A theoretical analysis of the gas-phase protonation of hydroxylamine, methyl-derivatives and aliphatic amino acids

Hydroxylamines and aliphatic amino acids present two active sites towards electrophilic attack by a proton, namely, the amino nitrogen and hydroxylic oxygen atoms. Site reactivity may be described in terms of local descriptors of chemical reactivity defined in the context of the density functional theory formulation of Parr, Pearson and Yang (DFT–PPY). We report a DFT–PPY analysis of site reactivity in hydroxylamine, methyl-derivatives of hydroxylamine and the aliphatic amino acids glycine, alanine and valine, using a simple formulation of the regional or condensed-to-atom k (fk−) Fukui function. A satisfactory qualitative correlation between this local descriptor of site reactivity and the experimental proton affinities is found. The nitrogen center appears as the preferential site of protonation. Enhancement of the site basicity at the nitrogens may be probed by variations of the Fukui function ΔfN−, with reference to the ammonia molecule; the correlation between proton affinity and the Fukui function difference is closely related to a local hard and soft acids and bases principle.