Gas-phase Reactions of Glycine, Alanine, Valine and their N-Methyl Derivatives with the Nitrosonium Ion, NO

The gas‐phase reactions of the nitrosonium ion, NO+ with the amino acids glycine, alanine and valine and their N‐methyl derivatives were investigated under chemical ionization mass spectrometric (CIMS) conditions. Two products were observed in all cases: the formation of the iminium ion and the formation of an [M‐H]+ ion. The latter product is consistent with a reaction channel involving hydride ion by NO+, and was confirmed by (i) examining the Ar+CI mass spectra of the same amino acids under similar source conditions and (ii) examining the unimolecular fragmentation reactions of the [M+H]+ ions of the N‐nitroso‐N‐methyl derivatives of each of the amino acids in a tandem mass spectrometer. Further insights into the reaction of glycine with NO+ were obtained by performing ab initio calculations (at the MP2/6–31G*//HF/6–31G* level). These results indicate that four reactions are thermodynamically viable for glycine: (i) hydride ion; (ii) iminium ion formation (with concomitant loss of HONO and CO); (iii) diazonium ion formation; and (iv) diazonium ion formation followed by loss of N2. Possible reasons why reactions (iii) and (iv) are not observed are discussed, and comparisons with solution reactivity and the gas‐phase reactivity of NO+ are also made.