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, Δ_fH^o_298K [CH₃NH₂(g)] = −21.8 ± 1.5 kJ mol^-1, Δ_fH^o_298K[C₂H₅NH₂(g)] = −50.1 ± 1.5 kJ mol^-1, Δ_fH^o_298K[C₃H₇NH₂(g)] = −70.8 ± 1.5 kJ mol^-1, Δ_fH^o_298K[C₃H₇^•] = 101.3 ± 1 kJ mol^-1, and Δ_fH^o_298K[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, Δ_fH^o_298K[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 Δ_fH^o_298K[CH₂NH₂⁺] = 750.3 ± 1 kJ mol^-1.