Correction: A topological path to the formation of a quasi-planar B70 boron cluster and its dianion

Correction for 'A topological path to the formation of a quasi-planar B70 boron cluster and its dianion' by Pinaki Saha et al., Phys. Chem. Chem. Phys., 2024, https://doi.org/10.1039/d2cp05452c.

Treatment of flexibility of protein backbone in simulations of protein-ligand interactions using steered molecular dynamics

To ensure that an external force can break the interaction between a protein and a ligand, the steered molecular dynamics simulation requires a harmonic restrained potential applied to the protein backbone. A usual practice is that all or a certain number of protein's heavy atoms or Cα atoms are fixed, being restrained by a small force. This present study reveals that while fixing both either all heavy atoms and or all Cα atoms is not a good approach, while fixing a too small number of few atoms sometimes cannot prevent the protein from rotating under the influence of the bulk water layer, and the pulled molecule may smack into the wall of the active site. We found that restraining the Cα atoms under certain conditions is more relevant. Thus, we would propose an alternative solution in which only the Cα atoms of the protein at a distance larger than 1.2 nm from the ligand are restrained. A more flexible, but not too flexible, protein will be expected to lead to a more natural release of the ligand.

Reversible Coupling of Germylone with Isocyanates

The germylone dimNHCGe (5, dimNHC=diimino N-heterocyclic carbene) undergoes a [2+2] cycloaddition with isocyanates RNCO (R=4-tolyl or 3,5-xylyl) to furnish novel alkyl carboxamido germylenes 7 (R=4-tolyl) and 8 (R=3,5-xylyl), featuring a C-C bond between the former carbene carbon and the isocyanate moiety. Heating a mixture of 8 with 4-tolyl isocyanate to 100 °C results in isocyanate metathesis, demonstrating reversible C-C bond formation on the reduced germanium compound. DFT calculations suggest that this process occurs via the reductive dissociation of isocyanate from 8 that regenerates the parent Ge(0) compound 5.

A reinvestigation of the boron cluster B15+/0/-: a benchmark of density functionals and consideration of aromaticity models

This study presents a thorough reinvestigation of the B15+/0/- isomers, first employing coupled-cluster theory CCSD(T) calculations to validate the performance of different DFT functionals. The B15+ cation has two planar lowest-lying isomers, while the first 3D isomer is less stable than the global minimum by ∼10 kcal mol-1. The PBE functional, within this benchmark survey, has proved to be reliable in predicting relative energies for boron isomers. Other functionals such as the TPSSh, PBE0 and HSE06 result in good energy ordering of isomers but warrant reconsideration when distinguishing between 2D and 3D forms. Caution is needed for structures having high spin contamination, as it may lead to significant errors. The anomalously lower stability of the B15- anion with respect to its neighbours, in terms of electron detachment energy, was explained through a competition between both rectangle and disk models for its geometry. This elucidates its stability with 12 electrons in rectangle model and instability with 10 electrons in disk-shaped structure, emphasizing the value of employing such geometric models. The proximity of the σ* LUMO to the π HOMO also contributes to the weakening of the B15- stability.

Binding Mechanism and Surface-Enhanced Raman Scattering of the Antimicrobial Sulfathiazole on Gold Nanoparticles

A combined study using the surface-enhanced Raman scattering (SERS) technique and quantum chemical calculations was carried out to elucidate the adsorption behavior of sulfathiazole, an antibiotic drug, on gold nanoparticles. The tetrahedral Au20 cluster was used as a simple model to mimic a nanostructured gold surface. Computations using density functional theory with the PBE functional were performed in both the gas phase and aqueous medium using a continuum model. The drug is found to bind to the Au metals via the nitrogen of the thiazole ring. The interaction is also partially stabilized by the ring-surface π coupling rather than a sideway adsorption as previously proposed. In an aqueous solution, the drug molecule mainly exists as a deprotonated form, which gives rise to a much greater affinity toward Au nanoparticles as compared to the neutral forms. The drug adsorption further induces a significant alteration on the energy gap of the gold cluster Aun, which could result in an electrical noise. Notable SERS signals below 1600 cm-1, which result from a coupling of several vibrations including the ring breathing, C-C stretching, and N-H bending, could be employed for both qualitative and quantitative detection and assessment of sulfathiazole at trace concentrations.

A Guanidine-Supported π-Complex of Germanium Amenable to Intramolecular C-C Cleavage in Arene and Ge Atom Transfer

The germylone dimNHCGe (dimNHC=diimino N-heterocyclic carbene) reacts with azides N3 R (R=SiMe3 or p-tolyl) to furnish the first examples of germanium π-complexes, i. e. guanidine-ligated compounds (dimNHI-SiMe3 )Ge (NHI=N-heterocyclic imine, R=SiMe3 ) and (dimNHI-Tol)Ge (R=p-tolyl). DFT calculations suggest that these species are formed by a Staudinger type replacement of dinitrogen in the azide by a nucleophilic germylone, leading to a transient carbene adduct of iminogermylidene. Heating a solution of compound (dimNHI-SiMe3 )Ge to 70 °C results in extrusion of the iminogermylidene that further aggregates to produce the known [Me3 SiNGe]4 tetramer, whereas the imidazolylidene fragment transforms into an unusual heptatriene species that can be considered as a product of carbene insertion into the C-C bond of a pendant Ar substituent at the imidazolylidene nitrogen of the dimNHC. Reaction of (dimNHI-SiMe3 )Ge with tetrachloro-o-benzoquinone results in the net transfer of a germanium atom and formation of the free diimino-guanidine ligand. This ligand also forms when (dimNHI-SiMe3 )Ge is treated with azide N3 (p-Tol), with the germanium product being [(p-Tol)NGe]n.

Formation of pyramidal structures through mixing gold and platinum atoms: the AuxPty2+ clusters with x + y = 10

The geometric and electronic structures of a small series of mixed gold and platinum AuxPty2+ clusters, with x + y = 10, were investigated using quantum chemical methods. A consistent tetrahedral pyramid structure emerges, displaying two patterns of structural growth by a notable critical point at y = 5. This affects the clusters' electron population, chemical bonding, and stability. For the Pt-doped Au clusters with y values from 2 to 5, the bonds enable Pt atoms to assemble into symmetric line, triangle, quadrangle, and tetragonal pyramidal Pty blocks, respectively. For the Au-doped Pt clusters, with larger values of y > 5, the structures are more relaxed and the d electrons of Pt atoms become delocalized over more centers, leading to lower symmetry structures. A certain aromaticity arising from delocalization of d electrons over the multi-center framework in the doped Pt clusters contributes to their stability, with Pt102+ at y = 10 exhibiting the highest stability. While the ground electronic state of the neutral platinum atom [Xe]. 4f145d96s1 leads to a triplet state (3D3), the total magnetic moments of AuxPty2+ are large increasing steadily from 0 to 10 μB and primarily located on Pt atoms, corresponding to the increase of the number of Pt atoms from 0 to 10 and significantly enhancing the magnetic moments. An admixture of both Au and Pt atoms thus emerges as an elegant way of keeping a small pyramidal structure but bringing in a high and controllable magnetic moment.

Theoretical approaches to defect mechanisms and transport properties of compounds used for electrodes and solid-state electrolytes in alkali-ion batteries

The transition from fossil fuels to cleaner energies employing different renewable sources constitutes one of the primary worldwide challenges. The search for appropriate solutions is becoming more urgent in view of the severe consequences of climate change. As for a perspective, stationary energy storage, alkali-ion batteries and hybrid supercapacitors are, among others, considered as efficient and affordable solutions. Alkali-ion batteries have proved to be the most investigated products in the past decade including optimizations for cost, energy density and safety. In this Perspective, a computational approach and its applicability in the inverse material design are presented. This approach includes density functional theory calculations, force field-based determinations and both static and molecular dynamics simulations. As for an illustration, the main properties of a selected series of battery materials, including oxides and sulfides Li2SiO3, Li2SnO3, SrSnO3, and A2B6X13 (A = Li+, Na+, K+; B = Ti4+, Sn4+; X = O2-, S2-), and mixed halide antiperovskite A3OX (A = Li+, Na+; X = Cl-, Br-) are explored in depth using these theoretical approaches. Doping strategies, new dopant incorporation mechanism, treatment with alkali insertion/de-insertion cycle in electrodes, transport properties, as well as thermodynamic stability, are discussed. Theoretical approaches reveal that the oxygen-sulfur exchange in alkali hexatitanates and hexastannates induces remarkable improvement of the required properties for electrode and electrolyte materials. In addition, doping of Li2SiO3 with low Na-concentration enhances the room temperature Li-diffusivity by a reduction of the activation energy. The effects of transition-metal and divalent dopants on the defect chemistry and transport properties of Li2SnO3 are also disclosed. The interstitial trivalent doping mechanism is a friendly synthesis strategy to improve the large-scale diffusion in Li2SnO3. The potential of SrSnO3 as an anode in alkali-ion batteries, and the influence of a particular grain boundary in nanocrystalline antiperovskite A3OX are also revealed by using advanced atomistic simulations. The computational approaches described here provide us with a convenient tool for the determination of the properties of battery materials with high accuracy and for the prediction of characteristics of a new generation of alkali battery materials that could be used in improved technologies.

Electron density mapping of boron clusters via convolutional neural networks to augment structure prediction algorithms

Determination and prediction of atomic cluster structures is an important endeavor in the field of nanoclusters and thereby in materials research. To a large extent the fundamental properties of a nanocluster are mainly governed by its molecular structure. Traditionally, structure elucidation is achieved using quantum mechanics (QM) based calculations that are usually tedious and time consuming for large nanoclusters. Various structural prediction algorithms have been reported in the literature (CALYPSO, USPEX). Although they tend to accelerate the structure exploration, they still require the aid of QM based calculations for structure evaluation. This makes the structure prediction process quite a computationally expensive affair. In this paper, we report on the creation of a convolutional neural network model, which can give relatively accurate energies for the ground state of nanoclusters from the promolecule density on the fly and could thereby be utilized for aiding structure prediction algorithms. We tested our model on dataset consisting of pure boron nanoclusters of varying sizes.

Small chromium-doped silicon clusters CrSin: structures, IR spectra, charge effect, magnetism and chirality

A series of small chromium-doped silicon clusters CrSi_n with n = 3-10 in the cationic, neutral and anionic charge states were investigated using quantum chemical methods. The CrSi_n⁺ cations with n = 6-10 were produced in the gas phase and characterized by far-IR multiple photon dissociation (IR-MPD) spectroscopy. Good agreement between experimental spectra in the 200-600 cm^-1 frequency range and those determined for the lowest-energy isomers by density functional theory calculations (B3P86/6-311+G(d)) provide a strong support for the geometrical assignments. An extensive structural comparison for the three different charge states shows that the structural growth mechanism inherently depends on the charge. While the structures of the cationic clusters are preferentially formed by addition of the Cr dopant to the corresponding pure silicon cluster, it favors substitution in both the neutral and anionic counterparts. The Si-Cr bonds of the studied CrSi_n^+/0/- clusters are polar covalent. Apart from a basket-like Cr@Si₉^- and an endohedral Cr@Si₁₀^- cage, the Cr dopant takes an exohedral position and bears a large positive charge in the clusters. The exohedrally doped clusters also have a high spin density on Cr, manifesting the fact that the intrinsic magnetic moment of the transition metal dopant is well conserved. Three CrSi_n clusters have a pair of enantiomeric isomers in their ground state, namely the cationic n = 9 and the neutral and anionic n = 7. Those can be distinguished from each other by their electronic circular dichroism spectra, calculated using time-dependent density functional theory. Those enantiomers, being intrinsically chiral inorganic compounds, might be used as building blocks of optical-magnetic nanomaterials because of their high magnetic moments and ability to rotate the plane of polarization.

A topological path to the formation of a quasi-planar B70 boron cluster and its dianion

In view of the competing assignments regarding the most stable isomer of the B70 boron cluster including the quasi-planar and bilayer structures, we reinvestigated the structural motifs of B70 using a genetic algorithm for structure search (MEGA) in conjunction with density functional theory computations using the PBE functional. The quasi-planar structure was also constructed using the topological leapfrog algorithm. The latter search aimed to give us unique insight into its formation and the growth pattern of boron clusters. Also, the di-anionic state of B70 was explored. Our extensive search suggested a competition between the quasi-planar, tubular and bilayer isomers for the ground state of B70 in both neutral and dianionic states. While the bilayer form is more stable in the neutral state, the quasi-planar counterpart becomes more stable in the dianionic B702-. The stability arises due to the fact that the B702- dianion possesses 50 π electrons that satisfy the disk aromaticity model rule. These results tend to extend the stabilization of the quasi-planar structure upon negative charge addition previously found in small size boron clusters to larger sizes.

A new look at the structure of the neutral Au18 cluster: hollow versus filled golden cage

The geometry of the neutral Au₁₈ gold cluster was probed by a combination of quantum chemical calculations and far-infrared multiple photon dissociation (FIR-MPD) spectroscopy of a Kr messenger complex. Two low-lying isomers are identified to potentially contribute to the experimental IR spectrum, both being derived from a star-like Au₁₇ structure upon capping with one extra Au atom either inside (18_1) or outside (18_5) the star. In particular, the present detection of structure 18_1 by DFT computations where a golden cage encapsulates an endohedral Au atom, is intriguing as a stable core-shell isomer has, to our knowledge, never been found before for such small neutral gold clusters. DFT and local coupled-cluster (DLPNO and PNO-CCSD(T)) computations indicate that both Au₁₈ isomers are close to each other, within ∼3 kcal mol^-1, on the energy scale. Although the exact energy ordering is again method-dependent and remains, at present, inconclusive, the most striking spectral signatures of both isomers are related to vibrational modes localized at atoms capping the inner pentaprism sub-structure that result in prominent peaks centered at ∼80 cm^-1, close to the most prominent experimental feature found at 78 cm^-1. The calculated IR spectra of both core-shell and hollow isomers are very similar to each other and both agree comparably well with the experimental FIR-MPD spectra of the Au₁₈Kr_1,2 complexes.

Facile iodine-promoted synthesis of bis(1-imidazo[1,5-a]pyridyl)arylmethanes and exploration of applications

A practical strategy for the iodine-promoted synthesis of bis(1-imidazo[1,5-a]pyridyl)arylmethane and its derivatives has been developed. These compounds exhibit high cytotoxicity toward various cancer cell lines and moreover they are promising ligands for the Cu-catalysed synthesis of quinolines.

Particle size as a driver of dewatering performance and its relationship to stabilization in fecal sludge

Poor and unpredictable dewatering performance of fecal sludge is a major barrier to sanitation provision in urban areas not served by sewers. Fecal sludge comprises everything that accumulates in onsite containments, and its characteristics are distinct from wastewater sludges and from feces. There is little fundamental understanding of what causes poor dewatering in fecal sludge. For the first time, we demonstrate that particle size distribution is a driver of dewatering performance in fecal sludge, and is associated with level of stabilization. Higher concentrations of small particles (<10 μm) and smaller median aggregate size (D50) corresponded to poor dewatering performance (measured by capillary suction time (CST) and supernatant turbidity) in field samples from Kenya and Uganda and in controlled laboratory anaerobic storage experiments. More stabilized fecal sludge (higher C/N, lower VSS/TSS) had better dewatering performance, corresponding to lower concentrations of small particles. Samples with the largest aggregates (D50 > 90 μm) had higher abundance of Gammaproteobacteria Pseudomonas, and samples with the smallest aggregates (D50 ≤ 50 μm) were characterized by higher abundance of Bacteroidetes Vadin HA17 and Rikenellaceae. Contrary to common perceptions, stabilization, particle size distribution, and dewatering performance were not dependent on time intervals between emptying of onsite containments or on time in controlled anaerobic storage experiments. Our results suggest that the stabilization process in onsite containments, and hence the dewaterability of sludge arriving at treatment facilities, is not dependent on time in containment but is more likely associated with specific microbial populations and the in-situ environmental conditions which promote or discourage their growth.

Binding mechanism and SERS spectra of 5-fluorouracil on gold clusters

The adsorption behaviour of the 5-fluorouracil (5FU) on small gold clusters Au _N with N = 6, 8, 20 was evaluated by means of density functional theory using the PBE-D3 functional in combination with a mixed basis set, i.e. cc-pVDZ-PP for gold atoms and cc-pVTZ for non-metal elements. The binding energies between 5FU and gold clusters were determined in the range of 16-24 and 11-19 kcal/mol in gas-phase and aqueous media, respectively. The corresponding Gibbs energies were found to be around -7 to -10 kcal/mol in vacum and sigificantly reduced to -1 to -6 kcal/mol in water solution, indicating that both the association and dissociation processes are likely spontaneous. An analysis on the charge density difference tends to confirm the existence of a charge transfer from the 5FU molecule to Au atoms. Analysis of the surface-enhanced Raman scattering (SERS) spectra of 5FU adsorbed on the Au surfaces shows that the stretching vibrations of N-H and C=O bonds play a major role in the SERS phenomenon. A mechanism for the drug releasing from the gold surfaces is also proposed. The process is triggered by either the low pH in cancerous tumors or the presence of cysteine residues in protein matrices.

Unravelling the alkali transport properties in nanocrystalline A3OX (A = Li, Na, X = Cl, Br) solid state electrolytes. A theoretical prediction

Transport properties of the halogeno-alkali oxides A3OX (A = Li, Na, X = Cl, Br) nanocrystalline samples with the presence of ∑3(111) grain boundaries were computed using large-scale molecular dynamic simulations. Results on the diffusion/conduction process show that these nanocrystalline samples are characterized with higher activation energies as compared to previous theoretical studies, but closer to experiment. Such a performance can be attributed to the larger atomic density at the ∑3(111) grain boundary regions within the nanocrystals. Despite a minor deterioration of transport properties of the mixed cation Li2NaOX and Na2LiOX samples, these halogeno-alkali oxides can also be considered as good inorganic solid electrolytes in both Li- and Na-ion batteries.

Boron Silicon B2Si3q and B3Si2p Clusters: The Smallest Aromatic Ribbons

The small binary boron silicon clusters B₂Si₃^q with q going from -2 to +2 and B₃Si₂^p with p varying from -3 to +1 were reinvestigated using quantum chemical methods. The thermodynamic stability of these smallest ribbon structures is governed by both Hückel and ribbon models for aromaticity. The more negative the cluster charge, the more ribbon character is shown. In contrast, the more positive the charge state, the more pronounced the Hückel character becomes. The ribbon aromaticity character can also be classified into ribbon aromatic, semiaromatic, antiaromatic, and triplet aromatic when the electron configuration of a ribbon structure is described as [...π²⁽ⁿ⁺¹⁾σ²ⁿ], [...π²ⁿ⁺¹σ²ⁿ], [...π²ⁿσ²ⁿ], and [...π²ⁿ⁺¹σ^2n-1], respectively. Geometry optimizations of the B₂Si₃ lowest-energy structure by some density functional theory (DFT) functionals result in a nonplanar shape because it possesses an antiaromatic ribbon character. However, its π aromaticity assigned by the Hückel rule is stronger in such a way that several other DFT and coupled-cluster theory CCSD(T) calculations show that B₂Si₃ is indeed stable in a planar form (C_s). A new global equilibrium structure for the anion B₂Si₃^2-, which is a ribbon semiaromatic species, was identified. Some benchmark tests were also carried out to evaluate the performance of popular methods for the treatment of binary B-Si clusters. At odds with some previous studies, we found that with reference to the high accuracy CCSD(T)/CBS method, the hybrid TPSSh functional is reliable for a structure search, whereas the hybrid B3LYP functional is more suitable for simulations of some experimental spectroscopic results.

Unexpected structures of the Au17 gold cluster: the stars are shining

The Au₁₇ gold cluster was experimentally produced in the gas phase and characterized by its vibrational spectrum recorded using far-IR multiple photon dissociation (FIR-MPD) of Au₁₇Kr. DFT and coupled-cluster theory PNO-LCCSD(T)-F12 computations reveal that, at odds with most previous reports, Au₁₇ prefers two star-like forms derived from a pentaprism added by two extra Au atoms on both top and bottom surfaces of the pentaprism, along with five other Au atoms each attached on a lateral face. A good agreement between calculated and FIR-MPD spectra indicates a predominant presence of these star-like isomers. Stabilization of a star form arises from strong orbital interactions of an Au₁₂ core with a five-Au-atom string.

First-row transition metal doped germanium clusters Ge16M: some remarkable superhalogens

A theoretical study of geometric and electronic structures, stability and magnetic properties of both neutral and anionic Ge₁₆M^0/− clusters with M being a first-row 3d transition metal atom, is performed using quantum chemical approaches. Both the isoelectronic Ge₁₆Sc⁻ anion and neutral Ge₁₆Ti that have a perfect Frank–Kasper tetrahedral T_d shape and an electron shell filled with 68 valence electrons, emerge as magic clusters with an enhanced thermodynamic stability. The latter can be rationalized by the simple Jellium model. Geometric distortions from the Frank–Kasper tetrahedron of Ge₁₆M having more or less than 68 valence electrons can be understood by a Jahn–Teller effect. Remarkably, DFT calculations reveal that both neutral Ge₁₆Sc and Ge₁₆Cu can be considered as superhalogens as their electron affinities (≥3.6 eV) exceed the value of the halogen atoms and even that of icosahedral Al₁₃. A detailed view of the magnetic behavior of Ge₁₆M^0/− clusters shows that the magnetic moments of the atomic metals remain large even when they are quenched upon doping. When M goes from Sc to Zn, the total spin magnetic moment of Ge₁₆M^0/− increases steadily and reaches the maximum value of 3 μ_B with M = Mn before decreasing towards the end of the first-row 3d block metals. Furthermore, the IR spectra of some tetrahedral Ge₁₆M are also predicted.

A theoretical study of geometric and electronic structures, stability and magnetic properties of both neutral and anionic Ge₁₆M^0/− clusters with M being a first-row 3d transition metal atom, is performed using quantum chemical approaches.

Evolution of Vibrational Spectra in the Manganese-Silicon Clusters Mn2Sin, n = 10, 12, and 13, and Cationic [Mn2Si13]. J Phys Chem A 2022;126:1617-1626. [PMID: 35238570 PMCID: PMC9084549 DOI: 10.1021/acs.jpca.1c10027]

A comparison of DFT-computed and measured infrared spectra reveals the ground state structures of a series of gas-phase silicon clusters containing a common Mn₂ unit. Mn₂Si₁₂ and [Mn₂Si₁₃]⁺ are both axially symmetric, allowing for a clean separation of the vibrational modes into parallel (a₁) and perpendicular (e₁) components. Information about the Mn–Mn and Mn–Si bonding can be extracted by tracing the evolution of these modes as the cluster increases in size. In [Mn₂Si₁₃]⁺, where the antiprismatic core is capped on both hexagonal faces, a relatively simple spectrum emerges that reflects a pseudo-D_6d geometry. In cases where the cluster is more polar, either because there is no capping atom in the lower face (Mn₂Si₁₂) or the capping atom is present but displaced off the principal axis (Mn₂Si₁₃), the spectra include additional features derived from vibrational modes that are forbidden in the parent antiprism.

Correction: The binary aluminum scandium clusters Al x Sc y with x + y = 13: when is the icosahedron retained?

Correction for ‘The binary aluminum scandium clusters Al_xSc_y with x + y = 13: when is the icosahedron retained?’ by Ngo Tuan Cuong et al., RSC Adv., 2021, 11, 40072–40084. DOI: 10.1039/D1RA06994B.

The lowest-energy structure of the gold cluster Au10: planar vs. nonplanar?

The onset of the transition from 2D to 3D structures in pure gold clusters remains a matter of continuing debate. In this theoretical study we revisit several planar and non-planar structural motifs of the size Au₁₀ with the aim to clarify this issue. Computations using a long-range corrected exchange-correlation functional LC-BLYP, coupled-cluster theories CCSD(T) and PNO-LCCSD(T)-F12 reveal that, at variance with previous reports on the preference of a planar elongated hexagon, both planar and nonplanar isomers of the neutral Au₁₀ are energetically degenerated up to 300 K. Its 3D equilibrium geometry is a core-shell structure which can be built up from a trigonal prism by capping four extra Au atoms outside. A comparison to the available experimental vibrational spectra allows us to argue that both lowest-lying 2D and 3D isomers of Au₁₀ likely coexist in the molecular beam during measurement of its FIR spectra. This result also suggests that the 2D-3D transition of neutral gold clusters occurs at the size Au₁₀.

Theoretical Aspects of Nonconventional Hydrogen Bonds in the Complexes of Aldehydes and Hydrogen Chalcogenides

Hydrogen bonds (H-bonds) in the complexes between aldehydes and hydrogen chalcogenides, XCHO...nH₂Z with X = H, F, Cl, Br, and CH₃, Z = O, S, Se, and Te, and n = 1,2, were investigated using high-level ab initio calculations. The C_sp2-H...O H-bonds are found to be about twice as strong as the C_sp2-H...S/Se/Te counterparts. Remarkably, the S/Se/Te-H...S/Se/Te H-bonds are 4.5 times as weak as the O-H...O ones. The addition of the second H₂Z molecule into binary systems induces stronger complexes and causes a positive cooperative effect in ternary complexes. The blue shift of C_sp2-H stretching frequency involving the C_sp2-H...Z H-bond sharply increases when replacing one H atom in HCHO by a CH₃ group. In contrast, when one H atom in HCHO is substituted with a halogen, the magnitude of blue-shifting of the C_sp2-H...Z H-bond becomes smaller. The largest blue shift up to 92 cm^-1 of C_sp2-H stretching frequency in C_sp2-H...O H-bond in CH₃CHO...2H₂O has rarely been observed and is much greater than that in the cases of the C_sp2-H...S/Se/Te ones. The C_sp2-H blue shift of C_sp2-H...Z bonds in the halogenated aldehydes is converted into a red shift when H₂O is replaced by a heavier analogue, such as H₂S, H₂Se, or H₂Te. The stability and classification of nonconventional H-bonds including C_sp2-H...Se/Te, Te-H...Te, and Se/Te-H...O have been established for the first time.

Generation and reactivity of an elusive base-stabilised phosphinidene

Reduction of phosphorus dichloride 6, supported by the diaryloxyphenyl group (OCO) featuring two bulky phenoxy wingtips, by PMe₃, generates a reactive intermediate that behaves as a base-stabilized phosphinidene (OCO)P (5). Warming up a solution of this species in toluene to room temperature results in trimerization to give the isolable cyclic triphosphine [(OCO)P]₃, whereas in situ trapping with 2,3-dimethylbutadiene-1,3 afforded a 3,4-dimethylphospholene-3. Investigation of the reduction of 6 by the phosphine PMe₃ by NMR led to the observation of a persistent species between -10 °C and 10 °C. A DFT study of this process suggests that this compound cannot be the proposed phosphinidene 5, and is more likely the diphosphine (OCO)ClP-PCl(OCO) (12). Attempted reduction of 5 by the bulky carbene IPr resulted in unusual electrophilic substitution in the carbene olefin backbone by the chlorophosphinyl group.

Another look at energetically quasi-degenerate structures of the gold cluster Au27 q with q = 1, 0, -1

Quantum chemical computations were used to reinvestigate the geometries, spectroscopic, and energetic properties of the gold clusters Au₂₇ ^q in three charge states (q = 1, 0, -1). Density functional theory (DFT) and the domain-based local pair natural orbital modification of the coupled-cluster theory DLPNO-CCSD(T) calculations revealed that, at variance with earlier reports in the literature, while the anion Au₂₇ ^- tends to exist in a tube-like form, both the lowest-energy Au₂₇ and Au₂₇ ⁺ isomers exhibit a pyramidal shape. However, several isomers were found to lie very close in energy, thus rendering a structural transition and their coexistence easy to occur. More specifically, the equilibrium geometry of the neutral Au₂₇ is a core-shell pyramid-like structure with one gold atom located inside. We also identified a novel ground state for the anion Au₂₇ ^- and located for the first time the global minimum of the cation Au₂₇ ⁺ . The vertical detachment energies of the neutral and anionic states were also computed and used to assign the available experimental photoelectron spectra. Although many Au₂₇ isomers were predicted to be energetically quasi-degenerate, the corresponding distinctive vibrational signatures can be used as fingerprints for the identification of cluster geometrical features.

The binary boron lithium clusters B12Lin with n = 1-14: in search for hydrogen storage materials

Molecular structures and properties of the binary clusters containing twelve boron atoms mixed with n lithium atoms, B₁₂Li_n with n = 1-14, were investigated using density functional theory with the TPSSh functional and the 6-311+G(d) basis set. Energetic parameters including relative energies, average binding energies and second-order energies of the entire series were predicted using the coupled-cluster theory (U)CCSD(T) in conjunction with the cc-pVTZ basis set. Several lowest-lying isomers were determined for each size B₁₂Li_n whose energies differ from each other by <3 kcal mol^-1, except for n = 1, 2 and 4 (≤5 kcal mol^-1), and particularly n = 8 (∼13 kcal mol^-1). Electronic structure and chemical bonding in some specific sizes such as B₁₂Li₄, B₁₂Li₈ and B₁₂Li₁₄ were analyzed in detail. We established the electron shells of some magic clusters such as the B₁₂Li₄ cone for which we proposed a mixed cone-disk electron shell model. Thanks to both the phenomenological shell and Clemenger-Nilsson models, B₁₂Li₈ which contains a specific set of shells of 44 valence electrons is a high stability species. The arrangement of Li atoms around a fullerene B₁₂ framework shows that the mixed B₁₂Li₈ emerges as the most suitable of this cluster series to adsorb molecular hydrogen. Up to 32 H₂ molecules can strongly be attached to the B₁₂Li₈ cluster which is thus predicted to be a realistic candidate for hydrogen storage material with gravimetric density reaching up to a theoritical limit of 26 wt%. Attachment of the fifth H₂ molecule to each Li atom of B₁₂Li₈ results in weaker average bonds but can give rise to a total of 40 H₂ molecules, corresponding to 30 wt% of hydrogen.

SERS Chemical Enhancement of 2,4,5-Trichlorophenoxyacetic Acid Adsorbed on Silver Substrate

Surface-enhanced Raman spectroscopy (SERS) was employed to gain an understanding of the chemical enhancement mechanism of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), an Agent Orange, adsorbed on a silver substrate surface. Experimental measurements were performed using a micro-Raman spectrophotometer with an excitation wavelength of 532 nm and successfully detected 2,4,5-T at a relatively low concentration of 0.4 nM. Density functional theory (DFT) calculations on the interactions of the 2,4,5-T molecule with some small silver clusters, Ag_n with n = 4, 8, and 20, as well as with extended Ag surfaces, demonstrate that the most stable adsorption configuration is formed via coordination of Cl9 sites and carbonyl C═O group on the 2,4,5-T ligand to the Ag atoms on surfaces. Analyses of charge transfer mechanism and frontier orbitals distributions show an electron transfer from 2,4,5-T to the cluster in the ground state, and an inversed trend occurs for the excited singlet state process, consequently leading to a chemical enhancement of SERS signals. The obtained results are of importance for subsequent work in guiding the design of mobile sensors specifically used for services of rapid screening and detection of these toxic compounds present in the environment, as well as agricultural and food products. Extensive computations pointed out that small silver clusters, in particular of Ag₂₀ size, can be used as appropriate models for a metal nanoparticle surface.

An octacoordinated Nb atom in the NbAl8H8+ cluster

The NbAl₈H₈⁺ cluster was formed in a molecular beam and characterized by mass spectrometry and infrared spectroscopy. Density functional theory calculations show the lowest-energy isomer is a high symmetry singlet with the Nb atom placed at the center of a distorted hexagonal Al ring and coordinated by two AlH moieties, therefore exhibiting octacoordination. The unprecedented high-symmetric geometry is attributed to the 20 valence electrons; the central Nb atom adheres to the 18-electron rule and two additional delocalized electrons stabilize the hexagonal ring.

Adsorption/Desorption Behaviors and SERS Chemical Enhancement of 6-Mercaptopurine on a Nanostructured Gold Surface: The Au20 Cluster Model

Computational approaches are employed to elucidate the binding mechanism and the SERS phenomenon of 6-mercaptopurine (6MP) adsorbed on the tetrahedral Au20 cluster as a simple model for a nanostructured gold surface. Computations are carried out in both vacuum and aqueous environments using a continuum model. In the gaseous phase and neutral conditions, interaction of 6MP with the gold cluster is mostly dominated by a covalent Au-S bond and partially stabilized by the Au⋅⋅⋅H-N coupling. However, in acidic solution, the nonconventional Au⋅⋅⋅H-S hydrogen-bond becomes the most favorable binding mode. The 6MP affinity for gold clusters decreases in the order of vacuum > neutral solution > acidic medium. During the adsorption, the energy gap of Au20 substantially declines, leading to an increase in its electrical conductivity, which can be converted to an electrical noise. Moreover, such interaction is likely a reversible process and triggered by either the low pH in sick tissues or the presence of cysteine residues in protein matrices. While N-H bending and stretching vibrations play major roles in the SERS phenomenon of 6MP on gold surfaces in neutral solution, the strongest enhancement in acidic environment is mostly due to an Au⋅⋅⋅H-S coupling, rather than an aromatic ring-gold surface π overlap as previously proposed.

Comment on 'Structural characterization, reactivity and vibrational properties of silver clusters: a new global minimum for Ag16' by P. L. Rodríguez-Kessler, A. R. Rodríguez-Domínguez, D. MacLeod Carey and A. Muñoz-Castro, Phys. Chem. Chem. Phys., 2020, 22, 27255, DOI: D0CP04018E

A recent paper by Rodríguez-Kessler et al., Phys. Chem. Chem. Phys., 2020, 22, 27255-27262, reported not only results of quantum chemical computations (using the PW91 density functional) on Ag₁₆ clusters as emphasized in the article's title, but also on the Ag₁₅ size. These authors confirmed previous results obtained by McKee and Samokhvalov (J. Phys. Chem. A, 2017, 121, 5018-5028 using the M06 density functional) that the most stable isomer of Ag₁₅ is a C_2v structure. We wish to point out that two low symmetry isomers of Ag₁₅ that have a similar energy content are even lower in energy than their reported C_2v global minimum. The relative energies between low-lying Ag₁₅ isomers were again found to be method-dependent, and within the expected accuracy of DFT and CCSD(T) methods they could be considered as energetically degenerate, and likely coexist in a molecular beam. The new lower-energy Ag₁₅ isomers appear to fit more consistently within the structural evolution of small silver clusters.

Gold nanoclusters as prospective carriers and detectors of pramipexole

Pramipexole (PPX) is known in the treatment of Parkinson's disease and restless legs syndrome. We carried out a theoretical investigation on pramipexole–Au cluster interactions for the applications of drug delivery and detection. Three Au_N clusters with sizes N = 6, 8 and 20 were used as reactant models to simulate the metallic nanostructured surfaces. Quantum chemical computations were performed in both gas phase and aqueous environments using density functional theory (DFT) with the PBE functional and the cc-pVDZ-PP/cc-pVTZ basis set. The PPX drug is mainly adsorbed on gold clusters via its nitrogen atom of the thiazole ring with binding energies of ca. −22 to −28 kcal mol⁻¹ in vacuum and ca. −18 to −24 kcal mol⁻¹ in aqueous solution. In addition to such Au–N covalent bonding, the metal–drug interactions are further stabilized by electrostatic effects, namely hydrogen-bond NH⋯Au contributions. Surface-enhanced Raman scattering (SERS) of PPX adsorbed on the Au surfaces and its desorption process were also examined. In comparison to Au₈, both Au₆ and Au₂₀ clusters undergo a shorter recovery time and a larger change of energy gap, being possibly conducive to electrical conversion, thus signaling for detection of the drug. A chemical enhancement mechanism for SERS procedure was again established in view of the formation of nonconventional hydrogen interactions Au⋯H–N. The binding of PPX to a gold cluster is expected to be reversible and triggered by the presence of cysteine residues in protein matrices or lower-shifted alteration of environment pH. These findings would encourage either further theoretical probes to reach more accurate views on the efficiency of pramipexole–Au interactions, or experimental attempts to build appropriate gold nanostructures for practical trials, harnessing their potentiality for applications.

Gold clusters as prospective carriers and detectors of pramipexole.

Theoretical Study of the Binding of the Thiol-Containing Cysteine Amino Acid to the Silver Surface Using a Cluster Model

Computational approaches within the framework of density functional theory (DFT) were employed to elucidate the binding mechanism of the cysteine amino acid on silver nanoparticles using several small silver clusters Ag_n with n = 2-10 as surface models. The long-range corrected LC-BLYP functional and correlation consistent basis sets cc-pVTZ-PP and cc-pVTZ were used to determine the structural features, energetics, and spectroscopic and electronic properties of the resulting complexes. In vacuum and highly acidic conditions, cysteine molecules prefer to adsorb on silver clusters via their amine group. In aqueous solution, the thiolate head turns out to be the most energetically favorable binding site. The cysteine affinity of silver clusters is greatly altered in different conditions, i.e., acidic solution < vacuum < aqueous solution, and is strongly dependent on the cluster size. As compared to free clusters, the frontier orbital energy gap of the ones capped by cysteine is significantly improved, which corresponds to stronger stability, especially in aqueous solution. The analysis of frontier orbitals also reveals that both forward and backward electron donations exhibit comparable contributions to the enhancement of stabilizing interactions. As for an application, a chemical enhancement mechanism of the surface-enhanced Raman scattering (SERS) procedure of cysteine by silver clusters was also analyzed.

Enhanced Li-ion transport in divalent metal-doped Li2SnO3

The improvement of Li-ion transport properties and doping engineering in Li-ion batteries are currently active research topics in the search for next-generation energy storage devices. In this theoretical work, the intrinsic defect formation and transport properties of divalent metal-doped Li2SnO3, which is being considered as an electrode and coating electrode material, are explored using atomistic simulations. Defect formation simulations reveal that all divalent dopants (Zn, Sc, Cd and Eu) occupy the Li site with charge compensation through Li vacancies. Molecular dynamics simulations show that the divalent dopants significantly reduce the activation energy for ionic diffusion and conduction compared to the undoped sample. The effects of both grains and grain boundaries on the Li-ion transport properties are investigated. Our calculated results demonstrate a marked improvement in the properties of Li2SnO3 that can be achieved either in current commercial and next-generation Li-ion battery technologies through divalent doping in mono- and polycrystalline Li2SnO3 samples.

A theoretical design of bipolar host materials for blue phosphorescent OLED

A series of host molecules have been designed and characterized for use in phosphorescent organic light-emitting diode devices. The parent host molecule was modified by adding nitrogen-containing group at different positions. The first triplet excited (T₁) and first singlet excited (S₁) states energies, frontier orbital energy levels, reorganization energies and injection barriers of the molecules designed were calculated in comparison to those of the reference host, emitter, hole and electron transport materials. Interestingly, our results suggest that addition of nitrogen group into the dibenzothiophene by a N-C linkage increases the triplet energy separation and decreases the injection barriers making them suitable for use as blue phosphorescent materials.

Structure, stability and bonding of the leapfrog B24 0 ,±1,±2

Two new structural motifs of the B₂₄ clusters are constructed by use of the leapfrog transformation. The resulting leapfrog B₂₄ has either a bowl shape with a square vacancy or a quasi-planar 2D close-packed triangular boron sheet. The neutral and ionic forms of the latter are found to be more stable than their homologous leapfrog bowl clusters, with the exception of the dicationic B₂₄ ⁺² . While the leapfrog isomer is less stable than the tubular double ring in the neutral state, it becomes competitive in some ionic states. The nucleus independent chemical shift, electron localization function, ring current maps and the electronic structure of leapfrog B₂₄ clusters reveal them to behave as aromatics.

The binary aluminum scandium clusters AlxScy with x + y = 13: when is the icosahedron retained?

Geometrical and electronic structures of the 13-atom clusters Al_xSc_y with x + y = 13, as well as their thermodynamic stabilities were investigated using DFT calculations. Both anionic and neutral isomers of Al_xSc_y were found to retain an icosahedral shape of both Al₁₃ and Sc₁₃ systems in which an Al atom occupies the endohedral central position of the icosahedral cage, irrespective of the number of Al atoms present. Such a phenomenon occurs to maximize the number of stronger Al–Al and Sc–Al bonds instead of the weaker Sc–Sc bonds. NBO analyses were applied to examine their electron configurations and rationalize the large number of open shells and thereby high multiplicities of the mixed clusters having more than three Sc atoms. The SOMOs are the molecular orbitals belonged to the irreducible representations of the symmetry point group of the clusters studied, rather than to the cluster electron shells. Evaluation of the average binding energies showed that the thermodynamic stability of Al_xSc_y clusters is insignificantly altered as the number y goes from 0 to 7 and then steadily decreases when y attains the 7–13 range. Increase of the Sc atom number also reduces the electron affinities of the binary Al_xSc_y clusters, and thus they gradually lose the superhalogen characteristics with respect to the pure Al₁₃.

The icosahedral structure of the Al_xSc_y clusters with x + y = 13.

Autocatalysis in Formose Reaction and Formation of RNA Nucleosides

Understanding of the abiotic formation of nucleosides under geochemical conditions is currently a major scientific challenge. In this study, free radical pathways for formation of RNA nucleosides with canonical nucleobases are proposed for the first time. The pathways proceed with relatively low energy barriers for the formation of ribose as well as all RNA nucleosides. The formose reaction proceeds either with or without Ca²⁺ and CaOH⁺ cations. An autocalytic cycle for the formation of both glycolaldehyde and glyceraldehyde is identified when Ca²⁺ or CaOH⁺ cations are involved in the reaction. The results suggest that Ca²⁺ cations are not involved in the formation of ribose from glyceraldehyde. In addition, these pathways lead to the formation of dihydroxyacetone and d-erythrose. Calculated results show that the glycosidic bond can be formed abiotically between the d-ribose and the nucleobase, where d-ribose forms a cyclic free radical that subsequently reacts with the neutral nucleobase. Involvement of proper nucleobase tautomer is important for the formation of RNA nucleosides. Our approaches provide a solution for the long-standing question of how the glycosidic bond is formed under the abiotic conditions with low energy barriers. The pathways for formation of the sugars without a catalyst are relevant to the formation of sugars in interstellar clouds. On the other hand, the autocalysis in the formose reaction followed by the formation of the nucleosides is appropriate for the abiotic synthesis taking place in the presence of water in the early Earth environment. The Ca²⁺ and CaOH⁺ cations appear to be the first nonenzymatic catalytic systems for formation of biomolecules.

Theoretical study on the interaction of iodide electrolyte/organic dye with the TiO2 surface in dye-sensitized solar cells

The iodide/triiodide interaction with the dye on a semiconductor surface plays a significant role in understanding the dye-sensitized solar cells (DSSCs) mechanism and improving its efficiency. In the present study, density functional theory (DFT) calculations were used to determine the interaction between the complexed iodide redox couple with dye/TiO2 for the relevance of DSSCs. Three new metal-free organic dyes noted as D1Y, D2Y and D3Y, featured with D-π-A configuration were designed by varying functional groups on the donor moiety. We analyzed the structural and electronic properties of these dyes when standing alone and being adsorbed on the oxide surface with the iodide electrolyte. Of the designed dyes, the incorporation of a strong donor unit in D1Y and D2Y sensitizers in conjunction with iodide electrolytes on the TiO2 surface provides better adsorption and electronic properties in comparison to those from the dye alone on the TiO2 surface. Analysis of density of states (DOS) indicates that the introduction of a strong electron-donating group into the organic dye, mainly D1Y and D2Y with an iodide electrolyte on the surface remarkably upshifts the Fermi energy, thereby improving the efficiency of the DSSCs by an increase of the open-circuit voltage (Voc). The present finding constitutes the basis for achieving a deeper understanding of the intrinsic interaction taking place at the electrolyte/dye/TiO2 interface and provides us with directions for the design of efficient dyes and redox electrolytes for improving DSSCs.

Theoretical investigation of protonated thiophene and two of its nitrile substituted derivatives (2-cyanothiophene and 3-cyanothiophene)

Theoretical and experimental spectroscopic data for protonated cyano-thiophenes (R-CNH⁺ with R = C₄H₃S), which are needed for their interstellar search and/or detection, are still lacking in the literature. Considering the high abundance and reactivity of H₃⁺ in the interstellar medium (ISM), a quantum chemical investigation on protonated thiophene and two of its nitrile-substituted derivatives (2-cyanothiophene and 3-cyanothiophene) is undertaken for their characterization. The geometrical structures for the title species are calculated at the M06-2X/6-31G(d,p) level of theory, followed by an empirical correction for systematic errors. At the same level of theory, IR and Raman spectra are explored and the rotational parameters are calculated. The proton affinity (PA) of R-CN and the enthalpy, entropy and Gibbs free energy changes (Δ_rH, Δ_rS and Δ_rG) of the reactions producing R-CNH⁺ are computed at the G2(MP2) and G3B3 levels of theory and at different temperatures. The PA calculations show that the protonation favors the nitrogen atom, while Δ_rH, Δ_rS, and Δ_rG reveal the spontaneous reactions producing R-CNH⁺ and their neutral forms. In addition, quadrupole hyperfine structures are predicted, while the region where the brightest lines fall at different temperatures is discussed. These results are expected to assist astrophysicists and astrochemists in the search for new species in the ISM.

Substituent Effects on the N-H Bond Dissociation Enthalpies, Ionization Energies, Acidities, and Radical Scavenging Behavior of 3,7-Disubstituted Phenoxazines and 3,7-Disubstituted Phenothiazines

The substituent effects on the N-H bond dissociation enthalpies (BDE), ionization energies (IE), acidities (proton affinity, PA), and radical scavenging behavior of 3,7-disubstituted phenoxazines (PhozNHs) and 3,7-disubstituted phenothiazines (PhtzNHs) were determined using density functional theory, with the M05-2X functional in conjunction with the 6-311++G(d,p) basis set. These thermochemical parameters calculated in both gas phase and benzene solution with respect to the changes in several different substituents including halogen, electron-withdrawing, and electron-donating groups at both 3 and 7 positions in both PhozNHs and PhtzNHs systems were analyzed in terms of the inherent relationships between them with some quantitative substituent effect parameters. The kinetic rate constants of hydrogen-atom exchange reactions between PhozNH and PhtzNH derivatives with the HOO• radical were also calculated, and the effects of the substituents on the kinetic behaviors of these reactions were thereby quantitatively evaluated.

Structure and Stability of a Trefoil Leaf Motif of Metal-Doped Silicon and Germanium Clusters: M3@E20 with E = Si and Ge and M = Fe, Ru, and Os

The first trefoil leaf-shaped cluster, a novel structural motif, was discovered by quantum chemical computations using density functional theory methods for both binary M₃@E₂₀ silicon and germanium clusters in which a metallic M₃ cycle is encapsulated within a D_3h Si₂₀ or Ge₂₀ cage. The triatomic transition metal Fe₃, Ru₃, and Os₃ cycles exhibit a suitable size to be placed inside both Si and Ge cages and satisfy electronic conditions, which thermodynamically stabilize the mixed clusters. Stabilizing orbital interactions between the M₃ cycle and D_3h E₂₀ cage crucially contributes to the high stability of the resulting M₃@E₂₀ trefoil cluster. The novel trefoil leaf-shaped clusters can be used as building units to create different nanoassemblies.

Molecular structure, IR, Raman and UV-VIS spectra of 2-cyanothiophene and 3-cyanothiophene: A comparative quantum chemical investigation

Spectroscopic data which are needed for detection of new species in the interstellar medium (ISM) are missing for five- and six-membered ring substituted derivatives. In order to supplement the existing literature information, quantum chemical calculations, including geometrical optimizations, simulated IR, Raman and UV-VIS spectra were carried out for 2-cyanothiophene (2CNT) and 3-cyanothiophene (3CNT). Structures for these molecular species were calculated from M06-2X/6-31G(d,p) computed results in conjunction with corrections for systematic deficiencies of basis sets and empirical adjustments for some geometrical parameters. A complete description of IR and Raman spectra including IR frequency assignment and potential energy distribution (PED) analysis is provided. IR and Raman spectra for deuterated species and UV-VIS spectra for thiophene, 2CNT and 3CNT were analyzed. The present computed molecular data are expected to assist astro-scientists in the search of these species in the ISM.

Boosting Li-Ion Transport in Transition-Metal-Doped Li2SnO3

Lithium stannate (Li₂SnO₃) is currently being considered as a material for electrode and electrode coating applications in Li-ion batteries. The intrinsic defect formation and Li-ion transport properties of Li₂SnO₃ doped with divalent and trivalent transition-metal dopants (Mn, Fe, Co, and Ni) are explored in this work using atomistic simulations. Defect formation simulations reveal that all divalent dopants occupy the Li site with charge compensation through Li vacancies. For trivalent doping, occupation of the Sn site is energetically preferred with charge compensation from Li interstitials. Molecular dynamics simulations reveal that divalent and trivalent dopants increase Li-ion diffusion and reduce its activation energy compared with the undoped system. We show that Li₂SnO₃ with Li excess or deficiency as a result of doping has improved Li-transport properties. This study highlights the substantial improvement in Li-ion diffusion of Li₂SnO₃ for both current commercial and next-generation Li-ion battery technologies that can be achieved through transition-metal doping.

Elucidating the binding mechanism of thione-containing mercaptopurine and thioguanine drugs to small gold clusters

Density functional theory methods were employed to clarify the adsorption/desorption behaviors of the thione-containing mercaptopurine and thioguanine drugs on the gold surface using both small Au₆ and Au₈ clusters as model reactants. Structural features, thermodynamic parameters, bonding characteristics, and electronic properties of the resulting complexes were investigated using the Perdew-Burke-Ernzerhof (PBE) and LC-BLYP functionals along with correlation-consistent basis sets, namely cc-pVDZ-PP for gold and cc-pVTZ for non-metals. Computed results show that the drug molecules tend to anchor on the gold cluster at the S atom with binding energies around -34 to -40 kcal/mol (in vacuum) and - 28 to -32 kcal/mol (in aqueous solution). As compared to Au₈ , Au₆ undergoes a shorter recovery time and a larger change of energy gap that could be converted to an electrical signal for selective detection of the drugs. Furthermore, interactions between the drugs and gold clusters are reversible processes and a drug release mechanism was also proposed. Accordingly, the drugs are able to separate from the gold surface due to either a slight change of pH in tumor cells or the presence of cysteine residues in protein matrices.

The teetotum cluster Li2FeB14 and its possible use for constructing boron nanowires

Systematic density functional theory (DFT) calculations using the TPSSh functional and the def2-TZVP basis set were carried out to identify the global energy minimum structure of the Li2FeB14 cluster. Keeping the double ring tubular shape of FeB14, capping of two Li atoms leads to a teetotum form at a low spin state, in which the Fe atom is endohedrally covered by two B7 strings, and both Li atoms are attached to Fe along the C7 axis at both sides. Calculated results show that strong electrostatic interactions between 2Li+ and Fe2- arising from Li electron transfer upon doping particularly provide a key driving force for stabilizing this charge-transfer structure. The bonding pattern of the teetotum can be understood from the hollow cylinder model (HCM). TD-DFT calculations demonstrate that this cluster can also be regarded as a useful material for transparent optoelectronic devices. Furthermore, the Li2FeB14 superatom can be used as a building block for making boron-based nanowires with metallic character. Replacement of Li atoms by Mg atoms was also found to lead to nanowires.

Interplay between σ Holes, Anion···H-C, and Cation-π Interactions in Dibromo[2,2]paracyclophane Complexes

Theoretical calculations were performed to investigate the interplay between σ-hole, anion-HC and cation-π interactions in the complexes of dibromo[2,2]paracyclophane (DBr[2,2]PCP) with alkali (Li⁺, Na⁺, K⁺), alkaline earth metal cations (Be²⁺, Mg²⁺, and Ca²⁺), and halogen anions (F^-, Cl^-, and Br^-) using the wave function (MP2) and density functional theory (M06-2X and B3LYP) methods with the 6-311++G(d,p) basis set. The study reveals that DBr[2,2]PCP behaves as amphoteric molecule with a predominance of basic character. It prefers to interact with hard cations and hard anions such as Be²⁺ and F^- through cation-π and anion···HC interactions, respectively. Substitution of Br by F and Cl atoms in DBr[2,2]PCP decreases slightly the interaction energies of DX[2,2]PCP-halogen complexes (X = F, Cl, and Br) by 2.0 and 0.3 kcal/mol (M06-2X), respectively. The anion-HC interactions in DBr[2,2]PCP complexes are ∼10 kcal/mol stronger (B3LYP; ∼15 kcal/mol at M06-2X and 7 kcal/mol at MP2) than the σ-hole interactions.

Structures, stabilities and aromatic properties of endohedrally transition metal doped boron clusters M@B22, M = Sc and Ti: a theoretical study

A genetic search algorithm in conjunction with density functional theory calculations was used to determine the lowest-energy minima of the pure B22 cluster and thereby to evaluate the capacity of its isomers to form endohedrally doped cages with two transition metal atoms M (M = Sc and Ti). An important charge transfer from metal atoms M to the boron cage takes place, stabilizing the endohedral compounds, as predicted with the genetic algorithm implemented. High-level coupled-cluster theory CCSD(T) calculations were carried out to confirm that the structures found are the lowest-energy isomers. For a deeper understanding of the doping effects and related charge transfer, the best structural motif of the B22 isomers was also determined when the bare cages are in anionic states, such as B222- and B224-. It was found that B22 has an appropriate size, geometric shape and electronic state to host the chosen metal atoms and, consequently, to form stable endohedrally doped compounds Ti@B22 (C2v, 4-Ti) and Sc@B22 (C2v, 5-Sc). The chemical bonding was analyzed in order to understand the molecular orbitals that these novel systems form. The cage aromaticity was evaluated by means of the nuclear independent chemical shift (NICS(0)iso) indices, the isochemical shielding surface (ICSSzz), the anisotropy of the current induced density (ACID) maps, and the magnetic ring current Gauge-Including Magnetically Induced Current (GIMIC) method, indicating that aromaticity plays a crucial role in the stabilization of endohedrally doped boron clusters. Finally, the thermodynamic stability of the latter, using parameters derived from density functional theory (DFT), was evaluated. Ab initio molecular dynamics (AIMD) simulations were performed to elucidate the stability, at high temperature, of the most stable endohedrally doped boron clusters 4-Ti and 5-Sc.