Homolytic Substitution at Phosphorus: An Ab initio Study of the Reaction of Hydrogen Atom and Methyl Radical With Phosphine and Methylphosphine

Abstract

Homolytic substitution reactions of hydrogen atom and methyl radical at the phosphorus atom in phosphine and methylphosphine have been examined 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 14-33 kJ mol-¹ and likely to involve hypervalent phosphoranyl radical intermediates. These intermediates, in turn, are found to have dissociative energy barriers of 10-31 kJ mol-¹, depending on leaving group, and are unlikely to undergo pseudorotation prior to dissociation. MP5/6-31G**//MP2/6-31G** calculations indicate that permutational isomerism of phosphoranyl radical is likely to involve barriers of 145 and 127 kJ mol-¹ for mechanisms involving transition states of D_4h and C_4v symmetry respectively.