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 (C₆H₅CO₂⁻ and CH₃OCO₂⁻) characteristic of radical, δ-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 H₂^+. 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 C₇H₆^−. ([M − 2H − CO₂]^−.) 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 − CO₂]^−. ion signals were quenched by reaction with O₂. Conditions required for production of O^−. spectra under NICI conditions were also examined.