Theoretical and experimental approaches to the barrier to rotation about the Csp2—Csp3 bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

Abstract

The 1H nuclear magnetic resonance spectra of benzyl silane and benzyl trichlorosilane, obtained in CS2 and benzene-d6 solutions, are analyzed. The long-range coupling constants between the methylene and para ring protons are used to derive apparent twofold barriers about the Csp2—Csp3 bonds of 7.4 ± 1.6 and 8.1 ± 1.1 kJ/mol for the silane and the trichlorosilane, respectively. These are higher than that for ethylbenzene and are attributed mainly to the stabilization of the perpendicular conformer, that with the C—Si bond in a plane perpendicular to the phenyl plane, by σ–π conjugation (hyperconjugation) of the C—Si bond and the π electron system. Molecular orbital computations confirm the predominantly twofold nature of the internal barrier in benzyl silane and also for benzyl germane and stannane. The calculated barriers for the silane derivatives are rather higher than the experimental values. The computed barriers have magnitudes that appear to change with X in much the same order as do the hyperconjugative interactions deduced in other ways for CH2X(CH3)3 groups (X = Sn, Ge, Si, C). The angles CCX in benzyl-X (X = CH3, SiH3, SiCl3, GeH3, SnH3) are all computed to decrease smoothly as sin2ψ, where ψ is the angle by which the C—X bond twists out of the phenyl plane. Key words: conformations, benzyl silane and trichlorosilane; NMR, benzyl silane and trichlorosilane; MO calculations, benzyl silane and trichlorosilane.