Dissociative photoionization of mono-, di- and trimethylamine studied by a combined threshold photoelectron photoion coincidence spectroscopy and computational approach

Abstract

Energy selected mono-, di- and trimethylamine ions were prepared by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Below 13 eV, the main dissociative photoionization path of these molecules is hydrogen atom loss. The ion time-of-flight (TOF) distributions and breakdown diagrams for H loss are analyzed in terms of the statistical RRKM theory, which includes tunneling. Experimental evidence, supported by quantum chemical calculations, indicates that the reverse barrier along the H loss potential energy curve for monomethylamine is 1.8 ± 0.6 kJ mol⁻¹. Accurate dissociation onset energies are derived from the TOF simulation, and from this analysis we conclude that Δ_fH°_298K[CH₂NH₂⁺] = 750.4 ± 1.3 kJ mol⁻¹ and Δ_fH°_298K[CH₂NH(CH₃)⁺] = 710.9 ± 2.8 kJ mol⁻¹. Quantum chemical calculations at the G3, G3B3, CBS-APNO and W1U levels are extensively used to support the experimental data. The comparison between experimental and ab initio isodesmic reaction heats also suggests that Δ_fH°_298K[N(CH₃)₃] = –27.2 ± 2 kJ mol⁻¹, and that the dimethylamine ionization energy is 8.32 ± 0.03 eV, both of which are in slight disagreement with previous experimental values. Above 13 eV photon energy, additional dissociation channels appear besides the H atom loss, such as a sequential C₂H₄ loss from trimethylamine for which a dissociation mechanism is proposed.