Pairing double hybrid functionals with a tailored basis set for an accurate thermochemistry of hydrocarbons

The pairing of the PBE-QIDH double-hybrid functional with a tailored split-valence basis set leads to a fast computational protocol for the accurate evaluation of hydrocarbon thermochemistry, without resorting to any empirical correction.

Ab initio and MNDO Calculations of the 35Cl NQR Parameters of Some Organic and Inorganic Molecules

The results of ab initio and MNDO calculations of the Cl2C = CHOCH3 , XCOCl (X = CH3 , OCH3 and COCl), 4-ClC6H4CH2Cl, (CNCl)3 and PCl5 with total optimization of their geometry are presented. The ab initio calculations were executed using Hartree-Fock theory and the split valence basis set 6-31G* (RHF/6-31 G*//RHF/6-31G*). Using the calculated p-orbital populations of the CI atoms in these molecules the 35Cl NQR frequencies and asymmetry parameters of the EFG at the 35Cl nuclei have been determined. When the populations of the less diffuse components of orbitals in the split valence basis set are used the calculated and experimenal ν and η values are in good agreement. Linear correlations between these calculated and corresponding experimental ν and η values are obtained. The causes of the nonconformity of the earlier calculated ν and η values and their experimental ones are analysed.

Contribution to the Semiempirical Interpretation of the Anisotropy of the 29Si NMR Shielding Tensor in Silicates. II. Influence of Changes in the O-Si-O Bond Angle in SiO44- Ion

The influence of variations in the OSiO bond angle α on the anisotropy of the 29Si shielding tensor of the monosilicate anion SiO44- in C3ν symmetry has been investigated by semiempirical CNDO/2 calculations. In the valence basis set of the silicon atom, no d-orbitals were employed. It could be shown that the terms characterizing the four Si-O bond contributions to ∆σ are of greatest importance for reproducing the experimentally observed trend of anisotropy signs in silicates. Both the increasing O(1)-Si-O(B) bond angle α, and the decreasing Si-O(1) bond length r(Si-O(1)) (and vice versa) exhibit an amplifying geometrical influence on ∆σ.

Ab initio studies of isocyanatoborane and difluoroisocyanatoborane; bent or linear BNCO chains?

Ab initio calculating at the 3-21G, 6-31G*, 6-31G**, 6-311G*, and 6-311G** Hartree–Fock levels, have been performed on the unknown H2BNCO and F2BNCO molecules in order to establish the geometries of these isoelectronic propadienone analogues. For H2BNCO the double split valence basis sets lead to linear BNCO chains, whereas either a triple split valence basis set, or the inclusion of correlation to second order (MP2/6-31G*) gives trans-bent structures. These have angles at nitrogen of 153.6° (6-311G*) or 149.9° (MP2/6-31G*), with the potential surface for angle bending extremely flat, and 0.5 kJ mol−1 (6-311 G*) or 1.12 kJ mol−1 (MP2/6-31 G*) separating the C2v and Cs structures. For the bent structures there is a small trans-bend (4–6°) at the carbon atom. The F2BNCO molecule is also linear at the 3-21G level, but is, however, already trans-bent (145.6° at nitrogen, 175.9° at carbon) at the 6-31G* Hartree–Fock level; 1.47 kJ mol−1 separates the bent and linear structures. The triple split valence basis set 6-311G* leads to a further decrease in the angle at nitrogen (141.0°), and a similar NCO angle (175.9°), with the bent structure favoured by 3.85 kJ mol−1. MP2/6-31G* calculations give a minimum with an angle at nitrogen of 140.2°, and a bent-linear energy difference of 3.58 kJ mol−1. Key words: ab initio calculations, isocyanatoboranes, structures, quasilinearity, propadienone analogues.

Zum Circulardichroismus von Disulfidbrücken in Proteinen. Teil 2. Vergleichende CNDO/S- und INDO/S-CI-Rechnungen

Rotatory strengths for H2S2 conformers based on X-ray data for ribonuclease A, 2-, and 4-zinc-insuline-dimers have been calculated with some CNDO/S- and INDO/S-versions. The best agreement between their calculated signs and those predicted by the quadrant rule and Rauk’s valence basis set ab initio calculations are found with the CNDUV-version of CNDO/S. A choice of β0s = - 11.5 eV within this method improves the calculated transition wavelengths compared to observed ones.

La liaison phosphore–azote. Étude quantochimique de modèles neutres et ioniques H3PNH, , , H2PNH− et

Abinitio molecular orbital calculations have been performed on the phosphazene bond using a split valence basis set on neutral and ionic compounds. The nature of the P—N multiple bond in H3PNH shows similarities with that of the P—C bond in phosphorus ylides. Electronic characteristics of cations (H2PNH3)+, (H3PNH2)+ and anions (HPNH2)−, (H2PNH)− related to H3PNH and its tautomer H2PNH2 were investigated. The results obtained were used to explain experimental observations for this type of compounds.