The Dimethylamine Dimer Ion Is an Ion−Radical Complex:  A Combined TPEPICO, Variational RRKM, and <i>ab Initio</i> MO Study of the Fragmentation of Ionized Dimers of Dimethylamine

Threshold photoelectron photoion coincidence spectroscopy has been used to study the fragmentation mechanism of dimethylamine dimer ions. The fragmentation rate constant data obtained from the experiment were modeled with variational RRKM theory using potential energies and vibrational frequencies from ab initio molecular orbital calculations at the MP2/6-31G* level. The results are consistent with the fragmentation of the dimethylamine dimer ion into protonated dimethylamine and the •CH2N(H)CH3 radical. This was supported by ab initio calculations in which the dimer ion was found to consist of a N−H−C hydrogen-bonded complex between the above two products. The RRKM fit to the experimental k(E) vs E data for the ion gave a dimethylamine dimer ion ΔfH°0 of 653 ± 11 kJ mol-1 and thus a dimer ion binding energy of 147 ± 16 kJ mol-1. In agreement with recent experimental results, the neutral dimethylamine dimer was calculated to be an N−H−N hydrogen-bonded dimer with a 0 K binding energy of 13 ± 3 kJ mol-1. The resulting neutral dimer adiabatic ionization energy is 6.8 ± 0.2 eV.