Molecular structure and vibrational spectra of indole and 5-aminoindole by density functional theory and ab initio Hartree–Fock calculations

The molecular geometry and vibrational frequencies of indole and 5-aminoindole in the ground state have been calculated by using the Hartree–Fock and density functional method with 6-311++G(d,p) basis set. The optimized geometrical parameters obtained by DFT calculations are in good agreement with the experimental values. Comparison of the observed fundamental vibrational frequencies of indole and 5-aminoindole with the calculated results by density functional and Hartree–Fock methods indicates that B3LYP is superior to the scaled Hartree–Fock approach for molecular vibrational problems. The theoretical spectrograms for FT-IR spectrum of 5-aminoindole have been constructed.     

Molecular structure and vibrational spectra of lepidine and 2-chlorolepidine by density functional theory and ab initio Hartree–Fock Calculations

The molecular geometry and vibrational frequencies of lepidine and 2-chlorolepidine in the ground state have been calculated by using the Hartree–Fock and density functional methods (B3LYP) with 6-31G (d) as the basis set. The optimized geometric bond lengths obtained by using B3LYP and bond angles obtained by HF that correspond to the experimental values of 2-cl-lepidine molecule were given. Comparison of the observed fundamental vibrational frequencies of lepidine and 2-chlorolepidine, and calculated results by density functional B3LYP and Hartree–Fock methods indicates that B3LYP is superior to the scaled Hartree–Fock approach for molecular vibrational problems.     

Molecular structure and vibrational spectra of N-mesylhydroxylamin and N-mesyl-O-methylhydroxylamin by density functional theory and ab initio Hartree–Fock calculations

The molecular geometry and vibrational frequencies of N-mesylhydroxylamin (N-MHN) and N-mesyl-O-methylhydroxylamin (N-MMHN) in the ground state have been calculated using the Hartree–Fock and density functional method (B3LYP) with 6-31G(d) basis set. The optimized geometric band lengths and bond angles obtained by using HF and DFT (B3LYP) show the best agreement with the experimental data. Comparison of the observed fundamental vibrational frequencies of N-MHN and N-MMHN and calculated results by density functional B3LYP and Hartree–Fock methods indicate that B3LYP is superior to the scaled Hartree–Fock approach for molecular vibrational problems.     

Conformational analysis of melatonin at Hartree–Fock ab initio level

Melatonin is a hormone, which is synthesized and secreted by the pineal gland during darkness and has been implicated in the regulation of several neural and endocrine processes. Conformational analysis of melatonin was performed at Hartree–Fock ab initio level. With the help of STO-3G and 6-31G* basis sets, 192 and 128 conformers were obtained, respectively. In both cases, there were eight conformers with nearly planar 3-amido-side chain and the most stable conformation found had folded structure in accord with previously published results. The present paper compares the Hartree–Fock conformational space of melatonin with the results of active conformation models and conformational analyses published in the literature.     

Chemisorption of HSi(OH)3 on silica surfaces: an ab initio periodic study

The study of the grafting of trialkoxysilane R′Si(OR)₃ molecules on amorphous silica has been undertaken at the Hartree–Fock level using a biperiodic model for the surface. Different types of slab cut out from the model system Edingtonite (a tetragonal silica structure with five SiO₂ groups per unit cell) have been used to simulate isolated and interacting silanol sites at the amorphous silica surface, while only the simple case of HSi(OH)₃ has been considered for the interacting molecule. In a first step, for each type of surface the geometrical parameters have been optimised and the surface formation energy determined. The geometrical structure of the grafted molecule is compared with that of the isolated one. The geometrical strains of the surfaces with either isolated or interacting silanols are also compared before and after the grafting reactions. The calculated values of the chemisorption energies show that the grafting process is favored on isolated silanols only if correlation effects are included.     

Theoretical determination of molecular structure and conformation : Part 12. Puckering of 1,3,5-cycloheptatriene, 1H-azepine, oxepine and their norcaradienic valence tautomers

Restricted Hartree—Fock calculations have been carried out for 1,3,5-cycloheptatriene, norcaradiene, 1H-azepine, benzenimine (azanorcardiene), oxepine and benzene oxide (oxanorcaradiene) employing the STO—3G, 4—31G and 6—31G^* basis sets. Theoretical geometries, conformations and barriers to ring inversion have been obtained and compared with the available experimental structural data. It has been found that the cyclotrienes possess a boat conformation with a constant admixture of 22% chair character leading to a flattening of the triene part and π-electron delocalization typical for a planar polyene. The structural data obtained in this work suggest that cyclic delocalization of either 6π (homoaromaticity) or 8π electrons (antiaromaticity) is not present in the three cyclotrienes. The former possibility, however, cannot be excluded in the case of the three norcaradienes.     

On the first principles Hartree—Fock and local density pseudopotentials

Angular momentum projected Hartree—Fock (HF) and local density functional (LDF) nonlocal atomic pseudopotentials are derived from first principles in     

A theoretical model investigation of the hydrogen-electrode processes of an alkaline H2---O2 fuel cell

Ab initio molecular orbital (MO) and self consistent field (SCF) computations have been carried out within the Restricted Hartree—Fock (RHF) formalism on the species involved in 22 possible reaction mechanisms associated with the anodic process: H₂ → 2H⁺ + 2e⁻ of the hydrogen—oxygen alkaline fuel cell. Such a model, which represents the gas phase reactions, may be regarded as a primary standard for any future study in which the catalyst of the electrode may be included.     

Structural and energetic predictions for anions from a modified Hartree—Fock procedure

The performance of a modifed Hartree—Fock (MHF) procedure which differs from conventional HF theory through the inclusion of continuum functions in t     

The molecular structure of bicyclo[1.1.0]tetraarsane from ab initio calculations using analytical gradients

The molecular structure of three different isomers of bicyclo[1.1.0]tetraarsane has been obtained by means of ab initio techniques including the use of non-empirical pseudo-potentials and analytical first derivatives of the total energy versus the nuclear coordinates. The results show unequivocally the existence of three different stable structures. The very small energy difference between the exo—exo and endo—exo is further evidence that these are in fact the two isomers found experimentally.     

The vibrational spectra, molecular structure and conformation of organic azides : Part IV. An ab initio study of hydrazoic acid, azidomethane, azidoethane, azidoethene and azidomethanal

Fully optimized geometries have been calculated for the title compounds at the Hartree—Fock SCF level and compared with existing experimental data. A basis set of double zeta quality has been employed. For hydrazoic acid, a calculation with a larger basis set, expected to give results near the Hartree—Fock limit, has also been performed. All of the calculations show the azide group to be slightly bent with a trans configuration around the central NN bond. Azidoethane is predicted to exist in two conformations, gauche (71°) and anti, with a negligible energy difference of 0.26 kJ mol⁻¹ between them. Azidoethene and azidomethanal both prefer the syn orientation of the azide group with respect to the C---C or C---O bonds, the computed energy difference between the anti and syn conformations being 3.31 and 30.3 kJ mol⁻¹ respectively.

The barrier to rotation around the C---N bond has been calculated to be 3.75 kJ mol⁻¹ in azidomethane while in azidoethane it was 3.30 and 9.40 kJ mol⁻¹ in the eclipsed anti-clinal (120°) and syn positions, respectively.

Complete harmonic force fields and dipole moment derivatives have been calculated for hydrazoic acid, azidomethane and for the two stable conformations of azidoethane. For azidoethane and azidomethanal only the azide part of the harmonic force field has been calculated. The theoretical harmonic force fields have been modified through scaling by a least squares refinement to the observed wavenumbers of hydrazoic acid, azidomethane and azidoethane (anti and gauche). Infrared vapour phase intensities have been calculated and theoretical spectra are presented for azidomethane and azidoethane.     

Which double-octet ABC molecules are bent? CI calculations on CaF2, and a softness criterion to predict bending

Hartree—Fock and configuration interaction calculations for CaF₂ yield a bond angle of 155° only after d-type polarization functions are included, suggesting that “anomalous” bent geometries can be rationalized by polarizability. A criterion using the chemical softness as a measure for both polarizability and low-lying valence states, completely separates the bent from the linear double-octet ABC molecules. Sixty yet unreported bent structures are predicted, and new experiments suggested.     

Theoretical study on the hydrogen migration reactions in ground state vinyl fluoride: Basis set and correlation energy effects

We performed ab initio molecular orbital (MO) calculations using Hartree—Fock SCF, and second- and fourth-order Møller—Plesset perturbation theory for hydrogen migration reactions on the singlet vinyl fluoride potential energy surface. We used different basis sets and polarization functions to obtain the stationary point geometries and activation barriers. Basis set and the polarization function extension have small effects, while the correlation energy evaluation leads to new conclusions for one of the studied reactions: the product singlet CHCH₂F is not a true local minimum on the potential energy surface.     

Isomers of C24. Density functional studies including gradient corrections

Density functional techniques are used to investigate the relative energies of seven different structural isomers of C₂₄. The traditional local density approximation yields the fullerene-like isomer to be the most stable. As in the case of C₂₀, the inclusion of gradient corrections has a dramatic effect on the relative energies. The gradient-corrected B-LYP method yields the monocyclic ring and graphite-like isomers to be almost isoenergetic (and most stable) while the bicyclic ring, fullerene-like, and bowl-like isomers are progressively higher in energy. The Hartree—Fock results are quite similar to the B-LYP results. Implications to fullerene growth mechanisms are pointed out.     

Photoionization cross sections involving an explicitly correlated initial state: Combination of multiconfigurational linear response and the stieltjes—tchebycheff moment theory

We have performed outer valence photoionization cross section calculations for N₂ and O₂. To do this we have combined several linear response techniques, in particular time-dependent Hartree—Fock (TDHF), multiconfigurational time-dependent Hartree—Fock (MC TDHF), and a modification of MC TDHF (MMC TDHF) with Stieltjes—Tchebycheff moment theory (STMT). To our knowledge, these MC TDHF and MMC TDHF calculations are the first which combine explicitly correlated Green function approaches with STMT. Since, in addition, these calculations are fully coupled, we expect the MC TDHF and in particular the MMC TDHF—STMT results to be highly reliable. For both N₂ and O₂ our MC TDHF—STMT and MMC TDHF—STMT results are in overall agreement with previous static exchange STMT results; however, there are a few significant differences and differences in detail in the partial and total photoionization cross sections. In particular, for example, for N₂ we note that the MMC TDHF—STMT does not give a “hump” resonance in the cross section for the (1π_u⁻¹)A²Π_u ionic state. In O₂ we note that the (3σ_g⁻¹) cross section obtained using MMC TDHF—STMT is substantially lower than the static exchange results.     

Correlation effects in the electronic coupling between π electrons through a cyclohexane spacer

The electronic coupling between symmetrically placed π electrons through a cyclohexane spacer is calculated using ab initio methods. In particular we have performed a multiconfigurational configuration interaction study involving the broken symmetry Hartree—Fock solutions. The value of Δ, in a simplified model the splitting between the symmetric and antisymmetric π orbitals, is moderately changed compared to the Koopmans theorem value. The influence of the basis set on the calculated splittings is also analyzed.     

A theoretical analysis of the explanation of the significant differences in antiferromagnetic interactions between homologous μ-alkoxo and acetate bridged dicopper(II) complexes: ab initio and semi-empirical calculations

A magnetostructural classification of dimmers, containing the Cu (μ-alkoxo) Cu core, based on data obtained from X-ray diffraction analysis reported in the literature has been performed. In these complexes, the local geometry around the copper ions is generally a square planar and each copper ion is surrounded by one N atom and three O atoms. The influence of the overlap interactions between the bridging ligands and the metal (Cu) d orbitals on the super-exchange coupling constant has been studied by means of ab initio Restricted Hatree–Fock molecular orbital calculations. The interaction between the magnetic d orbitals and highest occupied molecular orbitals of the acetate oxygens has been investigated in homologous μ-acetato-bridged dicopper(II) complexes which have significantly different −2J values (the energy separation between the spin-triplet and spin-singlet states). In order to determine the nature of the fronter orbitals, Extended Hückel molecular Orbital calculations are also reported. Ab initio restricted Hartree–Fock calculations have shown that the acetato bridge and the alkoxide bridge contribute to the magnetic interaction countercomplementarily to reduce antiferromagnetic interaction.     

Computational methods for conformational analysis of unsymmetrical 1,3-diamines: 3-aminotropanes

A comparative study has been performed to evaluate the ability of a range of computational theories to predict the relative basicity and the conformations of diamine systems. Specifically, molecular mechanics (MM3), semiempirical (AM1), and ab initio (Hartree–Fock) methods have been used in the conformational analyses of unprotonated, monoprotonated, and diprotonated 3-aminotropanes, a pair of isomeric 1,3-diamines. Use of the molecular mechanics force field, with the recently determined parameter set for protonated amines, affords results that are in agreement with experimental data, when corrected for water solvent (by setting the dielectric constant to 80). Ab initio and semiempirical calculations, in contrast, give inconsistent and incorrect results. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1371–1378, 1999     

Quantum chemistry investigation on the active site of Mn-SOD

In this paper, we carried out a theoretical study on the active site structures of the Mn-SOD with ab initio Hartree–Fock SCF method, and analyzed the molecular orbital energies, charges and atomic orbital contribution to the frontier molecular orbital.     

Computational electrochemistry: aqueous two-electron reduction potentials for substituted quinones

The electrode potentials of some quinone derivatives in aqueous solution have been calculated. The calculations are carried out at the Hartree–Fock and B3LYP levels with the inclusion of entropic and thermochemical corrections to yield free energies of redox reactions. The Polarisable Continuum Model (PCM) is used to describe the solvent. The average error of calculation of electrode potentials is less than 0.03 V and is decreased compared to the average error of methods presented previously. The role of relaxation energies and frequency calculations in improving the results has been investigated.