Restricted Hartree-Fock Method Research Articles

FTIR and Ab Initio Studies of Diisopropylsulfoxide and its Solutions

Fourier transform infrared (FTIR) measurements, ab initio quantum chemical calculations at the restricted Hartree–Fock (RHF) level and density functional theory (DFT) calculations have been performed to study molecular interactions in pure diisopropylsulfoxide (DiPSO) and the binary mixtures DiPSO/CCl4 and DiPSO/water. The optimized molecular geometry, vibrational wavenumbers, dipole moments and several thermodynamic parameters of free DiPSO and DiPSO/water 1:1 complex in the ground state were calculated using the RHF and B3LYP methods with the 6-31G(d) basis set. A fitting procedure has been performed for both SO and CH stretching regions and a detailed spectral interpretation has been done based on the data obtained from ab initio calculations, infrared spectra and band deconvolution analysis.

Synthesis, crystal and molecular-electronic structure, and kinetic investigation of two new sterically hindered isomeric forms of the dimethyl[methyl(phenylsulfonyl)amino]benzenesulfonyl chloride

Two new structural isomers – 2,4-dimethyl-5-[methyl(phenylsulfonyl)amino]benzenesulfonyl chloride (1) and 2,4-dimethyl-3-[methyl(phenylsulfonyl)amino]benzenesulfonyl chloride (2) were synthesized by interaction of N-(2,4-dimethylphenyl)-N-methyl-benzenesulfonamide or N-(2,6-dimethylphenyl)-N-methylbenzenesulfonamide with chlorosulfonic acid. Both compounds have been structurally characterized by X-ray single crystal diffraction at 100 K. The crystals of 1 are triclinic: space group P1¯, a = 8.1542(2), b = 11.0728(3), c = 11.2680(3) Å, α = 116.557(3), β = 95.155(2), γ = 108.258(2)°, V = 831.97(4) Å3, Z = 2, R = 0.0251 for 2429 reflections; the crystals of 2 are monoclinic: space group P21/c, a = 11.7428(2), b = 11.3518(2), c = 12.5886(2) Å, β = 93.659(2)°, V = 1674.66(5) Å3, Z = 4, R = 0.0269 for 2622 reflections. The structure of both isomers is organized as molecular crystals. These sterically hindered organic molecules are cross-linked into framework by means of hydrogen bonds of CH⋯O type (H⋯O distances are in range 2.27(2)–2.76(2) Å). The ab initio quantum-chemical calculations of an electronic structure of the isomeric molecules of 1 and 2 have been performed using the restricted Hartree-Fock method with a 6-31G* basis set. The calculated values of charge density concentrated on the electronegative atoms of the sterically hindered molecules are in good agreement with parameters of the intramolecular hydrogen bonds. The obtained data of the kinetic investigations of the substitution reactions in aqueous solution well correlate with stereo-chemical characteristics of the both molecules of the dimethyl[methyl(phenylsulfonyl)amino]-benzenesulfonyl chloride.

The experimental and theoretical study of tautomerism of 3-substituted 2-methyl-quinoline-4 (1h)-ones

4-Hydroxy-/4-oxo tautomerism in the series of 3-substituted 2-methyl-quinolin-4(1H)-ones has been studied by 13C NMR-spectroscopy and quantum-chemical methods in various approximations (restricted Hartree-Fock method, DFT and MP2) for the isolated molecules and for solutions using empirical correction of effects for solvents (PCM COSMO procedure). Substituents that are different in their nature have no significant influence on the value of the chemical shift of carbon in position C4 of the quinolone cycle. The only exception is the carbon shielding associated with the bromine atom in the molecule of 3-bromo-2-methyl-1,4-dihydroquinoline-4-one. Significant deshielding detected in all cases in 13C NMR-spectra of the carbon nuclei in position 4 of the ring is in favour of the existence of all derivatives studied as 4-oxo forms in DMSO-d6 solution. The experimental and calculated values for the chemical shift of carbon in position C4 of 4-oxo and 4-hydroxy isomers differ considerably and can be used as a criterion for assigning quinolin-4 (1H)-ones to a particular tautomeric form.

Substituent influence on the spectra of some benzo[f]quinoline derivatives

The aim of this study was to investigate the relations between the properties of some organic compounds that are cycloaddition derivatives of benzo[f]quinoline, namely benzo[f]pyrrolo[1,2-a]quinolines (BQCDs) and the structure of variable substituent in the addition cycle. The work was focused on the differences in the molecular parameters like frontier orbitals and dipole moment as well as electronic absorption spectra of substituted BQCDs.The optimized molecular structures of BQCDs were calculated using Gaussian 09, with DFT method, the frontier orbitals and electronic absorption spectra being modeled with restricted Hartree Fock method also implemented in Gaussian 09. Influence of substituted radical on the dipole moments and frontier orbital energies of the BQCDs was evidenced from calculated values. Substituent effect on the BQCDs recorded electronic absorption spectra in diluted solution and protonated diluted solution was also emphasized: different types of the transitions underlying absorption bands in the visible range were presumed based on the quantum chemical and experimental investigation.

Peculiarities of formation of hydroxonium ion and its small clusters

Density functional theory combined with the restricted open-shell Hartree-Fock method using the B3LYP, BLYP, and B3PW91 exchange-correlation functionals in the 6-311G** basis has been applied to determine equilibrium geometry of water-containing clusters 1(H3O+-nH2O) with n = 1–3, 5, 6 and 1(H3O+-3H2O)Cl−. The SA-MCSCF simulation has revealed a series of electronic terms of hydroxonium ion and energy profiles of its formation. Energy profiles of H3O+ interaction with H2O have been simulated and analyzed. Vibrational spectra have been simulated for all the mentioned complexes in the equilibrium state, and regularities of changes of the fundamental vibrational frequencies with the increasing nuclearity of the complexes have been discovered.

A new copper(II) chelate complex with tridentate ligand: Synthesis, crystal and molecular electronic structure of aqua-(diethylenetriamine-N, N′, N′′)-copper(II) sulfate monohydrate and its fire retardant properties

The crystals of a new aqua-(diethylenetriamine-N, N′, N′′)-copper(II) sulfate monohydrate have been synthesized by direct interaction of solid copper(II) sulfate pentahydrate with diethylenetriamine (deta). The crystal structure of [Cu(deta)H2O]SO4⋅H2O (1) has been determined by X-ray diffraction methods at 100K and characterized using X-ray powder diffraction pattern: space group P 1¯, a=7.2819(4), b=8.4669(4), c=8.7020(3)Å, α=83.590(3), β=89.620(4), γ=84.946(4)°, Z=2. The environment of the Cu(II) atom is a distorted, elongated square pyramid which consists of three nitrogen atoms of the deta molecule and oxygen atom of the water molecule in the basal plane of the square pyramid (the average lengths of the in-plane Cu–N and Cu–O bonds are 2.00Å). The apical position of the coordination polyhedron is occupied by complementary oxygen atom of the sulfate anion (the length of the axial Cu–O bond is 2.421(1)Å). The crystal packing is governed by strong hydrogen bonds of O–H⋯O and N–H⋯O types. The ab initio quantum-chemical calculations have been performed by the restricted Hartree–Fock method with a basis set 6-31∗G using the structural data of [Cu(deta)H2O]SO4⋅H2O. It has been ascertained that the degenerate d-orbitals of the Cu2+ ion split under the co-action of both the square-pyramidal coordination and the chelation. It is significant that visually observed crystals color (blue-violet) of the [Cu(deta)H2O]SO4⋅H2O complex is in good agreement with the calculated value of wavelength of visible light (λ=5735Å) which is closely related to the energy of the absorbed photon (Δ=2.161eV). Furthermore, the stereo-chemical aspect of influence of the CuSO4 upon combustibility of modified epoxy-amine polymers has been scrutinized.

General form of the total one-electron Hamiltonian in the restricted open shell Hartree-Fock method

The structure of the effective one-electron Hamiltonian \(\hat R\) in the Hartree-Fock equation \(\hat R \varphi _i = \varepsilon _i \varphi _i\) is discussed in many works. The most general definitions of \(\hat R\), satisfying all necessary conditions imposed by the variational principle for the energy in open shell systems are derived by Dyadyusha and Kuprievich and by Hirao and Nakasutji. In this work it is shown that these definitions cannot be concordant with additional variational conditions imposed by Koopmans’ theorem. A more general form of \(\hat R\) is proposed which provides a combination of the variational conditions imposed on the Hartree-Fock orbitals by the variational principle and Koopmans’ theorem.

Efficient molecular dynamics simulations of multiple radical center systems based on the fragment molecular orbital method.

The fully analytic energy gradient has been developed and implemented for the restricted open-shell Hartree-Fock (ROHF) method based on the fragment molecular orbital (FMO) theory for systems that have multiple open-shell molecules. The accuracy of the analytic ROHF energy gradient is compared with the corresponding numerical gradient, illustrating the accuracy of the analytic gradient. The ROHF analytic gradient is used to perform molecular dynamics simulations of an unusual open-shell system, liquid oxygen, and mixtures of oxygen and nitrogen. These molecular dynamics simulations provide some insight about how triplet oxygen molecules interact with each other. Timings reveal that the method can calculate the energy gradient for a system containing 4000 atoms in only 6 h. Therefore, it is concluded that the FMO-ROHF method will be useful for investigating systems with multiple open shells.

FT-IR and Raman spectra, ab initio and density functional computations of the vibrational spectra, molecular geometries and atomic charges of uracil and 5-methyluracil (thymine)

FT-IR (400–4000cm−1) and Raman spectra (200–4000cm−1) of uracil and 5-methyluracil (thymine) have been recorded and analyzed. The optimized molecular geometries, atomic polar tensor (APT) charges and vibrational characteristics have been studied theoretically using restricted Hartree–Fock (RHF) and density functional theory (DFT) methods. Using the Becke’s exchange in conjunction with Lee–Yang–Parr’s correlation functional and Becke’s three-parameter hybrid method (B3LYP), the ab initio and DFT calculations were carried out to study the optimized molecular fundamental vibrational frequencies for uracil and 5-methyluracil (thymine) by employing Gaussian-03 program. The fundamental vibrational frequencies along with their corresponding intensities in IR and Raman activities and depolarization ratios of the Raman lines have also been calculated using the RHF and DFT methods employing different basis sets. In quantum chemical calculations, most of the B3LYP/6-311++G∗∗ vibrational frequencies are in excellent agreement with the available experimental assignments and helped to propose in the reassignments of some missing frequencies in experimental study. Assuming under the Cs point group for both molecules, the distribution of normal mode of vibrations between the two species as planar (a′) and non-planar (a″) for all 39 normal vibrational modes of 5-methyluracil are given by 26a′+13a″, of which 30 modes (21a′+9a″) correspond to the uracil moiety and 9 modes (5a′+4a″) to the CH3 group. Consistent assignments have been made for the internal modes of CH3 group, especially for the anti-symmetric CH3 stretching and bending modes. A possible explanation could be the planarity of pyrimidine ring and non-planarity at carbon site of methyl group which might cause the splitting of frequencies including three components due to the substitution of CH3 group at the site of C5 atom on pyrimidine ring of uracil. The three non-equivalent CH bonds of CH3 group are distinctly separated from the CH/NH ring stretching frequencies. Kekule ring stretching mode is found to be comparatively higher frequency magnitude than the mode of uracil due to the involvement of hydrogen bonding of methyl group. But, the ring breathing is found to be lower frequency magnitude compared to those for uracil which could be due to mass effect of the CH3 group in place of the hydrogen atom at the site of C5 atom on pyrimidine ring of uracil.

The Fourier space restricted Hartree-Fock method for the electronic structure calculation of linear poly(tetrafluoroethylene)

Building on the pioneering work of Jean-Marie Andre and working in the laboratory he founded, the authors have developed a code called FT-1D to make Hartree-Fock electronic structure computations for stereoregular polymers using Ewald-type convergence acceleration methods. That code also takes full advantage of all line-group symmetries to calculate only the minimal set of two-electron integrals and to optimize the computation of the Fock matrix. The present communication reports a benchmark study of the FT-1D code using polytetrafluoroethylene (PTFE) as a test case. Our results not only confirm the algorithmic correctness of the code through agreement with other studies where they are applicable, but also show that the use of convergence acceleration enables accurate results to be obtained in situations where other widely-used codes (e.g., PLH and Crystal) fail. It is also found that full attention to the line-group symmetry of the PTFE polymer leads to an increase of between one and two orders of magnitude in the speed of computation. The new code can therefore be viewed as extending the range of electronic-structure computations for stereoregular polymers beyond the present scope of the successful and valuable code Crystal.

Semi-canonical MOs in the varied open shell method and related approaches

The self-consistency schemes previously developed in J. Struct. Chem., 30, N 5, p. 3 (1986) are shown to belong to the class of semi-canonical orbital schemes. In the derivation they explicitly involve only a part of canonical Koopmans orbitals in the iteration process. In particular, within the restricted Hartree-Fock method for a non-degenerate case a semi-canonical variational scheme based on the matrix elements of the unrestricted Hartree-Fock method is studied. Errors present in the above mentioned work in the description of the scheme of a varied open shell are corrected. A possible generalization for the case of Roothaan type degeneration is proposed and specific examples of low-spin and high-spin states with a violated Aufbau filling principle are considered.

IR and Raman spectra, ab initio and density functional computations of the vibrational spectra, molecular geometries and atomic charges of uracil and 5-aminouracil

Infrared (IR) and Raman spectra of uracil and 5-aminouracil have been recorded and analyzed between the region 200–4000cm−1. The optimized molecular geometries, atomic polar tensor (APT) charges and vibrational characteristics have been studied theoretically using restricted Hartree–Fock (RHF) and density functional theory (DFT) methods. Using the Becke’s exchange in conjunction with Lee–Yang–Parr’s correlation functional and Becke’s three-parameter hybrid method (B3LYP), the ab initio and DFT calculations were carried out to study the optimized molecular fundamental vibrational frequencies for uracil and 5-aminouracil by employing Gaussian-03 program. The fundamental vibrational frequencies along with their corresponding intensities in IR and Raman activities and depolarization ratios of the Raman lines have also been calculated using the RHF and DFT methods employing different basis sets. In quantum chemical calculations, the most of B3LYP/6-311++G** vibrational frequencies are in the excellent agreement with available experimental assignments and helped in the reassignments of some fundamental vibrational modes. On the basis of calculated results, the assignments of some missing frequencies in the experimental study are proposed. Assuming under the Cs point group for both molecules, the distribution of normal mode of vibrations between the two species as planar (a′) and non-planar (a″) are given by 25a′+11a″, of which 30 modes (21a′+9a″) correspond to the uracil moiety and 6 modes (4a′+2a″) to the NH2 group. Kekule ring stretching mode is found to be comparatively higher frequency magnitude than the mode of uracil due to the involvement of hydrogen bonding of amino group. But, the ring breathing is found to be lower frequency magnitude compared to those for uracil which could be due to mass effect of the NH2 group in place of the hydrogen atom. All other bands have also been assigned different fundamentals/overtones/combinations.

Electronic responses of long chains to electrostatic fields: Hartree-Fock vs. density-functional theory: A model study

The response to an electrostatic field is determined through simple model calculations, within both the restricted Hartree-Fock and density functional theory methods, for long, finite as well as infinite, periodic chains. The permanent dipole moment, μ0, the polarizability, α, and the hyperpolarizabilities β and γ, calculated using a finite-field approach, are extensively analyzed. Our simple model allows for treatment of large systems and for separation of the properties into atomic and unit-cell contributions. That part of the response properties attributable to the terminations of the finite system change into delocalized current contributions in the corresponding infinite periodic system. Special emphasis is placed on analyzing the reasons behind the dramatic overestimation of the response properties found with density functional theory methods presently in common use.

FT-IR and Raman spectra, ab initio and density functional computations of the vibrational spectra, molecular geometries and atomic charges of uracil and 5-halogenated uracils (5-X-uracils; X = F, Cl, Br, I)

Raman (200–4000cm−1) and FT-IR (400–4000cm−1) spectra of uracil and 5-halogenated uracils (5-X-uracils; X=F, Cl, Br, I) have been recorded and analyzed in the range 200–4000cm−1. The optimized molecular geometries, atomic polar tensor (APT) charges and vibrational characteristics have been studied theoretically using restricted Hartree–Fock (RHF) and density functional theory (DFT) methods. Ab initio and DFT calculations [using Becke’s exchange in conjunction with Lee–Yang–Parr’s correlation functional and Becke’s three-parameter hybrid method (B3LYP)] were carried out to study the optimized molecular fundamental vibrational frequencies for uracil and 5-halogenated uracils by employing Gaussian-03 program. Gauss View software was used to make the vibrational analysis. Raman and IR spectra have been computed theoretically for the uracil and 5-halogenated molecules. The fundamental vibrational frequencies along with their corresponding intensities in IR and Raman activities and depolarization ratios of the Raman lines have also been calculated using the RHF and DFT methods employing different basis sets. Quantum chemical calculations helped in the reassignments of some fundamental vibrational modes. Most of the B3LYP/6-311++G∗∗ vibrational frequencies are in excellent agreement with available experimental assignments. The ring breathing and kekule stretching modes are found to lower magnitudes compared to those for uracil which could be due to mass effect of halogen atom in place of the hydrogen atom. The C–X (X=F, Cl, Br, I) stretching frequency is distinctly separated from the CH/NH ring stretching frequencies on the pyrimidine ring. All other bands have also been assigned different fundamentals/overtones/combinations.

Quantum-mechanical predictions of electron-induced ionization cross sections of DNA components

Ionization of biomolecules remains still today rarely investigated on both the experimental and the theoretical sides. In this context, the present work appears as one of the first quantum mechanical approaches providing a multi-differential description of the electron-induced ionization process of the main DNA components for impact energies ranging from the target ionization threshold up to about 10 keV. The cross section calculations are here performed within the 1st Born approximation framework in which the ejected electron is described by a Coulomb wave whereas the incident and the scattered electrons are both described by a plane wave. The biological targets of interest, namely, the DNA nucleobases and the sugar-phosphate backbone, are here described by means of the GAUSSIAN 09 system using the restricted Hartree-Fock method with geometry optimization. The theoretical predictions also obtained have shown a reasonable agreement with the experimental total ionization cross sections while huge discrepancies have been pointed out with existing theoretical models, mainly developed within a semi-classical framework.

Zero field splitting of triplet levels of conjugated molecules: A comparison of exact and approximate π-schemes

An exact method (full configuration interaction) and approximate schemes for the calculation of the anisotropy spin coupling matrix and the corresponding zero field splitting D parameter are considered. Calculation algorithms for the exact method and a special case (valence bond (VB) method) are also presented. The results of specific π calculations give evidence that the VB method in fact cannot be recommended for the description of the spin-spin interaction, especially for biradicals with the triplet ground state. At the same time, the Tamm-Dancoff method (taking into account all singly excited configurations) provides on average a good approximation to an accurate description of non-degenerate triplet states. The description of fine spin effects, which is rougher, but better than that in VB, is given by the restricted Hartree-Fock method for open shells.

Molecular structure, vibrational spectra, UV–vis, NBO, and NMR analyses on nevirapine using ab initio DFT methods

Nevirapine is an anti-human immunodeficiency virus (HIV) agent that belongs to the class of the non-nucleoside inhibitors of the HIV-1 virus reverse transcriptase. Spectral characteristics of nevirapine have been probed into by methods of Fourier transform infrared (FTIR), FT-Raman, UV-visible, and quantum chemistry. The UV spectrum was measured in methanol. In order to gain some valuable insight into the recorded spectrum, the quantum mechanical calculations were performed for nevirapine using both ZINDO and time-dependent density functional theory at B3LYP/6-31G(d,p) level. The optimized molecular geometry, bond orders, natural bond order analysis, and harmonic vibrational wavenumbers of nevirapine were calculated by restricted Hartree-Fock and density functional B3LYP methods with the 6-31G(d,p) basis set using Gaussian 03 W program. The harmonic vibrational frequencies calculated have been compared with experimental FTIR and FT-Raman spectra. The restricted Hartree-Fock and density functional theory-based nuclear magnetic resonance (NMR) calculation procedure was also performed, and it was used for assigning the 13C and 1H NMR chemical shifts of nevirapine.

Electronic Structure of AlAs Nanocrystals

Ab initio restricted Hartree-Fock method within the framework of large unit cell formalism is used to investigate the electronic structure of the core and oxidized surface of AlAs nanocrystals. Large unit cells of 8, 16, 54, 64 and 128 atoms are used in the present analysis. Calculations are performed utilizing the Gaussian (03) software. The investigated properties include the cohesive energy, energy gap, valence bandwidth, conduction bandwidth and the density of states of the energy levels. Interesting results are obtained which reveal that the electronic structure of AlAs nanocrystals differs significantly from that of the bulk AlAs crystal. Also, it is found that the energy gap, valence band width and cohesive energy (absolute value) increase as the AlAs large unit cell size increases, for the core part. Whereas, the energy gap of oxygenated (001) - (1×1) surface decreases with increasing the large unit cell size. The energy gap is controlled by the surface part of the nanocrystal. The surface part has lower symmetry than the core part with smaller energy gap and wider valence bandwidth. The density of states of the core part is higher than that of the surface part. This is due to the broken bonds and the discontinuity at the surface and the existence of new kind of atoms (oxygen atoms). The present method has threefold results: the method can be used to obtain the converged electronic structure of bulk, surface, and nanocrystals.

Effects of protonation and deprotonation on the reactivity of quinolone: A theoretical study

Quinolones are the subject of much research as antibacterial compounds and as a new class of antitumor agents. The protonation (P) and deprotonation (D) sites and conformations of quinolone were investigated. The proton affinity (PA) on each of the possible sites in 4-oxo-1,4-dihydroquinoline has been calculated by the restricted Hartree-Fock (HF) and density functional theory (DFT) methods with the basis set 6–311G**. The O-site of protonation was found to be strongly favored over the N-site for the studied compound in the gas phase. Deprotonation takes place in quinolone by detachment of the N-H and COOH protons. The PA of the simple quinolone molecule was used to characterize quinolone reactivity with DNA binding sites. The relative stabilities of the syn and anti conformations were investigated at the B3LYP/6-311G** level of theory; the syn form was shown to be slightly more stable. Its conformation seems to be intrastabilized by hydrogen-bonds consisting of a hydroxyl proton with the O10 atom as the acceptor. We computed and discussed the charge-density distribution and electrostatic potential to explain the reactivity of quinolone.

Triphenyl moieties as building blocks for obtaining molecular glasses with nonlinear optical activity

The incorporation of trityl and triphenylsilyl groups into low molecular weight molecules allows the formation of stable molecular glasses. A series of materials based on the N-phenyldiethanolamine core was synthesized bearing different azobenzenes and benzylydene-1,3-indandione as active chromophores. Molecular hyperpolarizability of the synthesized compounds was calculated by a restricted Hartree–Fock method with basis 6-31G(d,p) and measured in solutions by hyper-Rayleigh scattering. Non-linear optical (NLO) activity of the thin glassy films was confirmed after a corona poling procedure. Thermal sustainability of the NLO response of up to 85 °C was achieved. Quantum chemical calculations of the compounds revealed increased steric bulk and conformational freedom of the triphenylsilyl moiety. While the presence of the triphenylsilyl group results in more stable glasses and increased material nonlinearity, in the case of trityl groups, measured glass transition temperatures are higher.