On the excited states ofp-quinones and an interpretation of the photocycloaddition ofp-quinones to alkenes

A theoretical investigation is made of the electronic states ofp-benzoquinone (PBQ), methyl substituted PBQ's and 1,4-naphthoquinone (NQ). In accord with experiment, the lowest triplet state of PBQ is calculated to be³ B _1g (n, *), while that for duroquinone (DQ) is³ B _3g (, *). The electron densities of these states are consistent with the hypothesis that³ n, * states lead to oxetan formation and³, * states to cyclobutanes. It is predicted that trimethyl PBQ might form both adducts, as the two states are calculated to be nearly degenerate.The photochemistry of NQ is more complex. The lowest excited triplet state is calculated to be ofn, * symmetry, in accord with experiment; however, several other states are predicted near in energy, and the photochemistry cannot be rationalized unambiguously.This work was supported in part by the National Research Council of Canada.     

An ab initio study of formamide

Calculated energy and molecular properties of the ground and low-energy excited states of formamide are presented at the ground state geometry. Satisfactory results are obtained except for the ¹* energy which remains too high by 1 eV (which is nevertheless a large improvement over previous calculations). The predicted triplet energies lie at 5.4 eV (³ n*) and 5.8 eV (³*).     

Ab initio SCF and CI study of the electronic spectrum of pyridine N-oxide

Configuration interaction (CI) studies of ground, n *, * * electronically excited states are reported for pyridine N-oxide. The transition energy to the lowest * excited ¹ B ₂ state is calculated at 4.35 eV, compared to the experimental spectrum range of 3.67–4.0 eV. This state lies below the lowest n * excited ¹ A ₂ state calculated at 4.81 eV above the ground state. The only experimentally reported triplet state at 2.92 eV above the ground state is predicted to be the ³ A ₁ (*) state. The calculated energy lies at 3.27 eV. Numerous other high-lying singlet states as well as the triplet states have also been calculated. The intramolecular charge transfer character of the ground and the excited states have been studied in terms of the calculated dipole moment and other physical properties.     

A theoretical study of the electronic spectra of pyridine and phosphabenzene

The electronic excitation spectra of pyridine and phosphabenzene have been studied using theoretical methods. The electronic states are described by wave functions derived from second-order perturbation theory based on multiconfigurational reference functions. The study includes singlet and triplet valences excited states as well as a number of Rydberg states. For both molecules the transition energies to the two lowest * excited singlet states are known from experiment and reproduced with an accuracy of 0.15 eV or better, while then * transition energies are predicted with a somewhat uncertain error of about 0.2 eV. The calculations suggest the lowestn * transition detected experimentally in pyridine corresponds to an adiabatic transition. 43 electronic states have been determined in each of the molecules.     

A theoretical determination of the electronic spectrum of formaldehyde

Summary The electronically excited states of formaldehyde are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis sets. The calculations comprised five valence excited states plus all singlet 3s, 3p, and 3d members of the Rydberg series converging on the first ionization. The computed vertical excitation energies were found to be within 0.2 eV of the available experimental energies. Full geometry optimization has been performed for five valence excited states. Assuming a planar geometry, the 0-0 transition for the valence¹A₁( *) state is calculated to appear near 7.9 eV, close to the (n _y 3p) region. This state is, however, not planar and the true adiabatic energy is 7.4 eV, which is 2.3 eV below the corresponding vertical transition.     

The Effect of cis-trans Isomerism on the Electronic-vibrational Structure of the Ground and Excited States and Reactivity of 2-(2-Furyl)- and 2-(2-Thienyl)oxazole

The fluorescent properties, structure, and electronic structure of the ground and excited singlet and triplet electronic states of the cis and trans forms of 4,5-dihydro-2-(2-furyl)oxazole, 4,4-dihydro-2-(2-thienyl)oxazole, 2-(2-furyl)oxazole (FO), and 2-(2-thienyl)oxazole (TO) have been studied. The orbital nature of the lower excited singlet and triplet states has been studied by the semiempirical INDO/S (valence approximation) and PPP/S ( approximation) methods. It was shown that for FO and TO molecules the lower triplet state is of the * type, for which delocalization of the electronic excitation on atoms is characteristic. In the singlet excitation state inversion was observed of the energy levels of the delocalized * states and n* states localized over several bonds (for the free TO and FO molecules the lower excited singlet states S₁* were assigned to * and n* types respectively). Owing to the low position of the T * and T _n* levels relative to the singlet level of * type, the rate constant for intercombination conversion is greater than the rate constant for radiative decay. Consequently an efficient population of the triplet states of the molecules occurs under conditions of electronic-vibrational excitation. The direction of reactions during synthesis was compared with the localization indices in the ground state for electrophilic, nucleophilic, and radical substitution, and also with the excitation localization numbers L for a wide selection of electronically excited states. It was concluded that the change in the structure of the azole molecule on replacing an O atom by an S atom, or on changing from a partially hydrogenated to a heteroaromatic system, was the main reason for the change of all the spectral parameters characterizing the electronic-vibrational or the spin-orbital interaction of the most reactive groups of atoms in the molecular structure.     

A CASSCF study of the singlet-singlet and triplet-triplet spectroscopy of naphthalene

Ab initio complete active space (CAS) SCF calculations have been carried out for the singlet and triplet excited states of naphthalene molecule. The CASSCF active space comprised 10-type molecular orbitals. The basis set was of ANO type and included both diffuse functions and polarization functions. The calculated excitation energies and transition moment provide a sound theoretical basis for the assignment of the experimental singlet-singlet and triplet-triplet spectra of the naphthalene molecule.     

A theoretical determination of the electronic spectrum of Methylenecyclopropene

The vertical electronic spectrum of methylenecyclopropene, the prototype of the nonalternant hydrocarbons known as fulvenes, has been studied using multiconfigurational second-order perturbation theory. The calculations comprise three valence states and the 3s, 3p, and 3d members of the Rydberg series converging to the first ionization limit. Vertical excitation energies to three valence states are found at 4.13, 6.12, and 6.82 eV. The second of them corresponds to an excitation from the highest occupied orbital to a ^* orbital, while the other two are ^* excitations. The third transition gives rise to the most intense feature in the electronic spectrum. The results are rationalized within the scheme of two interacting double bonds. Comparisons are made between this and the previous theoretical calculations of the electronic spectra of related systems and also between the available experimental data of methylenecyclopropene in solution.     

Electronic spectrum and π-electron delocalization of imidazole

The electronic spectra of imidazole chains involving one, two, and three molecules have been calculated within the framework of the semiempirical SCF LCAO MO CI method. The first - ^* singlet transition is in very good agreement with that known from experiment. The results indicate the possibility of existence of band structure, giving a bandwidth of the order of 0.6 eV. There is, however, no correlation between electrical parameters and the spectral data as could be expected from the Evans and Gergely model of delocalization of -electrons across the hydrogen bonds. Such a delocalization produces only a small extra effect on the system compared with the isolated molecule.

---

Zusammenfassung Die Elektronenspektren von Imidazolketten aus ein, zwei und drei Molekülen sind im Rahmen der semiempirischen SCF LCAO MO CI-Methode berechnet worden. Der erste ^* Singulettübergang stimmt sehr gut mit dem Experiment überein. Es wird auf die Möglichkeit einer Bandstruktur mit einer Breite von 0,6 eV geschlossen. Allerdings besteht keine Korrelation zwischen elektrischen Parametern und Spektraldaten, wie es das Delokalisationsmodell für -Elektronen von Evans und Gergely erwarten läßt. Solch eine Delokalisation über Wasserstoffbrücken hinweg zeigt lediglich kleine Rückwirkungen auf das System mit dem isolierten Molekül.

---

Résumé Calcul du spectre électronique de chaînes comportant une, deux et trois molécules d'imidazole, dans le cadre de la méthode SCF LCAO MO CI. La première transition singulet ^* est en très bon accord avec celle observée. Les résultats indiquent qu'une structure de bande, de largeur 0,6 eV environ, est possible. Il n'y a cependant aucune corrélation entre les paramètres électriques et les données spectrales comme on aurait pu en attendre du modèle d'électrons délocalisés à travers les liaisons hydrogène dû à Evans et Gergely. Une telle délocalisation n'introduit qu'une faible perturbation dans le système par rapport à la molécule isolée.

     

Molecular orbital studies of hydrogen bonds

The ground state, the lowest singlet and triplet n-* states, and the lowest triplet -* state of the formic acid monomer and dimer are studied with the ab initio molecular orbital theory. The two-configuration electron-hole potential method is used for calculations of excited states of dimers. The potential energy curves for the symmetrical simultaneous movement of two bridging protons are studied for all of the states. The barrier of the proton transfer in the ground state is found to be the smallest of the states studied. The association energy is analyzed in terms of various components.A preliminary account has been presented at the First International Congress of Quantum Chemistry, Menton, France, 1973, and has appeared in Ref. [3].     

Femtosecond dynamics of relaxation of photoexcited meso-tetraferrocenylporphyrin in the nonprotonated and diprotonated forms (Fc4PH2 and Fc4PH42+)

The relaxation of the Q¹(—*) excited state of the nonprotonated Fc₄PH₂ and diprotonated Fc₄PH₄ ²⁺ forms of meso-tetraferrocenylporphyrin was studied by femtosecond laser absorption spectroscopy. Transition from the Q¹(—*) state to the charge-transfer state was shown to occur within 208±10 fs for Fc₄PH₂ and 9±3 ps for Fc₄PH₄ ²⁺. A fast vibrational relaxation with a characteristic time of 120—140 fs was found for both forms. The relaxation time of Fc⁺—P^– charge-transfer state for Fc₄PH₂ was 17±4 ps.     

Ab initio studies of the low-lying π-states of borazine

Self-consistent-field and configuration-interaction studies were performed on borazine, using a double-zeta basis set augmented by six diffuse -functions. Low-lying singlet and triplet states of the A ₁ , A ₂ and E species were calculated, corresponding to ^* excited valence and Rydberg states. A selection out of singly and doubly excited configurations relative to a set of reference configurations was made for each species. Calculated vertical excitation energies (in eV) are 7.12 for ¹ A ₂(V), 8.45 for ¹ E (3p-R), 8.57 for ¹ A ₁(V), 8.9 for ¹ E(V-R), 9.55 for ¹ E(3d-R), 6.98 for ³ A ₁(_V), 7.27 for ³ E(V), 7.82 for ³ A ₂(V), 8.30 for ³ E(3p-R), and 9.5 for ³ E(3d-R), where V and R refer to valence or Rydberg character. The results are compared with experimental excitation energies, previous ab initio studies of borazine, and with recent ab initio studies of benzene.Dedicated to Professor Dr. H. Hartmann on the occasion of his 65th birthday.     

Triplet states via intermediate neglect of differential overlap: Benzene,pyridine and the diazines

The intermediate neglect of differential overlap technique is modified and applied to the calculation of excited triplet states. The resulting method generally reproduces the transition energies of the better-classified observations within a rms error of 1000 cm^–1. Trends are well reproduced, and the calculated orders ofn-* and -* triplet states are in good accord with the experimental information to date.The method is applied to benzene and the azines. The lowest four triplet states of benzene are calculated in good accord with experiment. Pyridine is calculated to have an-* triplet nearly degenerate with the lowest lying -* triplet, corroborating suggestions of Japar and Ramsay based on experimental information. A detailed analysis is made of the diazines, and assignments are suggested for the higher lying triplet states not yet classified or not yet observed.     

Diatomic interactions in momentum space. The 2pπ u and 3dπ g states of H 2 + system

The recently proposed method of momentum electron density for interatomic interactions is applied to the two states of the H ₂ ⁺ system. The processes of the attractive 2P _u and repulsive 3d _g interactions are analysed based on the behaviour of the momentum density and Compton profile. The results are compared with the previous ones for the 1 _{S g} and 2p _u states. The guiding principle of contraction and expansion for the energy-density relation in momentum space is shown to be common to both the and states.     

Comparison of the molecular structure and spectra of benzene and borazine

Ab initio SCF MO and CI calculations for two different gaussian basis sets are carried out for the isoelectronic molecules benzene C₆H₆ and borazine B₃N₃H₃ in order to investigate the effect of increasing the flexibility in the representation of their respective systems. In the process it is found from comparison of orbital charge density contour diagrams and inner shell orbital energies of borazine with analogous data for other systems that the BN bonds of this compound are considerably less polar (B⁺N^–) than that of ammonia borane BNH₆ (B^–N⁺). CI calculations employing the larger basis set produce generally better agreement with the experimental transition energies of benzene than do those using the smaller basis. In both treatments ¹ E _2g is predicted to lie lower than ¹ E _1u , in contrast to single excitation results; an analogous inversion in the calculated order of states is not observed in the study of borazine but the effect of double and triple excitation configurations is also found to be quite important for this system. For both benzene and borazine the CI calculations suggest in addition that transitions to ^* states occur at relatively low energy, perhaps within 0.5 eV of the strongest ^* absorption in each case.

---

Zusammenfassung Es wurden ab initio SCF-MO- und CI-Berechnungen für die isoelektrischen Moleküle Benzol C₆H₆ und Borazin B₃N₃H₆ durchgeführt; dabei werden zwei verschiedene Gaußfunktions-Basissätze von unterschiedlicher Güte hinsichtlich der Darstellung der entsprechenden -Systeme der Moleküle verwendet, und ihr Einfluß wird diskutiert. Beim Vergleich von Orbital-Elektronendichtediagrammen sowie den Orbitalenergien der inneren Schalen von Borazin mit entsprechenden Daten von anderen Systemen stellt sich heraus, daß die BN-Bindungen in B₃N₃H₆ bedeutend weniger polar (B⁺N^–) sind als die entsprechende Bindung (B^–N⁺) in BNH₆. Die CI-Rechnungen liefern bei Zugrundelegung der flexibleren AO-Basis Werte für die Übergangsenergien in Benzol, welche im allgemeinen in besserer Übereinstimmung mit den experimentellen Daten sind als bei Verwendung der kleineren Basis. In beiden Verfahren liegt der ¹ E _2g -Zustand niedriger als der ¹ E _1u , im Gegensatz zu den Ergebnissen, welche lediglich mit einer einfach angeregten Konfiguration erhalten werden; eine ähnliche Inversion in der berechneten Reihenfolge der Borazin-Zustände wird nicht gefunden, aber der Einfluß von zwei- und dreifach angeregten Konfigurationen stellt sich bei diesem Molekül ebenfalls als sehr wesentlich heraus. Die CI-Rechnungen legen weiterhin nahe, daß in den beiden Molekülen Benzol und Borazin ^*-Übergänge bei verhältnismäßig niedrigen Energien vorkommen, ungefähr 0,5 eV von der stärksten ^*-Absorption entfernt.

---

Résumé Calculs ab-initio dans deux bases de gaussiennes, SCF-MO et I.C., pour les molécules isolélectroniques de benzène C₆H₆ et de borazole B₃N₃H₆, aux fins d'étude de la flexibilité de représentation de leurs systèmes . De la comparaison des diagrammes de contour de densité de charge orbitale et des énergies des orbitales des couches internes du borazole avec des données analogues pour d'autres systèmes on déduit que les liaisons B-N de ce composé sont considérablement moins polaires (B⁺N^–) que celles du borure d'ammonium BNH₆ (B^–N⁺). Des calculs d'I.C. utilisant la plus grande base donnent généralement de meilleures énergies de transition pour le benzène que ceux utilisant la plus petite base. Dans les deux cas ¹ E _2g est trouvée en dessous de ¹E_1u contrairement à ce que donne l'approximation monoparticulaire; une inversion analogue de l'ordre des états calculés n'est pas observée dans l'étude du borazole mais l'effet des configurations diet tri-excitées est aussi assez important pour ce système. Pour le benzène comme pour le borazole les calculs d'I.C. suggèrent d'autre part que les transitions ^* se produisent à des énergies relativement basses, à 0,5 eV peut être de la plus forte absorption ^* dans chaque cas.

     

Electronic structure and aromaticity of planar singlet π-electron monocyclic π2 carbenes

Induced -electron ring currents and relative diamagnetic susceptibilities of planar singlet -electron monocyclic ² carbenes are calculated in the coupled variant of perturbation theory in the CNO method. It is shown that the parity theorem holds for charged planar ² and ² carbenes formally generated by the detachment of a proton or a hydride ion from alternant hydrocarbons. This leads, in such carbenes, to the same energies of singlet-singlet and singlet-triplet transitions, electric polarizabilities, conjugation energies, -electron ring currents, diamagnetic susceptibilities and current contributions to chemical shifts, whereas the charge contributions to the latter are of opposite signs.Donbas State Academy of Building and Architecture, 1 Derzhavina St., 339023 Makeevka, Donetsk Region, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 157–162, May–June, 1996. Original article submitted December 5, 1994.     

Correlation effects in the neutral and ionized ground states of acetylene

Correlation energies were calculated by Effective Pair Correlation Energy (EPCE) and minimal basis set configuration interaction (CI) methods for pairs of electrons in the occupied molecular orbitals for the ( ₄ ^u ) neutral ground state, ( ₃ ^u ) ground state of the positive ion, and ( ₄ ^u _g ) ground state of the hypothetical negative ion of acetylene.The EPCE values allow detailed breakdown of the ionization potential and electron affinity (for the unstable negative ion). It is seen that the SCF values for the former can be modified by the EPCE values to give estimates close to the experimental quantity. The EPCE values are compared against the pair-wise correlation energies obtained by minimal basis set CI and the percentages of the latter as compared to the former are interpreted by considering the form of the available excited configurations used in constructing the correlated functions. The MBSCI calculation accounts for only 20–30 % of the EPCE correlation energy.Supported by National Science Foundation Grant 31496X.     

Theoretical Study of the Products of Acetylene Addition to HgCl2

The ab initio MP2 method is used with the LANL2DZ basis to calculate the mercury chloride ,-complex with two acetylene molecules (1) and various isomeric forms of mercury di()-vinyl chloride -complexes (2): cis-cis (2A), cis-trans (2B), and trans-trans (2C). The ,-complex is the most stable form of all those considered; the difference between 1 and 2A is 24.9 kcal/mole. A relation between the total energies (kcal/mole) for isomeric forms 2 is established to be 2A (0) < 2B (0.98) < 2C (1.58). Complex 1 is shown to be transformed into 2A via the intermediate formation of 3, which is a hybrid form of the complex (,-complex of mercury chloride with two acetylene molecules). The structures of the transition states for the transformations of 1 into 3 (structure 4) and of 3 into 2A (structure 5) and the corresponding transition activation energies are determined. The interaction of 2A, 2B, and 2C with the Cl^- anion as a model nucleophile is considered. It is shown that the resulting anions (6A, 6B, 6C) have a planar structure with the relative stability increasing in the series 6A<6B<6C.     

π electron structure and reactivities of isomeric pyrrolo-sym-triazoles

The electron structures and energies of the singlet * transitions of a number of isoelectronic analogs of indolizine were calculated by the MO LCAO method within the semiempirical self-consistent field (SCF) approximation. In contrast to the calculations made by the simple MO LCAO method, the electron density distribution obtained is in better agreement with the direction of electrophilic substitution. On the basis of an analysis of the electronic spectra, it was concluded that the 6-phenyl group is not coplanar.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 261–267, February, 1974.     

Excited states of phenyl carbonyl compounds

In an attempt to clarify the origin of the dual phosphorescence in phenyl alkyl ketones, we have made some calculation (within the C.I.P.S.I. method in an excitonic scheme) to elucidate the conformation of both ground states and excited states of propiophenone. Our calculations have shown the presence of two stable isomers in the ground state, first n ^* state, and first ^* singlet and triplet states. So our work suggests that the origin of the dual phosphorescence of propiophenone could be related to the conformational change of the molecule in the n ^* state, because the most stable conformations in the n ^* state and in the ground state are different.