Dissociative Photoionization of X(CH3)3 (X = N, P, As, Sb, Bi): Mechanism, Trends, and Accurate Energetics

Abstract

Threshold photoelectron photoion coincidence spectroscopy is used to study the dissociation of energy-selected X(CH(3))(3)(+) ions (X = As, Sb, Bi) by methyl loss, the only process observed up to 2 eV above the ionization energy. The ion time-of-flight distributions and the breakdown diagrams are analyzed in terms of the statistical RRKM theory to obtain accurate ionic dissociation energies. These experiments complement previous studies on analogous trimethyl compounds of the N group where X = N and P. However, trimethylamine was observed to lose only an H atom, whereas trimethylphosphine was shown to lose methyl radical, H atom, and, to a lesser extent, methane in parallel dissociation reactions. Both kinetic and thermodynamic arguments are needed to explain these trends. The methyl radical loss has two channels: either a H transfer to the central atom, followed by CH(3) loss, or a direct homolytic bond cleavage. However, the H transfer channel is blocked in trimethylamine by an H loss channel with an earlier onset, and, thus, the methyl loss is not observed. Bond energies are defined based on ab initio reaction energies and show that the main thermodynamic reason behind the trends in the energetics is the significantly weakening C=X double bond in the ion in the N --> As direction. The first adiabatic ionization energies of Sb(CH(3))(3) and Bi(CH(3))(3) have also been measured by ultraviolet photoelectron spectroscopy to be 8.02 +/- 0.05 and 8.08 +/- 0.05 eV, respectively.