Category: 2002

Density functional theory (B3LYP) calculations were performed on the methyl and t-butyl substituted valence isomeric forms of silabenzene. The recently identified 12 valence isomeric minima on the potential energy surface of silabenzene were considered. The calculations reveal that substitution does not affect the skeleton of all the compounds including the unusual structures, V1a and V1b, located on the silabenzene potential energy surface. Similarly, the geometric parameters and relative energies show negligible differences upon substitutions by Me and t-Bu groups except for V1b. Chemical hardness and frontier orbital energy levels are used to assess the reactivity change upon substitution. The present study concludes that the substitution by Me or t-Bu will have only the steric effect and therefore, theoretical models employing the unsubstituted analogues should adequately describe the structural and energetic characteristics of the bulky group substituted silaaromatic compounds.

Computational studies using density functional theory (B3LYP) and coupled cluster (CCSD(T)) method were performed on a large number of isomers of disilabenzene, C₄Si₂H₄. In all, 78 stationary points were identified, wherein 61 of them correspond to the minima on the C₄Si₂H₆ potential energy surface. Although the planar forms correspond to the most stable isomers, several unconventional structures are highly competitive in energy. While the framework is an important factor, the substitution pattern seems to be equally important in deciding the relative stabilities of the isomers of disilabenzenes. The relative energies of valence isomeric forms are compared with those of benzene, silabenzene, and diphosphabenzene isomers. The relative energy orderings of all the isomers are analyzed, and conclusions about the stability are drawn. Several H-bridged isomers were computed to be very stable. Several monocyclic six-membered ring structures, where the 6π-aromaticity is disturbed, also indicated the flatness of the potential energy surface of disilabenzene and their propensity toward high reactivity and facile rearrangements.

Several endo-tricyclo[5.2.1.0^2,6]decan-10-ones and endo-tricyclo[5.2.1.0^2,6]dec-8-en-10-ones with hetero atom modifications at the distal C-4 position have been subjected to hydride reduction. π-Face selectivities in these systems are largely governed by the same electronic factors that were earlier identified in the case of the norbornyl system. A computational study demonstrates good predictability at the semi-empirical level.

Quantum mechanical calculations at the ab initio (MP2, CCSD(T)) and the hybrid density functional levels with an array of basis sets (6-31G*, 6-31G**, cc-pVDZ, 6-311+G**) were performed on 15 (CH)₅SiH structures. Of the 15 structures considered on the (CH)₅SiH potential energy surface, 12 structures were characterized as local minima. Two new valence isomeric forms of silabenzene, V1a_m and V1b_m, have been identified, which lie only about 20 kcal/mol higher than silabenzene (B1). PY, where Si is hexacoordinated, has been characterized as a stationary point, which lies only around 56 kcal/mol higher in energy than silabenzene. The relative energies are contrasted with those of the valence isomers of benzene. The lower magnitudes of the vibrational frequencies corresponding to the skeletal movements support the facile rearrangements witnessed in these compounds. The chemical hardness values were measured, and no direct correlation was obtained between the relative energy ordering and the chemical hardness values.

Ab initio molecular orbital theory at Hartree−Fock (HF), post-Hartree−Fock (MP2 and CCSD(T)), and the hybrid density functional theory (B3LYP) calculations were done on the mono-(CH)₅XH and diskeletally substituted (CH)₄(XH)₂ benzenes (X = B^–, N⁺, Al^–, Si, P⁺, Ga^–, Ge, and As⁺). The computed relative energies of the disubstituted isomers show interesting trends. While the ortho-isomer is the most stable for X = Ga^–, Ge, and As⁺, meta was found to be the most stable for X = B^–, N⁺, Al^– and Si, and para was found to be the most stable for X = P⁺. Various intricate factors that govern the relative stabilities, such as the sum of bond strengths in the twin Kekule forms, rule of topological charge stabilization (TCS), and electrostatic repulsion were critically examined. The sum of bond strengths in the twin Kekule forms was proved to be quite a successful measure in predicting the relative stability orders between ortho– and meta–/para-isomers. The rule of TCS breaks down especially in the presence of overwhelming factors such as the differences in the cumulative bond strengths of the two positional isomers; however, the stability ordering between the para– and meta-isomers is successfully predicted in most cases. The tendency for ring puckering increases a great deal especially when the substituents are from 3rd or 4th row. Extension of the popular inverse relationship between the thermodynamic stability and reactivity was found to be inapplicable for this class of compounds. The computed singlet−triplet energy differences and the chemical hardness (η) values indicate that the skeletal substitution weakens the π-strength of the benzenoid system and increases their reactivity.

Computations were performed on idealized retrosynthetic routes towards two model buckybowl compunds, sumanene (I) and pinakene (II). Two possible paths for sumanene (I) and six for pinakene (II) were analyzed. The computational results unequivocally predict that benzannulation is a significantly easier process compared to cyclopentannulation in the ring closure strategies in both cases. The suitability of the theoretical models for obtaining reliable trends is assessed and generalizations for the synthetic strategies directed towards buckybowls and C₆₀ were made.

Ab initio [HF, MP2 and up to CCSD(T)] and hybrid density functional B3LYP calculations were performed on benzene, group V heterobenzenes, (CH)₅X (X = N, P and As), and their corresponding valence isomers. Six valence isomers of benzene and ten valence isomers of (CH)₅X have been identified and their structures, energetics, relative stabilities and their reactivities were studied. The stability ordering observed in phosphinine and arsabenzene isomers is very similar to that in the benzene isomers (benzene > benzvalene > Dewar benzene > prismane > bicyclopropenyl > trans-Dewar benzene). In the pyridine isomers, one of the bicyclopropenyl isomers (C2N) is computed to be more stable than the prismane isomer (P1N). The framework is the main factor determining the relative stabilities and the nature of the substituent is of secondary importance. Exactly the same trend in the relative energies is observed for phosphinine and arsabenzene isomers.