Implications of hyperconjugative effects on bond lengths of allylic systems. An NBO investigation

The influence of hyperconjugative interactions on bond lengths of some allylic compounds (H₂CCH–CH₂–M(CH₃)₃; M=C, Si, Ge) has been investigated through NBO calculations using ab initio and density functional methods. The optimized structural parameters, at the B3LYP/6-31+G(d,p) and HF/6-31+G(d,p) levels, showed a good agreement with the resonance theory. Partial geometry optimization with orbital interactions removed confirmed the observations and revealed that σ→σ* interactions, together with the more common σ→π* ones, play an important role in determining the variations in bond lengths on going from C to Ge.