A comparison of semi-empirical and ab initio SCF-MO potential energy surfaces for the reaction of H2CO with R3PCH2 and RPCH2

Abstract

The reaction between HPCH₂ and H₂CO is predicted at the MNDO, PM3 and 3-21G*ab initio SCF-MO levels to proceed via a non-planar, unsymmetrical and relatively high energy transition state corresponding to a distorted _π2_s+_π2_a cycloaddition, in contrast to the planar, symmetrical and lower energy saddle point located for the allowed cycloaddition between H₃PCH₂ and H₂CO. The ‘allowed’ character of the latter reaction is attributed to contributions from the P–H bonds and not to asymmetry in the reactant orbitals or to ionic character in the wavefunction. The methyl analogue Me₃PCH₂ reveals a less synchronous transition state for cycloaddition, with a significant orbital contribution from the axial P–Me bond in particular.