Yu. F. Sigolaev

Quantum-chemical study of silacyclohexanes C5H10SiHCN, C5H10SiH(t-Bu), C5H10Si(t-Bu)CN, and C5H10SiHF

By quantum-chemical calculations at the M06-2X/aug-cc-pVTZ level of theory geometrical parameters, dipole moments, polarizabilities, first hyperpolarizabilities and relative energies of the axial and equatorial conformers in gaseous phase were determined for 1-cyano-1-silacyclohexane, 1-tert-butyl-1-silacyclohexane, 1-tert-butyl-1-cyano-1-silacyclohexane, and 1-fluoro-1-silacyclohexane. For the cyano group and fluorine atom the axial position is more preferable whereas for tert-butyl group, equatorial one. Polarizabilities of conformers are similar but optical anisotropy of equatorial conformers of C5H10SiHCN and C5H10SiH(t-Bu) molecules is much larger than that of axial conformers. Upon substitution in nitriles of C1 atom by Si atom the hyperpolarizability is many times increased.

Borabenzene and pentafluoroborabenzene adducts with dinitrogen, xenon and krypton : A quantum-chemical study

Structural parameters of borabenzene and pentafluoroborabenzene molecules and their adducts with N2, Xe, and Kr are calculated by the B3LYP quantum-chemical method. The adducts C6F5B·(N2, Xe, Kr) are characterized by the formation energies 36, 26, and 8 kcal mol−1, bond lengths 1.45, 2.23, and 2.21 Å, charges +0.2 (N2), +0.4 (Xe), and +0.3 au (Kr), and dipole moments 2.8, 7.0, and 6.0 D, respectively. The adducts C6H5B · (N2, Xe) have the total energies 18 and 11 kcal mol−1, bond lengths 1.48 and 2.45 Å, charges +0.1 (N2) and +0.3 au (Xe), and dipole moments 0.8 and 4.0 D, respectively. Unsubstituted borabenzene does not form a chemical bond with krypton.

Quantumchemical investigation of borabenzene adduct with pyridine

Atomic charges and structural parameters of borabenzene, pyridine and their adduct in free state, in liquid argon and in tetrahydrofuran are calculated by the quantum-chemical method B3LYP/6-311G(3d5f7,p) & PCM. Mutual polarization of the adduct and medium results in small increase in boron-nitrogen interatomic distance and dihedral angle between the aromatic heterocycles. Calculated dipole moment of the adduct (7.17 D) is by 4.55 D over than the sum of dipole moments of the free components. Internal rotation barriers are not high: 1 kcal mol−1 (0° and 180°) and 4 kcal mol−1 (90°). The borabenzene-pyridine bonding energy (46 kcal mol−1) is higher than that with dinitrogen (19 kcal mol−1) and xenon atom (6 kcal mol−1). The B-N bond length in the little stable adduct with dinitrogen is by 0.08 Å shorter than in the stable adduct with pyridine. The Lewis acid properties inherent in borabenzene are transferred on the π-electron system of pyridine fragment in the adduct. The electron transfer wavelength from borabenzene to pyridine fragment in argon matrix is by 109 nm higher than in tetrahydrofuran, as calculated by CIS CNDO/S method.

Computer simulation of the structure of large molecules: IV. 2D polybuckminsterfullerenes and their boraza analogs with bisingle nitrogen-boron bonds

The equilibrium structure of the dimer (C60)2, clusters (C60)9, and (C60)7 simulating the poly-C60 (VIII) and poly-C60 (XII), as well as their boraza analogs with bisingle nitrogen-boron bonds was determined by the quantum-chemical methods B3LYP/6-31G and PBE0/6-31G. The boraza analogs are characterized by the positive values of calculated (B3LYP/PBE0) energies of the interpolyhedral bonds: 10/20, 35/74, and 54/109 kcal mol−1 respectively per a pair of monomers. The PBE0 method predicts shorter bond than B3LYP, and, in accordance with calorimetric data, positive bonding energy of the polyhedra in (C60)n.

Quantum-chemical investigation of reactions of (dialkylamino)ethynylphosphonates with amines

Structural parameters and molecular energies of diethyl piperidylethynylphosphonate, dimethyl (diethylamino)ethynylphosphonate, dimethyl[2-(t-butylamino)-2-(diethylamino)vinyl]phosphonate, N,N-diethyl-N′-t-butyl(dimethoxyphosphinoyl)acetamidine, dimethyl [2-(diethylamino)-2-(phenylamino)vinyl]phosphonate, N,N-diethyl-N′-phenyl(dimethoxyphosphinoyl)acetamidine, and the cations formed by protonation of the phosphinoyl group, nucleophilic carbon atom, or nitrogen were determined by means of B3LYP/6-311G(d5,p)&6-31G(d5,p) quantum-chemical calculations. Acid-base properties of the related amines and their adducts with boron trifluoride in CCl4 are estimated. The mechanism of the reactions of (dialkylamino)ethynylphosphonates with amines is discussed.

Computer modeling of the structure of large molecules: III. Local excitations in the structure of 2D C60(XII) and C60(VIII) polymers

The quantum-chemical PM3 method was used to determine the structure of excited poly-C60(XII) and poly-C60(VIII) macromolecules containing diradical defects formed by cleavage of interpolyhedral C-C bonds and degradation of intrapolyhedral C=C bond. The relative energies of such excitations are estimated. The atomic free valences FA hybridization coefficients of valence-active atomic orbitals contributing maximum Fa ≈ FA to the FA value are calculated. The valent isomerism of the C60(XII) dodecaradical is discussed. The calculations are performed using the GAMESS computer program and a new SDIAG algorithm for diagonalization of large dense symmetric matrices.

Affinity of xenon(II) and carbon in the (heptafluorocyclohexa-1,4-dien-1-yl)xenonium ion for fluoride ion and acetonitrile

The results of B3LYP&PCM quantum-chemical calculations confirmed the possibility for formation of fluoro(heptafluorocyclohexa-1,3-dien-1-yl)xenon and (octafluorocyclohex-3-en-1-ylidene)xenon from (heptafluorocyclohexa-1,4-dien-1-yl)xenonium in liquid acetonitrile. The Xe(II) and C2 atoms in the cation are characterized by a strong affinity for fluoride ion, but the Xe(II) atom binds acetonitrile molecule. The addition of F− to C2 in [XeC6F7]+ is more favorable (by 6 kcal/mol) than replacement of the fluorine atom on Xe(II) by MeCN molecule. The charge on the xenon atom in (octafluorocyclohex-3-en-1-ylidene)xenon is small, the Xe-C bond is weakened, and its cleavage leads to ring contraction to perfluoro(3-methylidenecyclopent-1-ene).

Computer modeling of the structure of large molecules: II. Cluster C1500

The structure of a C1500 cluster modeling a “planar” polymeric buckminsterfullerene(VIII) is determined by the PM3 quantum-chemical method. The cluster consists of 25 oblate octa-, hexa-, and tetra-valent C60 polyhedra and has D2h symmetry. The calculated parameters of a rectangular unit cell are as follows: 9.016 Å along the bridging bonds in the polymer plane and 9.084 Å along the bridging bonds over and under the polymer plane. Hybrid σ orbitals of three-coordinate atoms are directed at an angle of 9.7°–9.8° to the polyhedron edges and those of four-coordinate atoms, at an angle of 8.3°–8.6° to the rectangular bridge edges. The s contribution into π orbitals is negligibly small. The GAMESS program with a new algorithm for diagonalization of large dense symmetrical matrices and a new VALENCY program were used for calculations.

7-oxabicyclo[2.2.1]heptane hydrochloride and hydrofluoride: Quantum-chemical investigation

Structural parameters, IR spectra and energy of H-bonds for binary complexes of 7-oxabicyclo[2.2.1]heptane molecule with molecules of HCl and HF are calculated by quantum-chemical B3LYP/cc-pVTZ method with inclusion to the basis of diffuse “aug”-orbitals of halogen, oxygen and bridgehead hydrogen.

Computational modeling of the structure of large molecules: I. Molecule C60H12 and cluster C852

Structural parameters of dodecahydro-Buckminster fullerene C60H12 and its derivative, the carbon cluster C852 consisting of a central C60 dodecaradical and twelve peripheral C60 diradicals connected by eighteen-C≡C-C≡C-bridges, are calculated using a PM3 quantum-chemical method. The C60H12 molecule and the central fragment of the cluster each contain eight quasiaromatic six-membered rings with bond lengths of 1.38 and 1.43 Å. The calculations we performed using the GAMESS package and an original algorithm for diagonalization of dense symmetric matrices.