An Ab initio study of the diadic prototropic tautomerism H3PX⇄H2PXH (X = O, NH, CH2)

Abstract

The diadic prototropci tautomerisms of three model systems H₃PX⇄H₂PXH (X = O, NH, CH₂) are investigated by ab initio methods. Stationary points corresponding to local minima and transition states are located using gradient techniques at HF/6-31G** level, and relative energies calculated at MP4/6-31G** including a zero-point energy (ZPE) correction, using the optimised geometries. Computation of vibrational frequencies shows a correlation between the norm of the transition-state imaginary frequencies and both the free energy of activation for hydrogen migration and the H_m–P–X angle. The intrinsic barriers ΔG_o^‡ calculated by the Marcus theory indicate that all three unimolecular hydrogen transfers belong to a homogenous series. The disparity between the calculated (gas-phase) stabilities and experimental observations based on solution studies is noted. However, it is shown that consideration of the computed net charges dipole moments and electric field gradients of the isolated species, in conjunction with a simple perturbation expression for the solvation energy, allows a successful rationalisation of tautomeric ratios measured in solution.