Hydrogen Bonding in Calcium Orthophosphates

Abstract

RECENT discussion^1,2 about the general formula of synthetic hydroxyapatites deficient in calcium has involved the nature of hydrogen bonding not only in the low calcium/phosphorus ratio defect apatites themselves but also in related stoichiometric calcium orthophosphates. In particular, the relationship between OH stretching frequency and the length of the oxygen–oxygen bridge connecting orthophosphate groups has been considered. The assignment by Stutman, Posner, and Lippincott¹ of the infra-red absorption band at 3,400 cm⁻¹ in synthetic phosphocalcic apatites to O–H … O stretching between hydrogen-bonded phosphate groups has been disputed by Winand and Dallemagne², who attribute this band to adsorbed water. We have recently re-examined three calcium orthophosphates crystallographically and from the P–O lengths and O – O separations have attempted to infer which oxygen atoms carry hydrogen atoms. These more detailed results point to some difficulties in the interpretation made by Stutman et al.¹, for example in their Table 2, which includes a comparison of a single O … O distance (derived from infra-red data) with unit-cell X-ray data for various orthophosphates.