An NMR study of steric and hyperconjugative barriers in benzyl X(CH3)3, X = C, Si, Ge, Sn, Pb

Abstract

The 1H nuclear magnetic resonance spectra and their analyses are reported for benzyltrimethyl X (X = C, Si, Ge, Sn, Pb) and for the 2,6-dichloro derivatives where X = Si, Ge, Sn, Pb. The steric barrier to rotation about the Csp2—Csp3bond for X = C, based on the long-range proton–proton coupling constant over six bonds, 6J(H,CH2), must be greater than 20 kJ/mol. Unexpectedly, 6J(H,CH2) implies that the barriers for X = Si, Ge, Sn, Pb have relatively minor steric components. If these barriers are dominated by hyperconjugative interactions of the C—X bond with the aromatic π system, then they are probably mainly twofold in nature. Furthermore, their magnitude is likely to be near 9.5 kJ/mol for X = Si, Ge, Sn and may be as high as 12 kJ/mol for X = Pb. Keywords: NMR, benzyl X(CH3)3, X = C, Si, Ge, Sn, Pb; NMR, conformations; NMR, hyperconjugation.