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

Abstract

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 (1a) 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.