An ab initio study of the comparative stabilities of the simple phosphine oxide tautomers

High level ab initio quantum mechanical model chemistries have been used to study the relative stabilities of the simple phosphine oxides tautomers OPH3/PH2OH and OPH(OH)2/P(OH)3 and related molecules. It is found that the two PH2OH isomers are virtually equi-energetic and only slightly more stable than OPH3 itself, and the oxides with one or more hydroxyl groups are significantly more stable than their hydroxide tautomers. The difference in stability arises mainly from the variation of the P=O bond energy among the various species, the P=O bond energy increasing with the number of hydroxyl groups present. The effects of solvation in aqueous solution are estimated by reaction field polarized continuum models and by explicit calculations of OPH3 and PH2OH hydrogen bound to a single water molecule. Both approaches indicate that the oxide form is significantly stabilized by its aqueous environment.