Theoretical pKa estimates for solvated P(OH)5 from coordination constrained Car–Parrinello molecular dynamics

Abstract

Car–Parrinello molecular dynamics simulations have been performed on P(OH)₅ and its monanionic and dianionic derivatives in liquid water, representing minimal models of the transition complex occurring in the hydrolysis of RNA. P(OH)₅ and PO₅H₄⁻ have been found to be stable in aqueous solution, while PO₅H₃²⁻ is reduced to PO₅H₄⁻ within femtoseconds by solvent–solute proton transfer. The deprotonated hydroxyl group of the monoanion exhibits pronounced equatoriphilicity. This observation has been underlined by theoretical pK_a determination of axial and equatorial hydroxyl groups of P(OH)₅, respectively, by means of coordination constrained ab initio molecular dynamics, which has shown the equatorial site to be more acidic by 4.4 units, the absolute values being 14.2 and 9.8.