Ionization of pyridine: Interplay of orbital relaxation and electron correlation

Deciphering the electronic structure and conformational stability of 2-pyridinecarboxaldehyde

The conformational structures and ionisation dynamics of 2-pyridinecarboxaldehyde (2-PCA) were explored using high-resolution vacuum ultraviolet mass-analysed threshold ionisation (VUV-MATI) spectroscopy, complemented by Franck-Condon (FC) simulations and quantum chemical calculations. The precise adiabatic ionisation energy of 2-PCA was determined to be 76 589 ± 4 cm-1 (9.4958 ± 0.0005 eV), which is notably lower than the previous values obtained from electron impact ionisation studies. The vibrationally resolved VUV-MATI spectrum of the molecule confirmed that ionisation predominantly originates from its s-trans conformer, with no significant contribution from its s-cis conformer, indicating that the interconversion barrier effectively limits the population of this species under supersonic expansion conditions. Molecular and natural bond orbital analyses revealed that the highest occupied molecular orbital of the s-trans conformer is primarily composed of a nitrogen nonbonding orbital, which interacts with the oxygen lone pairs of the formyl group. This interaction stabilises the electronic structure of the conformer, resulting in an increased ionisation energy compared with pyridine. FC analysis further demonstrated that vibrational excitations in the cationic state are predominantly associated with the in-plane ring and formyl bending modes, producing distinct vibrational progressions in the VUV-MATI spectrum. These findings provide not only valuable insights into the electronic structure, conformational stability, and ionisation dynamics of 2-PCA, but also a deeper understanding of the effect of functional-group substitution in pyridine derivatives. Moreover, the results underscore the effectiveness of VUV-MATI spectroscopy in resolving conformer-specific ionisation processes, paving the way for further investigations into the electronic properties of heterocyclic molecules.

One-pot three-component synthesis of stable pyrrole-3-selones using propargyl amines, acyl chlorides, and elemental selenium

Stable pyrrole-tailored selones, 1,2,5-trisubstituted-1,2-dihydro-3H-pyrrole-3-selones, have been synthesized in up to 75% yield from aminoacetylenic ketones and elemental selenium (KOH/EtOH, room temperature, 10-11 h). The work is mainly focused on the one-pot synthesis of these compounds (50-75% yields). This synthesis has been achieved via the reactions of propargyl amines with acyl chlorides in the presence of a Pd/Cu catalyst (toluene, 40-45 °C, 2-3 h) followed by the addition of a pre-heated mixture of Se/KOH (SnCl2)/EtOH at room temperature. Our quantum-chemical calculations predict an exceptionally large ground-state dipole moment of about 7.6 Debye and an anomalously low vertical ionization energy of about 6.5 eV for the molecules under study. According to our calculations, these remarkable properties are due to the fact that the CSe bond is incorporated into the common π-system of the 3H-pyrrole ring, allowing charge transfer from N to Se. The latter effect also contributes to the exceptional stability of the reported selones.

Investigating valence orbitals and cationic structure of 2,6-difluoropyridine via high-resolution VUV-MATI spectroscopy and Franck-Condon simulations

Pyridine derivatives are fundamental in fields such as organic chemistry, materials science, and pharmaceuticals, largely due to their versatile electronic properties. Fluorination of pyridine significantly alters these properties, yet the specific effects of the position and number of fluorine atoms on valence orbitals and cationic structures remain not fully understood. This study examines the impact of fluorine substitution on the valence orbitals and cationic structures of various pyridine derivatives, with a particular emphasis on 2,6-difluoropyridine (2,6-DFP). Using high-resolution vacuum ultraviolet mass-analysed threshold ionisation (VUV-MATI) spectroscopy, the adiabatic ionisation energy of 2,6-DFP was determined to be 78 365 ± 3 cm-1 (9.7160 ± 0.0004 eV). Franck-Condon simulations were conducted to interpret the VUV-MATI spectra, providing detailed insights into the molecular structure and vibrational modes of the cationic form. The analysis indicated a symmetry shift from C2V to C1 upon ionisation, highlighted by the presence of out-of-plane ring-bending modes. Natural bond orbital analysis identified the highest occupied molecular orbital (HOMO) and HOMO-1 as π-orbitals, with HOMO-2 being a nonbonding orbital. The introduction of two ortho-fluorine substitutions in 2,6-DFP significantly influenced this electronic configuration, stabilising the nonbonding orbital through interactions with the two fluorine σ-type lone pairs. This stabilisation notably altered the valence orbital ordering compared to that of 2-fluoropyridine, resulting in a substantial difference in the binding energies between the HOMO and HOMO-1. This research provides a deeper understanding of how halogen substitution affects the electronic properties of pyridine derivatives, promoting research in the field of physical chemistry.

Impact of chlorine substitution on valence orbitals and ionization dynamics in 3-chloropyridine: Insights from high-resolution vacuum ultraviolet mass-analyzed threshold ionization study

In this study, the effects of chlorine substitution on the valence orbitals and electronic states of 3-chloropyridine (3-CP) were investigated utilizing high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy and computational methods. High-quality vibrational spectra were obtained from the VUV-MATI spectra of 3-CP isotopomers (35Cl and 37Cl), revealing high-quality vibrational spectra for the lowest cationic states. The adiabatic ionization energies (AIEs) of these isotopomers were accurately determined, providing detailed information about the electronic structure and ionization dynamics. Intense spectra peaks were linked with the D1 excited state of the 3-CP cation, with vibronic transitions in this state closely matching those predicted by Franck-Condon simulations. This provided insights into the cationic structure and the roles of the highest occupied molecular orbital (HOMO) and the HOMO-1. The HOMO was primarily a π orbital of the pyridine ring, while the HOMO-1 consisted of nonbonding orbitals. The AIEs suggested that meta-chlorine substitution stabilizes nonbonding orbitals less effectively than ortho substitution, indicating closely spaced electronic states in the 3-CP cation. Minor discrepancies in vibrational frequencies and intensities, particularly above 800 cm-1, suggested the presence of vibronic coupling, warranting further investigation. Overall, this study provided a comprehensive understanding of the vibronic and ionization properties of 3-CP, emphasizing the influence of the position of the chlorine substitution on molecular orbitals and the value of advanced theoretical and experimental approaches for analyzing the vibrational spectra of complex molecules.

Fourth-Order Algebraic Diagrammatic Construction for Electron Detachment and Attachment: The IP- and EA-ADC(4) Methods

We present a non-Dyson fourth-order algebraic diagrammatic construction formulation of the electron propagator, featuring the distinct IP- and EA-ADC(4) schemes for the treatment of ionization and electron attachment processes. The algebraic expressions have been derived automatically using the intermediate state representation approach and implemented in the Q-Chem quantum-chemical program package. The performance of the novel methods is assessed with respect to high-level reference data for ionization potentials and electron affinities of closed- and open-shell systems. While only minor improvements over the corresponding third-order methods are observed for one-hole ionization and one-particle electron attachment processes from closed-shell systems (MAEIP-ADC(4) = 0.27 eV and MAEEA-ADC(4) = 0.05 eV), a significantly enhanced performance is found in case of open-shell reference states (MAEIP-ADC(4) = 0.11 eV and MAEEA-ADC(4) = 0.02 eV). A particularly appealing feature of the novel methods is their accurate treatment of satellite transitions. For closed-shell reference states, we obtain accuracies of MAEIP-ADC(4) = 0.81 eV and MAEEA-ADC(4) = 0.27 eV, while in case of open-shell reference states, mean absolute errors of MAEIP-ADC(4) = 0.15 eV and MAEEA-ADC(4) = 0.27 eV are found.

Revealing the Role of N Heteroatoms in Noncovalent Aromatic Interactions by Ultrafast Intermolecular Coulombic Decay

Despite the widely recognized importance of noncovalent interactions involving aromatic rings in many fields, our understanding of the underlying forces and structural patterns, especially the impact of heteroaromaticity, is still incomplete. Here, we investigate the relaxation processes that follow inner-valence ionization in a range of molecular dimers involving various combinations of benzene, pyridine, and pyrimidine, which initiate an ultrafast intermolecular Coulombic decay process. Multiparticle coincidence momentum spectroscopy, combined with ab initio calculations, enables us to explore the principal orientations of these fundamental dimers and, thus, to elucidate the influence of N heteroatoms on the relative preference of the aromatic π-stacking, H-bonding, and CH-π interactions and their dependence on the number of nitrogen atoms in the rings. Our studies reveal a sensitive tool for the structural imaging of molecular complexes and provide a more complete understanding of the effects of N heteroatoms on the noncovalent aromatic interactions at the molecular level.

The ionic and ground states of gamma-pyrone. The photoionization spectrum studied by synchrotron radiation and interpreted by configuration interaction and density functional calculations

A synchrotron-based photoionization spectrum up to 27 eV represents a considerable improvement in resolution over early He(I) and He(II) spectra. Symmetry-adapted coupled cluster calculations of the ionic state sequence give the sequence of state vertical ionization energies (VIE) as 1²B₂ < 1²B₁ < 1²A₂ < 2²B₁ < 1²A₁. Generally, these symmetry-adapted cluster configuration interactions VIE match reasonably well with the experimental spectrum over this wide energy range. Density functional calculations of the corresponding adiabatic terms (AIE) were also performed. Higher energy ionic states were determined by complete active space self-consistent field methods; these include all π-ionizations and some σ-ionic states. These were analyzed by Franck-Condon (FC) procedures and compared with an experiment. The spectral onset is complex, where two states, later shown to be the 1²B₂ and 1²B₁ states, are strongly overlapping. The superposition of the FC vibrational structure in the 1²B₂ and 1²B₁ states accounts for most of the peaks arising at the onset of the photoelectron spectra. However, the small separation between these two ionic states makes vibronic interaction fairly inevitable. In the absence of Herzberg-Teller analyses for ionic states, we have sought and determined a transition state between the 1²B₂ and 1²B₁ states, showing that vibronic coupling does occur. The lack of degradation in the vibrational envelope of the higher of the two states contrasts with our previous work on the halogenobenzenes, where overlapping state envelopes led to considerable widening of the line width at half-height of the higher energy states.

Differentiation of Seven Isomeric n-Pentylquinoline Radical Cations Based on Energy-Resolved Medium-Energy Collision-Activated Dissociation

Asphaltenes, a major and undesirable component of heavy crude oil, contain many different types of large aromatic compounds. These compounds include nitrogen-containing heteroaromatic compounds that are thought to be the main culprit in the deactivation of catalysts in crude oil refinery processes. Unfortunately, prevention of this is challenging as the structures and properties of the nitrogen-containing heteroaromatic compounds are poorly understood. To facilitate their structural characterization, an approach based on ion-trap collision-activated dissociation (ITCAD) tandem mass spectrometry followed by energy-resolved medium-energy collision-activated dissociation (ER-MCAD) was developed for the differentiation of seven isomeric molecular radical cations of n-pentylquinoline. The fragmentation of each isomer was found to be distinctly different and depended largely on the site of the alkyl side chain in the quinoline ring. In order to better understand the observed fragmentation pathways, mechanisms for the formation of several fragment ions were delineated based on quantum chemical calculations. The fast benzylic α-bond cleavage that dominates the fragmentation of analogous nonheteroaromatic alkylbenzenes was only observed for the 3-isomer as the major pathway due to the lack of favorable low-energy rearrangement reactions. All the other isomeric ions underwent substantially lower-energy rearrangement reactions as their alkyl chains were found to interact mostly via 6-membered transition states either with the quinoline nitrogen (2- and 8-isomers) or the adjacent carbon atom in the quinoline core (4-, 5-, 6-, and 7-isomers), which lowered the activation energies of the fragmentation reactions. The presented analytical approach will facilitate the structural characterization of nitrogen-containing heteroaromatic compounds in asphaltenes.

Concerted Double Hydrogen-Bond Breaking by Intermolecular Coulombic Decay in the Formic Acid Dimer

Hydrogen bonds are ubiquitous in nature and of fundamental importance to the chemical and physical properties of molecular systems in the condensed phase. Nevertheless, our understanding of the structural and dynamical properties of hydrogen-bonded complexes in particular in electronic excited states remains very incomplete. Here, by using formic acid (FA) dimer as a prototype of DNA base pair, we investigate the ultrafast decay process initiated by removal of an electron from the inner-valence shell of the molecule upon electron-beam irradiation. Through fragment-ion and electron coincident momentum measurements and ab initio calculations, we find that de-excitation of an outer-valence electron at the same site can initiate ultrafast energy transfer to the neighboring molecule, which is in turn ionized through the emission of low-energy electrons. Our study reveals a concerted breaking of double hydrogen-bond in the dimer initiated by the ultrafast molecular rotations of two FA⁺ cations following this nonlocal decay mechanism.

Studies of the First Electronically Excited State of 3-Fluoropyridine and Its Ionic Structure by Means of REMPI, Two-Photon MATI, One-Photon VUV-MATI Spectroscopy and Franck-Condon Analysis

3-Fluoropyridine (3-FP) has been investigated by means of two-photon resonance-enhanced multi photon ionization (REMPI), mass-analyzed threshold ionization (MATI) and one-photon vacuum-ultraviolet (VUV) MATI spectroscopy. The aim was the determination of the effect of m-fluorine substitution on the vibronic structure of the first electronically excited and ionic ground state. The S₁ excitation energy has been determined to be 35 064 ± 2 cm^-1 (4.3474 ± 0.0002 eV). Strong evidence of a distinct vibronic coupling via ν_16b and ν_{[Wag.out.,16a]} to one or both of the lowest ¹ππ* states has been found, which results in a warped S₁ minimum structure with C₁ symmetry. The adiabatic ionization energy of the ionic ground state (14a', n_N-LP orbital) has been determined to be 76 579 ± 6 cm^-1 (9.4946 ± 0.0007 eV), which is the first value reported for this state. The origin of the D₁ state (4a'', π-orbital) is located close by at 77 129 cm^-1 (9.5628 eV). As a result of the D₀-D₁ vicinity, the ionic ground state is coupled to the D₁ state via ν_{[Wag.out.,16a]} and ν_10a, which induces a twisted D₀ geometry with C₁ symmetry. Furthermore, for the first time two-photon and one-photon MATI spectra are presented together, which yield a much better understanding of the ionic vibronic structure in comparison to either of these experiments alone.

Investigation of the complex vibronic structure in the first excited and ionic ground states of 3-chloropyridine by means of REMPI and MATI spectroscopy and Franck-Condon analysis

3-Chloropyridine (3-CP) has been investigated by means of resonance-enhanced multi-photon ionization (REMPI) and mass-analyzed threshold ionization (MATI) spectroscopy to elucidate the effect of m-chlorine substitution on the vibronic structure of the first electronically excited and ionic ground states. The S₁ excitation energy has been determined to be 34 840 ± 2 cm^-1 (4.3196 ± 0.0002 eV) with a difference of less than 0.2 cm^-1 between both isotopomers, which is the first reported value for this transition in the gas phase so far. The S₁ state has been assigned to the ¹π* ← n transition. It is subject to strong vibronic coupling via ν_16b to one or both of the lowest ¹ππ* states. In addition, strong coupling via at least one more non-totally symmetric vibration is very likely to exist but the vibration could not be identified yet. Overall, the coupling results in a minimum S₁ structure with C₁ symmetry. The adiabatic ionization energy of the n_N-LP orbital (14a') has been determined to be 75 879 ± 6 cm^-1 (9.4078 ± 0.0007 eV) with a difference of less than 2 cm^-1 between the two isotopomers, which is the first value reported for this state so far. The ionic ground state exhibits a distinct vibronic coupling via ν_16a and ν_10a to either the D₁ state (4a'') and/or D₂ state (3a''), which results in a twisted D₀ geometry with C₁ symmetry. As a consequence of the warped geometry in both S₁ and D₀ states, very complicated MATI spectra were obtained when exciting S₁ states at higher wavenumbers.

Valence shell photoelectron angular distributions and vibrationally resolved spectra of imidazole: A combined experimental-theoretical study

Linearly polarized synchrotron radiation has been used to record polarization dependent valence shell photoelectron spectra of imidazole in the photon energy range 21-100 eV. These have allowed the photoelectron angular distributions, as characterized by the anisotropy parameter β, and the electronic state intensity branching ratios to be determined. Complementing these experimental data, theoretical photoionization partial cross sections and β-parameters have been calculated for the outer valence shell orbitals. The assignment of the structure appearing in the experimental photoelectron spectra has been guided by vertical ionization energies and spectral intensities calculated by various theoretical methods that incorporate electron correlation and orbital relaxation. Strong orbital relaxation effects have been found for the 15a', nitrogen lone-pair orbital. The calculations also predict that configuration mixing leads to the formation of several low-lying satellite states. The vibrational structure associated with ionization out of a particular orbital has been simulated within the Franck-Condon model using harmonic vibrational modes. The adiabatic approximation appears to be valid for the X ²A″ state, with the β-parameter for this state being independent of the level of vibrational excitation. However, for all the other outer valence ionic states, a disparity occurs between the observed and the simulated vibrational structure, and the measured β-parameters are at variance with the behavior expected at the level of the Franck-Condon approximation. These inconsistencies suggest that the excited electronic states may be interacting vibronically such that the nuclear dynamics occur over coupled potential energy surfaces.

Koopmans'-Type Theorem in Kohn-Sham Theory with Optimally Tuned Long-Range-Corrected (LC) Functionals

In the present study, we have investigated the applicability of long-range-corrected (LC) functionals to a Kohn-Sham (KS) Koopmans'-type theorem. Specifically, we have examined the performance of optimally tuned LCgau-core functionals (in combination with BOP and PW86-PW91 exchange-correlation functionals) by calculating the ionization potential (IP) within the context of Koopmans' prediction. In the LC scheme, the electron repulsion operator, 1/r₁₂, is divided into short-range and long-range components using a standard error function, with a range separation parameter μ determining the weight of the two ranges. For each system that we have examined (H₂O, CO, benzene, N₂, HF, H₂CO, C₂H₄, and five-membered ring compounds cyclo-C₄H₄X, with X = CH₂, NH, O, and S, and pyridine), the value of μ is optimized to minimize the deviation of the negative HOMO energy from the experimental IP. Our results demonstrate the utility of optimally tuned LC functionals in predicting the IP of outer valence levels. The accuracy is comparable to that of highly accurate ab initio theory. However, our Koopmans' method is less accurate for the inner valence and core levels. Overall, our results support the notion that orbitals in KS-DFT, when obtained with the LC functional, provide an accurate one-electron energy spectrum. This method represents a one-electron orbital theory that is attractive in its simple formulation and effective in its practical application.

Ground and excited electronic states of AuH2via detachment energies on AuH2− using state-of-the-art relativistic calculations

AuH₂ is not as simple as it may seem at first glance!

Determination of the highest occupied molecular orbital and cationic structure of 2-chloropyridine by one-photon VUV-MATI spectroscopy and Franck–Condon fitting

Substitution of a chlorine atom for the H in pyridine alters the HOMO of the molecule, which ultimately affects the cationic structure.

Spectroscopic and quantum chemical study of difluoroboron β-diketonate luminophores: Isomeric acetylnaphtholate chelates

The electronic structure and optical properties of the isomeric difluoroboron β-diketonates, 2,2-difluoro-4-methylnaphtho-[2,1-e]-1,3,2-dioxaborin (I) and 2,2-difluoro-4-methylnaphtho-[1,2-e]-1,3,2-dioxaborin (II), were studied by means of X-ray photoelectron, absorption and luminescence spectroscopies. The experimental results were interpreted using high-level ab initio quantum chemical computations, including the algebraic-diagrammatic construction method for the polarization propagator of the second and third orders (ADC(2) and ADC(3)), the outer-valence Green's function (OVGF) method, and the time-dependent density functional (TDDFT) approach. The X-ray photoelectron measurements were assigned in the entire energy range using the results of the Kohn-Sham orbital calculations which employed the B3LYP functional. Pronounced hypsochromic shift of crystal-state fluorescence was observed in I upon the lowering of temperature, which can be explained by the deterioration of the conditions for excimers formation. According to our results, remarkable feature of II, absent in I, is its phosphorescence at room temperature. Basing on our calculations, a decay mechanism for the S₁ state was proposed, explaining the observed differences in the phosphorescence of I and II.

Assignment of photoelectron spectra of intramolecular silicon complexes: 1-vinyl- and 1-phenylsilatranes

The problematic photoelectron spectra of 1-vinyl- and 1-phenylsilatranes were theoretically studied and the correlation between the VIE and thed_SiNwas proposed to be used for the identification of orbitals in the Si←N coordination bond in XSi[OCH₂CH₂]₃N.