STRUCTURE AND RING CLEAVAGES OF THE PHOSPHIRANE CATION - A THEORETICAL ABINITIO SCF-CI STUDY

The structure of phosphirane cation 1 (PC2H4+) and the relative stabilities of 1 and some isomeric structures have been investigated by using ab initio SCF-CI calculations. The C-C disrotatory ring opening giving bis(methylene)phosphorane cation 2 has a 15.2 kcal.mol-1 activation energy, and 2 is 17.5 kcal.mol-1 more stable than 1. Ethylene plus P+, in their ground states, lie 60.7 kcal.mol-1 above 1. The lowest vertical excitation energies of 1 and 2 have been calculated. The linear phosphallene cation CH2=P=CH2+ and CH2CH2P+ cation are not minima on the potential energy surfaces (PES). However, a C2v triplet exciplex having rather long P-C bonds is found only 7.9 kcal.mol-1 above 1. The preceding data together with semiquantitative calculated PESs provide a background for a general discussion of the thermal and photochemical reactivity of cation 1.