The electronic structure of some heterocycles with bridgehead nitrogen: photoelectron spectra and ab initio molecular orbital calculations

He(I) and He(II) photoelectron spectra are reported for the cycl[3,3,3]azine (1), cycl[3,2,2]azine (2), indolizine (6) and imidazo[1,2-a] pyridine (7), as well as He(I) spectra for related compounds (3–5). Ab initio molecular orbital calculations have been used to assign the spectra of 1, 2, 3, 6 and 7, and to give information about the nature of the π-electron energy levels. The first IP for 1 is singularly low (5.86 eV), and this has been interpreted in terms of occupancy of the 1a₁'' orbital which is normally vacant in related compounds. In the cyclazines, the nitrogen lone pair seems to be split into two π-levels.     

Electronic absorption spectra of some nicotinamides and nicotinic acids. Molecular orbital treatment

The electronic absorption spectra of the position isomers nicotinamide and isonicotinamide, nicotinic acid, and isonicotinic acid were investigated, together with the spectra of thionicotinamide, N-methyl nicotinamide and nicotinic acid N oxide. Apparent differences in the spectra of the position isomers were interpreted in terms of the torsion angle between the planes of the molecule, the height of the barrier to internal rotation, and the results of molecular orbital (MO) calculations. The largest perturbation effect was observed in the case of thionicotinamide whereas the smallest effect was observed in the case of nicotinic acid N oxide. MO calculations have indicated the existence of overlapping transitions. The observed transitions proved to be π-π* transitions, none of the n-π* was observed. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64 : 689–701, 1997     

Electronic structure and spectroscopy of nucleic acid bases: ionization energies, ionization-induced structural changes, and photoelectron spectra

We report high-level ab initio calculations and single-photon ionization mass spectrometry study of ionization of adenine (A), thymine (T), cytosine (C), and guanine (G). For thymine and adenine, only the lowest-energy tautomers were considered, whereas for cytosine and guanine we characterized the five lowest-energy tautomeric forms. The first adiabatic and several vertical ionization energies were computed using the equation-of-motion coupled-cluster method for ionization potentials with single and double substitutions. Equilibrium structures of the cationic ground states were characterized by DFT with the ωB97X-D functional. The ionization-induced geometry changes of the bases are consistent with the shapes of the corresponding molecular orbitals. For the lowest-energy tautomers, the magnitude of the structural relaxation decreases in the following series, G > C > A > T, the respective relaxation energies being 0.41, 0.32, 0.25, and 0.20 eV. The computed adiabatic ionization energies (8.13, 8.89, 8.51-8.67, and 7.75-7.87 eV for A, T, C, and G, respectively) agree well with the onsets of the photoionization efficiency (PIE) curves (8.20 ± 0.05, 8.95 ± 0.05, 8.60 ± 0.05, and 7.75 ± 0.05 eV). Vibrational progressions for the S(0)-D(0) vibronic bands computed within double-harmonic approximation with Duschinsky rotations are compared with previously reported experimental photoelectron spectra and differentiated PIE curves.     

Study of the electronic structure of molecules. XV. Comments on the molecular orbital valency state and on the molecular orbital energies

The valency state (vs) concept is analyzed in the Hartree–Fock approximation. A valency state “standard” is defined for atoms at infinite separation. A molecular orbital valency state (Movs) is defined from a partitioning technique (bond energy analysis) previously introduced for the Hartree–Fock molecular wave functions. The Movs for a given atom in a molecule is much higher in energy than the vs and its energy varies from molecule to molecule depending on the exact field of the surrounding atoms. The examples selected in the discussion are the CH₄ CH₃F, CH₂F₂, CHF₃ and CF₄ molecules. An analysis of the orbital energies is then given in terms of the bond energy. The importance of the rearrangement effects following ionization of inner shell electrons (simulation of ESCA type experiments) is illustrated with computations of the positive ion for methane and its fluoroderivatives. It is concluded that rearrangement following ionization from inner shells is as important as rearrangements following ionization from valency electrons. A direct consequence is that the orbital energies should not be equated to the inner shell ionization potentials. The computation of such ionization potentials agrees to about 99.5% with ESCA data, when the energy of both the neutral and ionic species are computed; the use of the orbital energies limits this agreement to about 95%.     

The electronic structure of aromatic molecules: ab inito molecular orbital studies and photoelectron spectra for the aza-derivatives of indole,benzofuran and benzothiophen

The He(I) photoelectron spectra for the aza-derivatives of benzofuran (4 and 7), benzothiophen (8) and indole (3a, 3b, 6a, 9b), and combined He(I) and He(II) spectra of the 1,2-benzisothiazole (5), 1,2,3-benzothiadiazole (11) and benzotriazole (9a) have been obtained and assigned by a combination of heteroatom substituent effects and ab initio molecular orbital calculations. The variations in lone pair levels (LP_NLP_O, LP_S) and π-levels, between these and the monocyclic compounds are discussed.     

Electronic structure and absorption spectra of yttrium quinaldinates with an island and polymer structure

The electronic absorption spectra and electronic structure of yttrium quinaldinates with an island and polymer structure were studied. A comparative analysis of the energies of the singlet and triplet excited states, the total energies of complex compounds in the ground state, and Mulliken’s bond overlap populations was performed by the TD-DFT method with the B3LYP density functional. For yttrium quinaldinate with a polymer structure, the distance between the highest occupied and lowest unoccupied orbitals was found to be longer than for the compound with an island structure. The transition energies were shifted to the blue region, which accounts for the higher stability of the polymer compound.     

Semiempirical molecular orbital studies on the electronic structure of molecules in excited states: The INDO/2-VN— 1 potential model: Part I.

The performance of the V_N—1 potential model at the INDO/2 level of approximation in the calculation of transition energy, singlet—triplet splitting, change in molecular structure, inversion barrier and electron-density distribution in the excited electronic states of a few simple carbonyls is analysed. The method turns out to be reasonably successful in many ways.     

Electronic structure of the peptide linkage. II. A molecular orbital treatment of the electronic spectra of benzohydroxamic acids

The electronic absorption spectra of some benzohydroxamic acids have been analyzed and interpreted using the results of molecular-orbital calculations. Charge density distributions, dipole moments, and the extent of delocalization of the MOS were used to interpret the observed solvent effects. Comparison between the spectra of benzohydroxamic acids and that of simple amides led to conclusions regarding the structural differences between the two classes of compounds.     

Electronic structure and molecular conformation of furan and pyrrole azomethines. An ab initio molecular orbital study

STO-3G minimal basis set ab initio molecular orbital calculations were employed to study the electronic structure and conformational preferences in furan-2-N-methylmethyleneimide ( 1 ) and pyrrole-2-N-methylmethyleneimide ( 2 ). The theoretical results were examined by comparison with the parent molecular systems through a population analysis and molecular orbital interactions considerations. The OCCN-trans and the NCCN-cis forms were found to be the most stable structures in 1 and 2 , respectively. Comparisons were made with available experimental data. The theoretical results indicate thatπ-electron interactions and molecular orbital interactions are not significant factors in determining the conformational preferences which most likely depend on dipole-dipole interactions.     

Electronic absorption spectra and the structure of the ligand in metal complexes with purpurin

Purpurin complexes with metals are formed due to metal coordination at the peri- or ortho-hydroxycarbonyl group involving anthraquinoid tautomers. The _l,* bands of metal purpurinates correspond to 1,10- and 1,4-anthraquinoid anions, 9,10-anthraquinoid structures are not typical of them.Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 3, 2005, pp. 237–240.Original Russian Text Copyright © 2005 by Fain, Zaitsev, Ryabov.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Electronic and molecular structure of some substituted azomethines: A gas-phase u.v. photoelectron spectroscopic investigation

Four azomethinic molecules, three of which present a seven-member ring with two carbon atoms shared with an aromatic system, have been studied by gas-phase u.v. photoelectron spectroscopy with the aim of correlating their electronic and molecular structure. The analysis of the experimental findings, supported by quantum-mechanical calculation (EHT and HAM/3) on model molecules, showed that no appreciable interactions occur between the systems of the aromatic and of the seven-member ring, and that the electronic structure of these molecules depends critically on the geometry. The comparison with smaller parent molecules pointed out that the formation of the seven-member ring induces drastic changes in the electronic structure of the valence levels.     

CNDO/2 molecular orbital studies on the electronic structure and conformational modes of 4-amino-tetrolic acid

4-aminotetrolic acid and its zwitterion have been studied using the CNDO/2 method. The molecular electronic structures, dipole moments and barriers to internal rotation are discussed. A simple procedure to overcome difficulties. encountered with the calculations on the zwitterionic form is described.     

Molecular spectra and molecular structure of heterocyclic compounds. Infrared and near ultraviolet absorption spectra of pyridine N-oxide derivatives

The infrared and ultraviolet spectra of many pyridine N-oxide derivatives have been studied. Ultraviolet spectra show a strong solvent effect in going from aprotic to hydroxylic solvents due to strong hydrogen bonding. Values of δ H° for several systems CH₃OH/XPyNO in tetra-chloroethylene solution were evaluated by infrared techniques and a linear correlation δ H° vs. δ v OH has been found. Features of the infrared and ultraviolet characteristic absorption bands were also investigated.     

Electronic absorption spectra of pyridine and nicotine in aqueous solution with a combined molecular dynamics and polarizable QM/MM approach

The electronic absorption spectra of pyridine and nicotine in aqueous solution have been computed using a multistep approach. The computational protocol consists in studying the solute solvation with accurate molecular dynamics simulations, characterizing the hydrogen bond interactions, and calculating electronic transitions for a series of configurations extracted from the molecular dynamics trajectories with a polarizable QM/MM scheme based on the fluctuating charge model. Molecular dynamics simulations and electronic transition calculations have been performed on both pyridine and nicotine. Furthermore, the contributions of solute vibrational effect on electronic absorption spectra have been taken into account in the so called vertical gradient approximation. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.     

Theoretical studies on molecular structure and vibrational spectra of 8-hydroxyquinolinium picrate

Quantum chemical calculations of geometrical structure and vibrational wavenumbers of 8-hydroxyquinolinium picrate (8-HQP) were carried out by ab initio HF and density functional (DFT/B3LYP) method with 6-31++G(dp) basis set. The calculated geometric parameters of 8-HQP are presented. A detailed interpretation of the infrared spectra of 8-hydroxyquinolinium picrate (8-HQP) are also reported. Theoretical molecular frontier orbital energies of the title compound have been calculated using the method mentioned above in order to understand this phenomenon in the context of molecular orbital picture. The molecular HOMOs and LUMOs generated via HF and B3LYP method have been outlined.     

Electronic structure of pentatomic heterocyclic compounds containing N,O, S and Se. Gas-phase UV photoelectron spectra and quantum-mechanical calculations

The gas-phase UV photoelectron spectra of a series of pentatomic heterocyclic molecules having the hydantoin skeleton, i.e.

are discussed on the ground of comparison with the spectra of analogous molecules and with the results of HAM 3 and/or CNDO/S calculations. Evidence was found of the presence of significant interactions between the π orbitals of the chalcogen atoms when X and Y were sulfur and/or selenium. The π orbitals of the nitrogen atoms are on the contrary pretty much localised in the whole series, particularly when X=Y=O and in the monosubstituted derivatives.