Electronic accessibility of dissociation channels in an amide: N,N-dimethylformamide photodissociation at 193 nm

Measurement of the photofragment velocity and angular distributions from the photodissociation of N,N-dimethylformamide at 193 nm in its pnbp* absorption evidences three competing dissociation channels: HCON~CH3!2→HCO(X̃ A8)1N~CH3!2(X̃ B1); HCO(X̃ A8)1N~CH3!2(Ã A1); and HCONCH31CH3. ~H atom eliminations are not probed.! These products are formed in a ratio of 0.1560.04:0.4960.09:0.3660.07, determined by use of trimethylamine as a calibrant molecule. Nitrogen–carbonyl bond fission occurs on a rapid time scale with an angular distribution of the dissociation products given by b51.260.2. Excited state N~CH3!2 products are formed quasidiabatically from the initial planar geometry, whereas symmetry-breaking vibrations allow one-electron matrix elements to couple the initial electronic configuration to the ground state N~CH3!21HCO channel. Competition of nitrogen–methyl bond fission is evidence of the strong coupling between the pnbp* excitation and the nitrogen–methyl reaction coordinate; ab initio calculations confirm that the electronic excitation is not localized on the N–C5O moiety. We also include here an advance report of the excited state energy of the N~CH3!2(Ã A1) radical, which is found to be 1.59 eV. © 1999 American Institute of Physics. @S0021-9606~99!01218-0#