Gas phase protonation of alkyl and phenyl azides

Gas-phase protonation of azides has been investigated with mass spectrometric and computational techniques. The proton affinities of MeN3, EtN3 and PhN3, derived from bracketing experiments, are respectively 199±3, 210±5, and 196±3 kcal mol−1. The structures and the energetics of MeN3 and MeN3H+ have been investigated by ab initio MO techniques, a proton affinity of MeN3 of 195.3 kcal mol−1 at the 6-31G **//6-31G* level of theory being obtained, in agreement with the experimental value. The loss of N2 from MeN3H+, yielding the MeNH+ nitrenium ion, has also been theoretically estimated (by ab initio techniques) to be endothermic by ca. 15 kcal mol−1, again in accord with experimental observations. In fact, MeN3H+ is fairly stable, being the base peak in the CH4 chemical ionization spectrum of MeN3, while PhN3H+ is but a minor peak in the CH4 chemical ionization spectrum of PhN3, undergoing extensive N2 loss. Collisional Activation (CA) mass spectrometry of the charged fragment formed upon N2 loss from CH3N3H+ suggests rearrangement to CH2=NH+2, while the PhNH+ fragment retains the nitrenium-ion structure, reacting with benzene to give a C12H12N+ derivative whose CA spectrum is closely similar to that of protonated diphenylamine.