| Valence bond(VB) theory and molecular orbital(MO) theory are parallel moden chemical bond approaches.In the past two decades,great importance was attached to VB theory again due to the rapid progress in computer technology.The MO explanation of the essential concept of conjugation,aromaticity and antiaromaticity is not comparable to the VB explanation.When aromaticity of a system is studied,the uniqueness of the VB theory lies in its rigid definition of strictly localized resonance structures with VB functions and thus provides an ideal reference to evaluate the strength of electronic delocalization within molecules.In this pater,the ab inito VB theory is applied to small clusters of boron family and cyclopropane,and the work is summarized into two sections as follows:1.VB study on the aromaticity and bonding modes of B3- and B4 clustersThe ab initio valence bond(VB) methods,VBSCF,BOVB and VBCIS,have been performed for B3- and B4 clusters in order to discuss theπ-aromaticity andσ-aromaticity of the two species.The results show that strong aromatic character of bothπsystem andσsystem has been observed in B3- and B4 clusters.Besides the significantly higherπresonance energies than that of theπsystem in benzene at the same level of theory,the calculated values ofσresonance energy are also considerable and close to the corresponding values ofπresonance energy.Furthermore,theπandσelectron delocalization of B3- and B4 can be qualitatively described using several Lewis resonance structures and the favorable bonding mode of localizedσelectrons in those two species can be determined using VB calculations.2.VB study on the aromaticity of cyclopropane and trisilacycloproaneCyclopropane has long been regarded as the prototype for the concept ofσ-aromaticity.But recent analysis of its magnetic properties challenged theσ-ring current model which centers on theσ-aromaticity paradigm.To estimate the aromatic stabilization in cyclopropane which truly defines aromaticity in chemistry,we applied ab initio valence bond(VB) theory to cyclopropane and its linear references including propane and butane.Computational results confirmed that there is no apparentσ-aromaticity in cyclopropane.In contrast,remarkableσ-aromatic stabilization is found in trisilacycloproane,due to the diffuse nature of the valence orbitals on silicon atoms.Thus,trisilacycloproane instead of cyclopropane may be used as a paradigm for the concept ofσ-aromaticity... |
