AB initio calculations of the electronic structure and doubling parameters of BS

Ab initio calculations have been performed to understand the influence of spin—orbit interaction upon the fine structure of the observed valence doublet states of the BS molecules: spin—orbit splittings, γ-type and Λ-type doublings. Configuration interaction is shown to be an essential feature in order to account for the observed data. Other properties of the electronic states (transition energies) have also been calculated.     

Rotational barrier and structure of aminoborane determined by AB initio calculations

Ab initio calculations have been used to determine the structural parameters of both the planar and the orthogonal form of H₂BNH₂. The BN bond lengths were found to be 1.378 Å and 1.469 Å, respectively. The arrangement about nitrogen is pyramidal in the orthogonal form. The best estimate of the barrier to internal rotation is 33.3 kcal/mole.     

Ab initio calculations of the electronic structure of helical polymers

An ab initio self-consistent-field (SCF ) algorithm taking into account all the features of the one-dimensional translational periodicity and the helical symmetry is presented. This algorithm includes the long-range correction to the Coulomb potential and is designed to calculate the band structure of periodic one-dimensional polymers (planar or helical). Its efficiency in terms of computing time and numerical accuracy is tested via applications on a (LiH)_n chain, polyethylene, and four conformers of polypropylene.     

AB initio calculations of the dipole moments in low-lying electronic states of the ccn radical

The structures and dipole moments of the four low-lying electronic states (X²Π, A²Δ, B²Σ⁻ and C²Σ⁺) of the linear CCN radical are investigated by ab initio calculations at SDCI/DZP and TZP levels. For all the electronically excited states, the dipole moments are calculated to be ≈ 3.0 D. However, a significantly smaller dipole moment, ≈ 0.6 D, is predicted for the ground state. This result is consistent with the recent experiment by Suzuki, Saito and Hirota, where the MODR signals are observed for the A state CCN but not for the X state. Electronic correlation is important in determining both equilibrium bond lengths and dipole moments.     

AB initio effective spin—orbit operators for use in atomic and molecular structure calculations. Results for carbon and silicon

Ab initio effective spin-orbit operators for carbon and silicon are derived from relativistic effective core potentials based on Dirac—Fock wavefunctions. Transferability of these operators to electronic states other than the one used in the original derivation is treated by calculating spin—orbit splittings of various neutral and ionic atomic states.     

Interpreting ultrafast molecular fragmentation dynamics with ab initio electronic structure calculations

High-level ab initio electronic structure calculations are used to interpret the fragmentation dynamics of CHBr(2)COCF(3), following excitation with an intense ultrafast laser pulse. The potential energy surfaces of the ground and excited cationic states along the dissociative C-CF(3) bond have been calculated using multireference second order perturbation theory methods. The calculations confirm the existence of a charge transfer resonance during the evolution of a dissociative wave packet on the ground state potential energy surface of the molecular cation and yield a detailed picture of the dissociation dynamics observed in earlier work. Comparisons of the ionic spectrum for two similar molecules support a general picture in which molecules are influenced by dynamic resonances in the cation during dissociation.     

Photoelectron spectra andab initio calculations of the electronic structure of amino-n-sulfenyl chlorides

The electronic structure of some amino-N-sulfenyl chlorides and related compounds is studied by photoelectron spectroscopy and ab initio calculations. Similar values of IP(n _S ) and IP(n _N ) and the total energy minimum indicate that in stable conformations the n _S and n _N orbitals are orthogonal. These conformers are characterized by an effective n_N-σ _S-Cl ^* interaction. The relationship between the intramolecular shift of charge and ionization potential values and the spatial structure of amino-N-sulfenyl chlorides is analyzed. A. E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 69–73, March–April, 1994. Translated by L. Smolina     

AB initio molecular orbital calculations on the pentadienyl radical

In this publication we present the results of ab initio Gaussian orbital calculations of ESR coupling constants in the pentadienyl radical. The results are compared with experiment and with the results of similar semi-empirical calculations.     

Ab initio calculations on the electronic structure of the divalent lead-water complex

We studied the electronic structure of the Pb (2+)-4H 2O system. Analysis of the complex orbital evidenced no mixing between the 6s lone pair orbital of the lead and the 6p orbital components. Moreover, we found that the HOMO is widely described by the mixture of the 6p components with the 7s valence orbital of the lead. This orbital shows an important elliptical electron charge density around the lead ion and opposite the direction of the short lead-water bonds. From these results, we demonstrated that the hemidirected conformation of the Pb (2+)-4H 2O system could be easily explained by the shape of the electron charge density distribution of the HOMO rather than by the stereochemically active character of the 6s (2) lone pair of lead electrons.     

Steric and electronic structure of 2-alkoxyphenyltrichlorostannanes according to results of ab initio calculations

Steric and electronic structure of 2-methoxy- and 2-ethoxyphenyltrichlorostannanes, as well as of 2-methoxyphenyltrichlorostannanes substituted in the ring, was studied using the RHF and B3LYP levels with the 3?C21G* basis set. The results of calculations were compared with experimental ³⁵Cl NQR data. In all studied molecules the Sn atom is pentacoordinated. The structure of the coordination polyhedron is a highly distorted trigonal bipyramid. Replacing methyl group in the alkoxy substituent involved in the Sn??O coordinating interaction by a more electron-donor ethyl group increases the strength of the Sn??O coordination bond. The same occurs also at the introduction of an electron-releasing substituent in the aromatic ring.     

AB initio SCF study of the electronic structure and spectrum of CuF2

MC SCF and contracted CI calculations have been performed for the three ligand-field states of CuF₂ and also for two charge-transfer states. With the most extensive basis set the calculated d-d transition energies, including a Davidson correctior for cluster effects, are 4150 cm^?1 (²11g) and 10560 cm^?1 (²Δg). These calculations were made with 98 basis functions, including of orbitals on Cu and d orbitals on F. To check the charge distribution in the molecule, calculations of the ESR g factors were also made at the SCF and CI levels of approximation. Resulting CI values are g_| = 1.93 (1.91) and g₁ = 2.76 (2.60). with corresponding experimental numbers in parentheses.     

Bond functions for AB initio calculations on polyatomic molecules hydrocarbons

Transferable gaussian CH and CC bond functions and a C lone-pair function were optimized and compared to conventional polarization functions. The exponents (α) and position (γ,β) of these functions are α = 0.9, γ = 0.27 for CH bonds; α = 1.2, γ = 0.50 for CC bonds; α = 0.3, β = 0.47 for C lone-pair.     

Flexible H2O2 in water: electronic structure from photoelectron spectroscopy and ab initio calculations

The effect of hydration on the electronic structure of H(2)O(2) is investigated by liquid-jet photoelectron spectroscopy measurements and ab initio calculations. Experimental valence electron binding energies of the H(2)O(2) orbitals in water are, on average, 1.9 eV red-shifted with respect to the gas-phase molecule. A smaller width of the first peak was observed in the photoelectron spectrum from the solution. Our experiment is complemented by simulated photoelectron spectra, calculated at the ab initio level of theory (with EOM-IP-CCSD and DFT methods), and using path-integral sampling of the ground-state density. The observed shift in ionization energy upon solvation is attributed to a combination of nonspecific electrostatic effects (long-range polarization) and of the specific interactions between H(2)O(2) and H(2)O molecules in the first solvation shell. Changes in peak widths are found to result from merging of the two lowest ionized states of H(2)O(2) in water due to conformational changes upon solvation. Hydration effects on H(2)O(2) are stronger than on the H(2)O molecule. In addition to valence spectra, we report oxygen 1s core-level photoelectron spectra from H(2)O(2)(aq), and observed energies and spectral intensities are discussed qualitatively.     

ab initio calculations of the geometrical and electronic structure of thiophene and 2-chlorothiophene molecules

Features of the structure of thiophene and 2-chlorothiophene molecules have been analyzed from the results of ab initio calculations using the RHF/6-31G^* method.Institute for Technical Chemistry, Ural Department, Russian Academy of Sciences, Perm 614000, Russia; email: cheminst@mail.psu.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 40–43, January, 1999.     

Methyaluminoxane (MAO) polymerization mechanism and kinetic model from ab initio molecular dynamics and electronic structure calculations

MAO is the co-catalyst in metallocene catalytic systems, which are widely used in single-site olefin polymerization due to their high stereoselectivity. To date, the structures of the catalytically active compound or compounds in MAO have eluded researchers. Although many structural models have been proposed, none are generally accepted. In this study, aspects of the formation mechanism of MAO are addressed. Molecular dynamics simulations at the MP2 level of theory were carried out for presumed elementary steps in MAO formation via hydrolysis of trimethylaluminum (TMA). Methane production was observed, in agreement with experiment, as well as intermediate species that are consistent with the known structural features of MAO and similar to isolated and structurally characterized aluminoxanes. A (CH3)3Al-OH2 species, which we denote as TMA-OH2, containing a stable Al-O single bond emerged as the building block molecule. From this species, a hexameric cage was formed and activation barriers for the various reactions were calculated. Three distinct channels were identified for growth beyond the hexameric cage. It was concluded that MAO formation is a step polymerization through a bifunctional monomer, with [(CH3)Al-O] as the structural unit and a kinetic model was proposed. The structures that emerged were in agreement with the crystallographic evidence for aluminoxanes and support the experimental data regarding the MAO chemical composition.     

Glycine in an electronically excited state: ab initio electronic structure and dynamical calculations

The goal of this study is to explore the photochemical processes following optical excitation of the glycine molecule into its two low-lying excited states. We employed electronic structure methods at various levels to map the PES of the ground state and the two low-lying excited states of glycine. It follows from our calculations that the photochemistry of glycine can be regarded as a combination of photochemical behavior of amines and carboxylic acid. The first channel (connected to the presence of amino group) results in ultrafast decay, while the channels characteristic for the carboxylic group occur on a longer time scale. Dynamical calculations provided the branching ratio for these channels. We also addressed the question whether conformationally dependent photochemistry can be observed for glycine. While electronic structure calculations favor this possibility, the ab initio multiple spawning (AIMS) calculations showed only minor relevance of the reaction path resulting in conformationally dependent dynamics.