Gas-phase reactivity of hydroxylamine. A mass spectrometric and computational study of the protonation and methylation of NH2OH.

The gas-phase proton affinity (PA) of HzNOH evaluated by the FT-ICR “bracketing” technique and by the kinetic method based on the unimolecular fragmentation of proton-bound adducts amounts to 193.7 f 2 kcal mol-’ at 298 K, in excellent agreement with the 193.8 kcal mol-’ PA computed at the GAUSSIAN 1 level of theory for the N atom, consistent with the view that N protonation is energetically favored. The computed PAS of the N atom of CH3NHOH and HzNOCH3, i.e. 203.9 and 201.0 kcal mol-’, respectively, agree also with the values from FT-ICR equilibrium measurements, 205.1 f 2 and 202.7 f 2 kcal mol-’, respectively, whereas the PAS calculated for the 0 atom are much lower, 175.1 and 179.8 kcal mol-’, respectively. Methylation of HzNOH by (CH&Cl+ ions gives a charged product whose structure, assayed by MIKE and CAD spectrometry, indicates predominant, if not exclusive, N alkylation. The relative reactivities of the N and the 0 atom of HzNOH in gas-phase protonation and methylation are compared, and the observed site selectivity of (CH3)2C1+ is discussed.