Ab initiopost-Hartree-Fock study on tautomeric stabilities of phosphine oxide, phosphinous acid and their thio analogues

Abstract

The prototropic tautomerism H3PX ⇌ H2PXH (X = O, S) is investigated by ab initio quantum-mechanical calculations. The relative internal energies at 0 K computed at different levels of theory (electronic energies calculated at the MP4(SDTQ)/6-311G**//MP2/6-31G** level + zero-point energies from MP2/6-31G** vibrational frequencies) undoubtedly suggest that in a gas phase H2PSH predominates over H3PS much more distinctly than H2POH over H3PO. Temperature-dependent thermochemical contributions to the free energies calculated at T = 298·15 K do not change the conclusions concerning the stabilities of the studied species in a gas phase. The effect of the polar environment on the shift of the H3PX ⇌ H2PXH equilibria is briefly discussed.