A quantum chemical study of the protonation of phenylphosphine and its halogenated derivatives

The protonation and methylation of phenylphosphine (C(6)H(5)PH(2)) and its mono-halogenated derivatives have been studied using ab initio quantum chemical calculations. Density functional theory (B3LYP) calculations using the 6-311++G(d,p) basis set consistently confirm that protonation of phenylphosphines takes place at the phosphorus atom; the C(4)-protonated phenylphosphine lying about 66 kJ mol(-1) above the P-protonated isomer. Similarly, methylation of phosphines consistently occurs at phosphorus. The proton and methyl cation affinities are estimated as follows: PA(phenylphosphine) = 863 +/- 10 kJ mol(-1) and MCA(phenylphosphine) = 515 -/+ 12 kJ mol(-1). Mono-halogen substitution appears to reduce the proton affinites by up to 20 kJ mol(-1). In this context, following P-protonation of either ameta- or a para-X-C(6)H(4)-PH(2), an elimination of the halogen X-atom under collisional activation (CA) conditions is expected to lead to a distonic radical cation, a low-energy isomer being 50 kJ mol(-1) above ionized phenylphosphine.