The Nature of the Silicon–Oxygen Bond

Even though siloxanes form the basis for the entire worldwide silicone industry, the nature of silicon–oxygen bonding has not been well understood. In the present work we employ correlated and highly polarized basis levels and natural bond orbital techniques to analyze the anomalous structural and basicity properties of inorganic Si–O–Si linkages compared to organic C–O–C congeners. We find that vicinal hyperconjugative interactions of nO → σ*XH type are primarily responsible for promoting the increased torsional flexibility and reduced basicity that strongly distinguish disiloxanes from ethers. Paradoxically, the key to enhanced siloxane hyperconjugation lies in the inherently larger X–O–X bond angle for X = Si (consistent with electronegativity and hybridization variations predicted by Bent’s rule), even though such resonance-type corrections are generally stronger in ethers (X = C) at any particular X–O–X angle. These findings have important implications for many aspects of silicone chemistry as well as for improved general understanding of basicity and H-bonding phenomena in terms of resonance-type donor–acceptor (rather than “electrostatic”) concepts.