Photoion Photoelectron Coincidence Spectroscopy of Primary Amines RCH2NH2 (R = H, CH3, C2H5, C3H7, i-C3H7):  Alkylamine and Alkyl Radical Heats of Formation by Isodesmic Reaction Networks

Alkylamines (RCH2NH2, R = H, CH3, C2H5, C3H7, i-C3H7) have been investigated by dissociative photoionization by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). The 0 K dissociation limits (9.754 ± 0.008, 9.721 ± 0.008, 9.702 ± 0.012, and 9.668 ± 0.012 eV for R = CH3, C2H5, C3H7, i-C3H7, respectively) have been determined by preparing energy-selected ions and collecting the fractional abundances of parent and daughter ions. All alkylamine cations produce the methylenimmonium ion, CH2NH2+, and the corresponding alkyl free radical. Two isodesmic reaction networks have also been constructed. The first one consists of the alkylamine parent molecules, and the other of the alkyl radical photofragments. Reaction heats within the isodesmic networks have been calculated at the CBS-APNO and W1U levels of theory. The two networks are connected by the TPEPICO dissociation energies. The heats of formation of the amines and the alkyl free radicals are then obtained by a modified least-squares fit to minimize the discrepancy between the TPEPICO and the ab initio values. The analysis of the fit reveals that the previous experimental heats of formation are largely accurate, but certain revisions are suggested. Thus, ΔfHo298K [CH3NH2(g)] = −21.8 ± 1.5 kJ mol-1, ΔfHo298K[C2H5NH2(g)] = −50.1 ± 1.5 kJ mol-1, ΔfHo298K[C3H7NH2(g)] = −70.8 ± 1.5 kJ mol-1, ΔfHo298K[C3H7] = 101.3 ± 1 kJ mol-1, and ΔfHo298K[i-C3H7] = 88.5 ± 1 kJ mol-1. The TPEPICO and the ab initio results for butylamine do not agree within 1 kJ mol-1; therefore, no new heat of formation is proposed for butylamine. It is nevertheless indicated that the previous experimental heats of formation of methylamine, propylamine, butylamine, and isobutylamine may have been systematically underestimated. On the other hand, the error in the ethyl radical heat of formation is found to be overestimated and can be decreased to ± 1 kJ mol-1; thus, ΔfHo298K[C2H5·] = 120.7 ± 1 kJ mol-1. On the basis of the data analysis, the heat of formation of the methylenimmonium ion is confirmed to be ΔfHo298K[CH2NH2+] = 750.3 ± 1 kJ mol-1.