Internal Chemical Bonding in Solutions of Simple Phosphates and Vanadates

The chemical bonding within structurally related phosphates and vanadates in aqueous solution is compared on the basis of vibrational frequencies obtained by classical Raman spectroscopy. To do this, an empirical relationship between the stretching frequency of [formula omited] and [formula omited] or [formula omited] groups and bond strength is developed such that the sum of the PO bond strengths, expressed in terms of average number of electron pairs per bond, is as close as possible to 5.0 for phosphoric acid and various anions and esters thereof. The same approach is used for the corresponding vanadates. The internal bonding in phosphates involves a greater bond strength for [formula omited] and a smaller strength for [formula omited] than might be expected from a simple consideration of canonical resonance forms. In vanadates, [formula omited] and [formula omited] are closer to single and double bonds, respectively, than in phosphates, and the force constant for V=O is considerably smaller than for P=O, although that for V—OH and P—OH is similar. Since treating the [formula omited] and [formula omited] groups of simple tetrahedral phosphates and vanadates as independent diatomic oscillators provides good correlations between the respective frequencies and bond strengths, the same correlations are used to approximate the expected stretching frequencies for distorted phosphates and vanadates. The distortions considered are those that presumably characterize associative and dissociative transition states for a concerted transfer of the (PO₃⁻) fragment of a dianionic phosphate group between donor and acceptor oxygens with similar character. The results are compared with measured frequencies for ground-state and transition-state analog complexes of phosphoglucomutase in the accompanying paper [Deng, H., Ray, W. J., Jr., Burgner, J. W., II, & Callender, R. (1993) Biochemistry (following paper in this issue)].