O–H stretching vibrations in ice clathrate

Abstract

In H-bonded materials, the frequency of intramolecular vibrations is considered to depend only on the strength of the H-bond, or equivalently, the distance between the H-bonded molecular pairs. Therefore, in vibrational spectroscopic studies of the molecular structure of condensed phases with O–H⋯O bonds, the frequency of O–H stretching vibrations, ν, is generally taken as being directly proportional to the average distance, Ro–o, between the nearest oxygen atoms. Thus (∂ν/∂Ro–o) obtained from a study of various polymorphs of ice (Ro–o range, 2.75–2.83 A) has been used as a scaling factor to suggest: (1) a continuous random network structure of amorphous solid and liquid water1,2, and (2) the possibility of transformation of ice VII to a structure with centrosymmetric hydrogen bonds3. While such scaling of Ro–o from changes in ν may seem a good approximation, studies of hexagonal ice and ice clathrate suggest that factors other than the intermolecular distances affect the frequency of O–H stretching vibrations. A study of the Raman spectrum of ice clathrate, reported here, shows that in condensed phases where the O–H⋯O bonds are nonlinear and/or the intermolecular distances are distributed over a range (as in water, various amorphous and crystalline forms of ice, ice-like structures, alcohols, carboxylic acids and aqueous solutions), a treatment based on such scaling is an oversimplification; the geometry of the H-bond affects the frequency more than the strength of the H-bond.