The nature of resonance and hyperconjugation for cyclic β-silyl substituted carbocations: NBO, NRT, EDA, and NMR studies

Abstract

The stabilities of various cyclic β-silyl carbocations and their desilylated analogues, relative to tert-butyl cation, were calculated in the gas phase at B3LYP/6-311++G** level of theory. Natural bond orbital (NBO) theory was used for the calculation of charges and bond orders. The contribution of carbenium ion [R1R2C+–CR3R4Si(Me)3] and silylium ion [R1R2C=CR3R4 Si(Me)3+] to the resonance structures was investigated by natural resonance theory (NRT). The localized molecular orbitals energy decomposition analysis (LMO-EDA) was employed to estimate the bond dissociation energy of β-silyl carbocations to R1R2C=CR3R4 and Si(Me)3+ fragments via the MP2/6-311++G** method. The silylium ion is the most important resonance structure of β-silyl cycloalkyl (cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl) and cyclopentadienyl cations. In the case of β-silyl phenylcyclopropyl, benzocyclobutyl, cyclopropenyl, and tropylium ions, the carbenium ion is the major contributor (due to the delocalization of positive charge). The calculated 29Si and 13C nuclear magnetic resonance (NMR) chemical shifts complement these results.