Unimolecular decay of energy-selected dimethylformamide cations: a combined molecular orbital and RRKM analysis

Abstract

The unimolecular dissociation chemistry of N,N-dimethylformamide cations, (CH[sub 3])[sub 2]NCHO[sup +], has been investigated by photoelectron-photoion coincidence in the energy range 11.11-11.49 eV. The measured rates for CHO and CH[sub 3] loss were modeled with RRKM statistical theory. The vibrational frequencies for both the molecular ion and two transition states were obtained from ab initio MO calculations so that the only adjustable parameters in the RRKM analysis were the activation energies. This analysis shows that these apparently simple dissociation reactions proceed via substantial reverse activation energies and tight transition states. Ab initio MO calculations (3-21G* basis set) of the transition states suggest that these reactions proceed via an intramolecular (not associated with the expelled radical) hydrogen atom transfer to produce the CH[sub 3]NHCH[sub 2][sup +] and CH[sub 3]NHCO[sup +] product ions, rather than (CH[sub 3])[sub 2]N[sup +] and CH[sub 3]NCHO[sup +] expected from simple loss of CHO and CH[sub 3]. The measured kinetic energy release was found to be only slightly greater that that predicted by statistical theory. 27 refs., 6 figs., 2 tabs.