A comparison of semi-empirical and ab initio SCF-MO potential energy surfaces for the reaction of H2C
O with R3P
CH2 and RP
CH2
Abstract
The reaction between HPCH2 and H2C
O 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 π2s+π2a cycloaddition, in contrast to the planar, symmetrical and lower energy saddle point located for the allowed cycloaddition between H3P
CH2 and H2C
O. 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 Me3P
CH2 reveals a less synchronous transition state for cycloaddition, with a significant orbital contribution from the axial P–Me bond in particular.