Hypervalent ammonium radicals. Competitive N-C and N-H bond dissociations in methyl ammonium and ethyl ammonium.

The title hypervalent ammonium radicals were investigated by neutralization-reionization mass spectrometry and quantum chemical calculations. Methyl ammonium (1) forms a small fraction of metastable radicals from isotopomers CH3ND3 (la) and CD3NH3 (1b) when these are produced by femtosecond electron transfer to vibrationally excited precursor cations. The branching ratios for dissociations of the N-C and N-(H,D) bonds in 1 favor the latter, k(N-C)/k(N-H) = 0.39. The experimental results are in accord with ab initio/RRKM calculations that quantitatively reproduce the branching ratios for dissociations of 1. A small fraction of high-energy 1 dissociates to form ammonium methylide, -CH2NH3+. Ethyl ammonium (2) and its CH3CH2ND3 isotopomer (2a) dissociate completely on the microsecond time scale. The branching ratios for dissociations of the N-C and N-(H,D) bonds favor the former, k(N-C)/k(N-H) = 2.04. This result is incompatible with the calculated potential energy surface of the ground doublet electronic state in 2 and is attributed to the formation and dissociations of excited electronic states.