Basicity of siloxanes,alkoxysilanes and ethers toward hydrogen bonding

Thermodynamic constants have been determined for hydrogen bonding of phenol to several siloxanes and alkoxysilanes, as well as to Me₃SiSCMe₃ and Me₃SiSSiMe₃. Alkoxysilanes are slightly weaker bases toward phenol than the isostructural ethers. Unstrained siloxanes have low values of △H for hydrogen bonding of 3.5–3.9 kcal/mol and are therefore distincatly weaker bases than either alkoxysilanes or ethers. Siloxanes with small SiOSi bond angles show increased basicity. Me₃SiSCMe₃ (△H 1.9 kcal/mol) and Me₃SiSSiMe₃ (△ 0.8 kcal/mol) are markedly less basic than dialkyl sulfides. The results are discussed using a molecular orbital model.