Hydrogen bonds of extremely high polarisability between molecules adsorbed on silica gel — an IR investigation

Abstract

The adsorption on silica gel of a series of electron pair donors with a variation of the pKa value from 12 up to the range of negative values was subjected to IR spectroscopic investigation. Adsorption is effected via hydrogen bonds with the OH groups of the surface, indicated by the disappearance of the band of the free OH groups (3750 cm −1) and by the occurrence of a broad band at smaller wave numbers. The shift is roughly proportional to the pKa value. Furthermore, a continuous absorption is observed as the pKa value increases. This shows that protons become detached from the SiOH groups and hydrogen bonds of type NH +…N form between the adsorbed molecules. A double minimum potential well occurs in these hydrogen bonds, which are extremely easily polarisable. The anomalous proton conductivity to be expected sometimes in the presence of such groupings is discussed.