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

Abstract

Measurement of the photofragment velocity and angular distributions from the photodissociation of N,N-dimethylformamide at 193 nm in its nb * absorption evidences three competing dissociation channels: HCON͑CH 3 ͒ 2 →HCO(X 2 AЈ)ϩN͑CH 3 ͒ 2 (X 2 B 1); HCO(X 2 AЈ)ϩN͑CH 3 ͒ 2 (Ã 2 A 1); and HCONCH 3 ϩCH 3. ͑H atom eliminations are not probed.͒ These products are formed in a ratio of 0.15Ϯ0.04:0.49Ϯ0.09:0.36Ϯ0.07, 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 ␤ϭ1.2Ϯ0.2. Excited state N͑CH 3 ͒ 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 N͑CH 3 ͒ 2 ϩHCO channel. Competition of nitrogen-methyl bond fission is evidence of the strong coupling between the nb * excitation and the nitrogen-methyl reaction coordinate; ab initio calculations confirm that the electronic excitation is not localized on the N-CϭO moiety. We also include here an advance report of the excited state energy of the N͑CH 3 ͒ 2 (Ã 2 A 1) radical, which is found to be 1.59 eV.

Photodissociating trimethylamine at 193 nm to probe dynamics at a conical intersection and to calibrate detection efficiency of radical products

“…Such small amounts of translational energy following trimethylamine photodissociation mean formation of products with high internal energy, perhaps N͑CH 3 ͒ 2 (Ã 2 A 1 ), although Kawasaki et al considered the quantum yield of this channel to be very small. We originally undertook the present study as a means of determining an absolute branching ratio for the dissociation channels in N,N-dimethylformamide; 6 our results on trimethylamine photodissociation were, however, so different than those obtained by Kawasaki et al that we report them here.…”

“…͓There could be a slight contribution from N͑CH 3 ͒ 2 (X 2 B 1 ) at m/eϭ43, but it must be very minor in order to avoid overfitting the fast edge of the m/eϭ43 TOF spectra.͔ This cracking pattern following 200 eV electron impact ionization is in agreement with the cracking patterns observed from ground state N͑CH 3 ͒ 2 radicals produced by the photodissociation of dimethylformamide at 193 nm. 6 In recent experiments at the ALS, 14 we photodissociated trimethylamine at 193 nm, but used softer photoionization in the detection scheme. Even by ionizing close to threshold, we were unable to observe signal at m/eϭ44, implying that the N͑CH 3 ͒ 2 ϩ ion is unstable and promptly dissociates.…”

“…The CH 2 NCH 3 molecules resulting from secondary decomposition of unstable N͑CH 3 ͒ 2 * radicals exhibit the same cracking pattern as primary N͑CH 3 ͒ 2 radicals with a similar distribution of internal energy, observed following photodissociation of dimethylformamide at 193 nm. 6 Thus, it is likely that in the studies of dimethylformamide, the species assigned to primary formation of N͑CH 3 ͒ 2 (Ã 2 A 1 ) and said to arrive at the detector as intact N͑CH 3 ͒ 2 , underwent H-atom loss to give CH 2 NCH 3 . The compelling electronic arguments in Ref.…”

“…The compelling electronic arguments in Ref. 6 for this channel to arise from primary N͑CH 3 ͒ 2 (Ã 2 A 1 ) radicals lead us to consider whether this could be the case in trimethylamine. We analyze the relevant energetics in the next paragraph.…”

Ultrafast excited-state dynamics in a prototype of the peptide bond: Internal conversion of the isolated N,N-dimethylformamide

“…The early gas-phase experimental study was published by Butler and co-workers [5,6]. They found that the C-N bond cleavage proceeded along the excited-state pathway or through decay channel leading to fragments at 193 nm excitation to the second excited singlet state.…”