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 πnbπ* absorption evidences three competing dissociation channels: HCON(CH3)2→HCO(X̃ 2A′)+N(CH3)2(X̃ 2B1); HCO(X̃ 2A′)+N(CH3)2(Ã 2A1); and HCONCH3+CH3. (H atom eliminations are not probed.) These products are formed in a ratio of 0.15±0.04:0.49±0.09:0.36±0.07, 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 β=1.2±0.2. 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)2+HCO channel. Competition of nitrogen–methyl bond fission is evidence of the strong coupling between the πnbπ* excitation and the nitrogen–methyl reaction coordinate; ab initio calculations confirm that the electronic excitation is not localized on the N–C=O moiety. We also include here an advance report of the excited state energy of the N(CH3)2(Ã 2A1) radical, which is found to be 1.59 eV.