Reactions of O −· with methyl benzoate: a negative ion chemical ionization and Fourier transform ion cyclotron resonance study

Abstract

The reactions of O −· with methyl benzoate have been examined by the measurement of negative ion chemical ionization (NICI) mass spectra using a CI source, with confirmatory studies carried out on a Fourier transform ion cyclotron resonance mass spectrometer. Reaction mechanisms have been elucidated using isotopically labeled esters. Nucleophilic attack at the carbonyl carbon and the aromatic ring were important reaction pathways. Nucleophilic attack at the carbonyl carbon was followed by the production of products (C 6H 5CO 2 − and CH 3OCO 2 −) characteristic of radical, β-fragmentation. Using 18O-labeled methyl benzoate, the S N2 reaction was found to account for a smaller percentage, 21(±1)%, of the benzoate product. Aromatic ring attack resulted in formation of [M + O − H] − and [M − 2H] −· ions. Although aryl hydrogens accounted for most H 2 +· abstracted by O −·, evidence for abstraction of H arylH alkyl +· and H alkylH alkyl +· was also found. Although present at much lower abundance, dehydrobenzoate, dehydrophenoxy, and C 7H 6 −· ([M − 2H − CO 2] −·) radical anions were also observed. An H aryl/H alkyl exchange associated with formation of the benzoate anion was attributed to an H alkyl abstraction that occurred within the methanol/dehydrobenzoate ion–dipole complex. The [M − 2H] −·, dehydrobenzoate, dehydrophenoxy, and [M − 2H − CO 2] −· ion signals were quenched by reaction with O 2. Conditions required for production of O −· spectra under NICI conditions were also examined.