HOMOLYTIC SUBSTITUTION AT PHOSPHORUS - AN AB-INITIO STUDY OF THE REACTION OF HYDROGEN-ATOM AND METHYL RADICAL WITH PHOSPHINE AND METHYLPHOSPHINE

Homolytic substitution reactions of hydrogen atom and methyl radical a t the phosphorus atom in phosphine and methylphosphine have been exami ned by high-level ab initio molecular orbital theory. MP4SDTQ/6-31G(* )//MP2(FC)/6-31G(*) calculations predict that free-radical attack at the phosphorus atom in phosphines is facile, with energy barriers of 1 4-33 kJ mol(-1) and likely to involve hypervalent phosphoranyl radical intermediates. These intermediates, in turn, are found to have dissoc iative energy barriers of 10-31 kJ mol(-1), depending on leaving group , and are unlikely to undergo pseudorotation prior to dissociation. MP 5/6-31G(*)//MP2/6-31G(**) calculations indicate that permutational is omerism of phosphoranyl radical is likely to involve barriers of 145 a nd 127 kJ mol(-1) for mechanisms involving transition states of D-4h a nd C-4v symmetry respectively.