Characterization of [M–H] cations, radicals and anions of glycine in the gas phase: a combined experimental and ab initio study

Abstract

The gas phase structures of the [M–H] cations and anions of glycine have been studied by using a combination of ab initio calculations (at the MP2(FC)/6–31+G∗ level of theory) and tandem mass spectrometry (MS/MS). It was found that the ab initio stability order for the anions is [H 2NCH 2CO 2] − > [H 2NCHCO 2H] − > [HNCH 2CO 2H] −. In contrast, the cations exhibit different behaviour, whereas [H 2NCHCO 2H] + is predicted to be a stable structure, [H 2NCH 2CO 2] + spontaneously fragments to the ion–molecule complex [H 2NCH 2 + ⋯ (OCO)] and the singlet [HNCH 2CO 2H] + isomer is predicted to undergo a skeletal rearrangement to form [CH 2NHCO 2H] +. MS/MS spectra of [M–H] + cations of various glycine isotopomers were obtained via: (i) collisional activation of electron impact generated cations and (ii) charge reversal of anions formed via HO − negative ion chemical ionization. The resulting spectra were significantly different, suggesting different structures were involved. Neutralization–reionization experiments were performed on [M–H] − anions in order to gain insights into the structures of the intermediate radicals.