The Vibrations of Protons in Minerals

Abstract

The disappearingly small proton radius (∼10⁻⁵ Å) imparts peculiar properties to the hydrogen atom. Even the most polar A-H bonds (A is an electronegative atom) are mainly covalent. However, in interactions between an acid group A-H and a basic atom B, a "hydrogen bond" A-H…B is formed in which the hydrogen atom plays the role of an electron acceptor. Current concepts concerning the nature of H-bonds and the influence they exert on the structure and physical properties of substances have been discussed in a number of reviews and monographs (Sokolov, 1964; Pimentel and McClellan, 1960; Hamilton and Ibers, 1968; et al.). For this reason, we shall restrict ourselves to a few remarks on the mechanism of H-bond formation in inorganic compounds. It is known that for the formation of a strong H-bond A-H…B, the atom B must possess an unshared electron pair on an extended hybrid orbital. It is necessary also that the proton screening by the unshared electron pairs of the A atom be sufficiently weak.¹ A schematic representation of the two most frequently observed mechanisms of increasing the proton-donor ability of the A-H group may be given as follows: Variant I corresponds to the case when the unshared electrons of atom A are accepted by another proton-donor molecule A'-H. Variant 11, more effective under certain conditions, corresponds to the case when one or several X atoms connected with the A-H group possess vacant orbitals which are well overlapped by the orbitals of