Category: 2004

Quantum chemical calculations at B3LYP/6-31G* and semiempirical levels have been performed on a series of sterically unbiased ketones, where facial differentiation during nucleophilic additions is electronically induced through distal functional groups. The face selectivity data for fifty-four substrates representing nine different skeleta were computed and compared with the available experimental data on thirty-eight of them. The predictive abilities of various computational methods such as, charge model, hydride model, LiH transition state model, Cieplak hyperconjugation effect estimated by NBO analysis and the cation complexation model have been evaluated.

B3LYP/6-31G* calculations were done on seven C₂₁H₉^Z fragments, where the charge, Z, is varied from −3 to +3; doublet states of the open shell systems and both singlet and triplet states of closed shell species are considered. Among them, only the D_3h and C_3v forms of the singlet states of C₂₁H₉³⁻ and C₂₁H₉¹⁺, and triplet-C₂₁H₉¹⁻ correspond to the bowl-to-bowl inversion transition states and minimum energy structures, respectively. 6-31+G* basis set, which employs a set of diffuse functions on the non-hydrogen atoms, was employed on anionic systems. The rest encountered symmetry breaking problem, which necessitated us to go for lower symmetry structures to obtain the local minima and the bowl-to-bowl inversion transition states. The curvature, ascertained by π-orbital axis vector angles (POAV), is lowest for the trianion and highest for the cations. The triplet states of the closed shell species are found to be competitive in energy with the singlet states when Z=−1 and +3, whereas for Z=−3 and +1, the triplet states lie above the corresponding singlet states over 30 kcal/mol. The inversion barriers vary from 12.2 to 40.2 kcal/mol with C₂₁H₉³⁻ and C₂₁H₉²⁺ having the lowest and highest inversion barriers, respectively. The electron affinities and ionization energies were calculated to assess the electron receiving and donating ability of the C₃-fragment in comparison to corannulene. The curvature and redox behavior of the C₂₁H₉ fragment are compared to those of C₆₀.

Hybrid density functional theory (B3LYP) and ab initio (CCSD(T)) calculations were performed to explore the reaction energy pathways of the valence isomerizations among the disilabenzene isomers, namely, bis(silacyclopropenyl) (1), disila(Dewar benzene) (3) and disilabenzvalene (4). The computational study predicts that the interconversion of 1 to 3 and 4 occurs spontaneously. However, the interconversion between Dewar benzene (3) and benzvalene (4) analogs is a multistep process involving a new intermediate structure 2a, with energy barriers lying above 30 kcal/mol in energy compared to 3 and 4.

Isomerization from bis(silacyclopropenyl) is very facile either to disila(Dewar benzene) or disilabenzvalene.

Density functional theory (B3LYP) calculations with double and triple-ζ quality basis sets were performed on the Li⁺ and Na⁺ π-complexes of corannulene 2, sumanene 3CH₂, heterosumanenes 3X, triphenylene 4 and heterotrindenes 5X. The metal ions bind to both convex and concave faces of buckybowls, with a consistent preference to bind to the convex surface by about 1–4 kcal/mol. The metal ion complexation with the π-framework of the central six-membered ring span wider range compared to benzene, indicating the control of size, curvature and electronic perturbations over the strength of cation–π interactions. Computations show that the bowl-to-bowl inversion barriers are only slightly altered upon metal complexation, indicating the continuity of bowl-to-bowl inversion despite metal complexation. We have calculated the binding energies of model systems, triphenylene (4) and heterotrindenes (5X), which indicate that the interaction energies are controlled by electronic factors. While the inversion barrier is dependent mainly on the size of the heteroatom, the extent of binding is independent of the size of the atom or the bowl depth.

Ab initio and density functional theory calculations indicate that a benzene dication isomer (1: C₆H₆²⁼) with three contiguous planar tetracoordinate carbons is at a minimum on the potential energy surface. The remarkable preference for the planar structure for 1 is traced to the aromatic stabilization present in the three membered ring formed by the three planar tetracoordinate carbon atoms.

A novel structure, C₆H₆²⁼, with three contiguous planar tetracoordinate carbon atoms is proposed as a viable candidate.

Hartree–Fock, post-Hartree–Fock (MP2 and CCSD(T)) and density functional theory (BLYP, B3LYP, BPW91 and B3PW91) methods were employed to obtain the relative stabilities of eight energetically competitive C₂S₂H₂ isomers. Peculiarly high dependency on the basis set has been observed and the convergence of the relative stability orderings is slow even at the quintuple-ζ levels. Basis sets starting from 6-31G*, 6-311G*, 6-311+G*, 6-311+G**, 6-311++G**, 6-31G(2df), 6-31G(3df), 6-31G(3df,3pd), cc-pVDZ, cc-pVTZ, cc-pVQZ and cc-pV5Z were used in conjunction with various methodologies. A critical comparison of the performance of the density functional methods with MP2, CCSD(T) and G2 methods was made.

Cyclopropyne (C₃H₂) and related spiro bicyclic structures show unusual structural preference for the planar tetracoordinate structure over the conventional tetrahedral arrangement. C₅H₄ has been proposed as the smallest neutral hydrocarbon with planar tetracoordinate carbon . NICS values indicate substantial stabilization of the planar arrangements over the tetrahedral counterparts.