The origin of the decreasing basicity from monofunctional alkoxysilane to tetrafunctional alkoxysilane: A combined IR and theoretical study

• A sharp decrease in oxygen basicity was observed from Me3SiOEt to Si(OEt)4.
• NBO analysis suggest delocalization of oxygen lone pair to adjacent σ∗Si–R is the key factor.
• The relative basicity of alkylalkoxysilane was established by a combined IR and DFT study.

By a combined IR and theoretical study, the oxygen basicity was found to be sharply decreased from mono- and di-functional alkoxysilanes to tri- and tetra-functional alkoxysilanes. An analysis of the natural bond orbital interactions in Me4−nSi(OMe)n (n = 1–4) and its 1:1 hydrogen bond complex with chloroform indicate that the basicity properties of Si–O–C unit may determined by the competitive hyper-conjugations from the adjacent silicon and carbon centre. The increased contribution from the delocalization of oxygen lone pair to the adjacent Si–R anti-bonding orbitals may be the main origin for decreasing basicity for Si(OEt)4 compared to Me3SiOEt.

The basicity of mono-functional alkoxysilanes was found to be remarkably stronger than that of tetra-alkoxysilanes by a combined IR and theoretical studies. NBO analysis suggests that the basicity properties of Si–O–C unit are determined by the competitive hyper-conjugations from the adjacent silicon and carbon centre. The increased contribution from the delocalization of oxygen lone pair to the adjacent Si–R anti-bonding orbitals may be the main origin for decreasing basicity.Figure optionsDownload as PowerPoint slide