THF)][UV(O t Bu)6] and [UVI(O t Bu)6]. This is attributed to competing ligand to metal charge transfer absorption processes shifted to lower energy upon oxidation of the actinide center, which mask the relevant f-f transitions in the visible region of the electronic absorption spectrum.

Theoretical Investigations on the Molecular Magnetic Behavior of Actinide Molecules [AnPc2]0/- (An = U, Cf): Prediction of the High Magnetic Blocking Barrier and Magnetic Blocking Temperature in [CfPc2]. J Phys Chem A 2025;129:717-732. [PMID: 39780501 DOI: 10.1021/acs.jpca.4c06757] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Searching for single-molecule magnets (SMM) with large effective blocking barriers, long relaxation times, and high magnetic blocking temperatures is vitally important not only for the fundamental research of magnetism at the molecular level but also for the realization of new-generation magnetic memory unit. Actinides (An) atoms possess extremely strong spin-orbit coupling (SOC) due to their 5f orbitals, and their ground multiplets are largely split into several sublevels because of the strong interplay between the SOC of An atoms and the crystal field (CF) formed by ligand atoms. Compared to TM-based SMMs, more dispersed energy level widths of An-based SMMs will give a larger total zero field splitting (ZFS) and thus provide a necessary condition to derive a higher Ueff. In combination of the density functional theory (DFT) as well as the CF model Hamiltonian and ab initio calculation, we have investigated the structural stability and electronic structures as well as the magnetodynamic behavior of [AnPc2]0/- (An = U, Cf) molecules. We find that An atoms can strongly interact with its ligand N atoms in forming An-N ionic bonds, and 5f electrons are more localized in the Cf atom than in the U atom, giving U4+(5f2) and Cf3+(5f9) valence states. Although the UPc2 molecule has a modest value of Ueff = 514 cm-1, it is not a good SMM due to the easy occurrence of quantum tunneling of magnetization (QTM). Based on the consistent results of CF Hamiltonian and ab initio calculations on the [CfPc2]- molecule, we propose that almost prohibited QTM within the Kramers doublets (KDs) as well as very low transition probabilities between different states via hindered spin-flip transitions would result in a high Ueff = 1401 cm-1. The estimated high magnetic blocking temperature (TB) of 58 K renders [CfPc2]- an excellent SMM candidate, implying that magnetic hysteresis could be observed in future experiments.

Reassessing the role and lifetime of Q x in the energy transfer dynamics of chlorophyll a

Chlorophylls are photoactive molecular building blocks essential to most photosynthetic systems. They have comparatively simple optical spectra defined by states with near-orthogonal transition dipole moments, referred to as B x and B y in the blue/green spectral region, and Q x and Q y in the red. Underlying these spectra is a surprisingly complex electronic structure, where strong electronic-vibrational interactions are crucial to the description of state characters. Following photoexcitation, energy-relaxation between these states is extremely fast and connected to only modest changes in spectral shapes. This has pushed conventional theoretical and experimental methods to their limits and left the energy transfer pathway under debate. In this work, we address the electronic structure and photodynamics of chlorophyll a using polarization-controlled static - and ultrafast - optical spectroscopies. We support the experimental data analysis with quantum dynamical simulations and effective heat dissipation models. We find clear evidence for B → Q transfer on a timescale of ∼100 fs and identify Q x signatures within fluorescence excitation and transient spectra. However, Q x is populated only fleetingly, with a lifetime well below our ∼30 fs experimental time resolution. Outside of these timescales, the kinetics are determined by vibrational relaxation and cooling. Despite its ultrashort lifetime, our theoretical analysis suggests that Q x plays a crucial role as a bridging state in B → Q energy transfer. In summary, our findings present a unified and consistent picture of chlorophyll relaxation dynamics based on ultrafast and polarization-resolved spectroscopic techniques supported by extensive theoretical models; they clarify the role of Q x in the energy deactivation network of chlorophyll a.

A Density Functional Valence Bond Study on the Excited States

The accurate description of excited states is crucial for the development of electronic structure theory. In addition to determining excitation energies, strong state interactions arise when electronic states with the same symmetry are degenerate or nearly degenerate, often requiring a multi-state treatment. These strong correlation effects and state interactions can be effectively handled by the Hamiltonian matrix correction-based density functional valence bond (hc-DFVB) method, a multi-reference density functional theory capable of accurately describing electronic state interactions. In this paper, we explore the low-lying excited states of four isoelectronic systems (C2H, CN, CO+, BO) using valence bond methods, including the valence bond self-consistent field (VBSCF) and hc-DFVB methods. Our results show that the hc-DFVB method provides significantly better excitation energies compared to VBSCF. Furthermore, hc-DFVB can reliably predict the correct ordering of excited states, whereas VBSCF shows some ordering inconsistencies. By categorizing the VB structures into groups based on point group symmetry, we can extract the key structural contributions and bonding pictures of each state from the weight distribution of these groups. Additionally, we study the potential energy curves for lithium fluoride (LiF) and a mixed-valence spiro cation, demonstrating the superior performance of hc-DFVB when applied to the study of near-degenerate excited states in the avoided crossing region.

Fine-Tuning the Anisotropies of Air-Stable Single-Molecule Magnets Based on Macrocycle Ligands

Air-stable single-molecule magnets (SMMs) can be obtained by confining DyIII ion in a D6h coordination environment; however, most of the current efforts were focused on modifying the rigidity of the macrocycle ligand. Herein, we attempt to assemble air-stable SMMs based on macrocycles with a replaceable coordination site. By using an in situ 1 + 1 Schiff-base reaction of dialdehyde with diamine, three air-stable SMMs have been obtained in which one of the equatorial coordination sites can be varied from -NH- (for Dy-NH), -O- (for Dy-O), and -NMe- (for Dy-NMe). Complex Dy-NH shows a less distorted D6h symmetry and an anisotropy energy barrier of 1270 K. For complex Dy-O, the coordination site of -O- gives a relatively longer coordination bond but a comparable energy barrier in contrast with that of Dy-NH. In the case of complex Dy-NMe, although the -NMe-group gives a very long coordination bond, the large steric effect on the -NMe- group enforces a larger distortion of the D6h coordination geometry, resulting in the fast quantum tunneling of the magnetization that shortcuts the thermal relaxation process; therefore, Dy-NMe shows a lower energy barrier. This study provides a new strategy for modifying the coordinate site on the equatorial plane of D6h symmetry to fine-tune the structure and magnetic anisotropy of SMMs.

Absorption Spectrum of Hydroperoxymethyl Thioformate: A Computational Chemistry Study

Hydroperoxymethyl thioformate (or HPMTF) is a compound relevant to the chemistry of sulfur in the marine atmosphere. The chemical cycling of this molecule in the atmosphere is still uncertain due in part to the lack of accurate knowledge of its photolytic behavior. Only approximations based on the properties of its chromophores are used in previous studies. In this work, we calculated the absorption spectra of the molecule in gas and aqueous phases using the Nuclear Ensemble Approach (NEA) and the CASPT2 method. Furthermore, we used such information to obtain relative photolysis rates. We found that the chromophore approximation overestimates the photolysis rates in the gas phase by twice the value obtained with the NEA-CASPT2 protocol. Furthermore, for the aqueous phase, we predict a lower role of photolysis as compared to the gas phase.

Competing Photocleavage on Boron and at the meso-Position in BODIPY Photocages

BODIPY photocages (photocleavable protective groups) have stirred interest because they can release biologically active cargo upon visible light excitation. We conducted combined theoretical and experimental investigations on selected BODIPY photocages to elucidate the mechanism of the competing photocleavage at the boron and meso-position. Based on the computations, the former reaction involves elongation of the B-C bond, yielding a tight borenium cation and methyl anion. These ions are intercepted by CH3OH, enabling an efficient proton-coupled electron transfer (PCET) to produce the methane and isolated ether photoproducts. Singlet and triplet excited-state lifetimes were measured in CH3OH and CD3OD to probe the kinetic isotope effects (KIEs). The resulting KIEs are small, implying that the kinetic bottleneck is due to the C-B bond scission rather than the subsequent PCET. The introduction of a methoxy group in the meso-phenoxy substituent redirects the photosubstitution toward the meso-position. The corresponding regiochemistry was explained computationally. On elongating the C-O bonds in the S1 state, it is found that the unproductive conical intersection is encountered much earlier for the alkyl-O bond than for the phenyl-O bond. The current findings are valuable for the rational design of new BODIPY photocages with tailored biological applications.

Predicting Magnetic Barriers in Lanthanide Complexes with Electrostatic Potential Charges

Single-molecule magnets (SMMs) with slow relaxation of magnetization and blocking temperatures above that of liquid nitrogen are essential for practical applications in high-density data storage devices and quantum computers. A rapid and accurate prediction of the effective magnetic relaxation barrier (Ueff) is needed to accelerate the discovery of high-performance SMMs. Using density functional theory and multireference calculations, we explored correlations between Ueff, partial atomic charges, and the anisotropic barrier for a series of sandwich-type lanthanide complexes containing cyclooctatetraene, substituted cyclopentadiene, phospholyl, boratabenzene, or borane ligands. Our results show a correlation between the electrostatic potential charge of the lanthanide ion in the complex and Ueff. Systematic ligand modifications show that reducing ligand nucleophilicity and incorporating soft bases enhance magnetic anisotropy and Ueff values. This work identifies a correlation to predict Ueff values and optimization of ligand coordination environments in lanthanide-based SMMs.

DC24: A new density coherence functional for multiconfiguration density-coherence functional theory

In this study, we explored several alternative functional forms to construct more accurate and more physical density coherence (DC) functionals for multiconfiguration density-coherence functional theory. Each functional is parameterized against the same database as used in our previous work. The best DC functional, which is called DC24, has a more physical interpretation, and-as a side benefit-it also has a mean unsigned error of 1.73 kcal/mol, which is a 9% improvement as compared to the previous functional. The article also contains a new definition of the unpaired electron density, which may be useful in other contexts as well.

Excited state relaxation mechanisms of paracetamol and acetanilide

The photochemical pathways of acetanilide and paracetamol were investigated using the XMS-CASPT2 quantum chemical method and the cc-pVDZ (correlation consistent polarized valence double- ζ ) basis set. In both compounds, the bright state is the second excited state, designated as a1 ( ππ * La) state. Through a detailed exploration of the potential energy profile and the conical intersection structure between the1 ( ππ * La) and ground states, we gained a better understanding of how cleavage might occur in both molecules upon photoexcitation. Other potential relaxation mechanisms, including crossings with the dark1 n π * and1 ( ππ * La) states, are also discussed in detail.

A cross-entropy corrected hybrid multiconfiguration pair-density functional theory for complex molecular systems

Hybrid density functionals, such as B3LYP and PBE0, have achieved remarkable success by substantially improving over their parent methods, namely Hartree-Fock and the generalized gradient approximation, and generally outperforming the second-order Møller-Plesset perturbation theory (MP2) that is more expensive. Here, we extend the linear scheme of hybrid multiconfiguration pair-density functional theory (HMC-PDFT) by incorporating a cross-entropy ingredient to balance the description of static and dynamic correlation effects, leading to a consistent improvement on both exchange and correlation energies. The B3LYP-like translated on-top functional (tB4LYP) developed along this line not only surpasses the accuracy of its parent methods, the complete active space self-consistent field (CASSCF) and the original MC-PDFT functionals (tBLYP and tB3LYP), but also outperforms the widely used complete active space second-order perturbation theory (CASPT2). Remarkably, while remaining satisfactory for general purpose, tB4LYP shows superior accuracy for challenging cases like the Cr2 dissociation and the associated low-lying vibrational energies, the ethylene torsional rotation and the ethyne diabatic colinear dissociations, with the significantly lower computational cost than CASPT2.

Entanglement and Mutual Information in Molecules: Comparing Localized and Delocalized Orbitals

The use of the mutual information (MI) as a measure of the entanglement in quantum systems has gained a consensus in recent years, even if there is an ongoing effort to distinguish the classical and quantum contributions contained therein. This quantity has been first introduced in condensed matter physics, in particular, in studies based on the density matrix renormalization group method. This method has been successfully adapted to quantum chemistry problems, opening the way to compute MI also in molecular systems. A key aspect of this quantity is its dependence on the one-electron (orbital) basis set, even for wave functions that are invariant under unitary transformation of the orbitals. In this work, we investigate the role of the orbital basis set (delocalized or localized, following different strategies) for wave functions expressed as linear combinations of Slater determinants and we give the analytic expression for the MI for a few special cases. This study aims to improve the knowledge of the relationship between the characteristics of the chemical bond (considering a few paradigmatic molecules, H2, F2, N2, and short linear polyenes) and the properties of interest in the field of quantum information theory.