Gas-Phase Heats of Formation for Alkylimmonium Cations by Photoionization Mass Spectrometry

Photoionization mass spectrometry has been used to measure appearance energies for immonium cation formation from 25 alkyl amine precursors. A number of the unimolecular fragmentation processes are shown to involve excess energy at threshold so that, of the 11 different cations investigated, it is only possible to derive reliable 298 K heats of formation for CH2NH2 + (749.0 ± 0.9 kJ mol-1), CH3CHNH2 + (666.1 ± 1.1 kJ mol-1), C2H5CHNH2 + (636.8 ± 2.5 kJ mol-1), CH2NH(CH3)+ (706.1 ± 1.0 kJ mol-1), CH2NH(C2H5)+ (668.4 ± 1.3 kJ mol-1), and CH2N(CH3)2 + (668.0 ± 2.5 kJ mol-1). When these are compared to those calculated by the G3, G3B3, G2, G2(MP2), CBS-APNO, and W1U composite ab initio methods, it is found that the smallest mean absolute deviation of 1.2 ± 0.8 kJ mol-1 is obtained from the G2 calculations.