Generation and characterization of distonic dehydrophenoxide radical anions under electrospray and atmospheric pressure chemical ionizations

We have explored the possibilities of generating radical anions under electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) conditions. By using different sets of ortho-, meta-, and para-isomers of nitrobenzoic acids, methylphenols, and nitrophenols, and m-, and p-isomers of hydroxybenzaldehydes and hydroxyacetophenones as the precursor molecules, we have successfully generated the isomeric distonic dehydrophenoxide radical anions (m/z 92) using the ESI process by applying relatively high capillary voltages, the in-source dissociation (ISD) condition. Under the same conditions, the o-hydroxybenzaldehyde and the o-hydroxyacetophenone yielded the even-electron dehydrophenoxide anion (m/z 93) due to the well-known ortho-effect. The distonic phenoxide radical anions at m/z 92 were also generated under APCI-ISD conditions by using m- and p-isomers of nitrobenzaldehydes and nitroacetophenones. While the o-nitrobenzaldehyde and the o-nitroacetophenone mainly yielded the phenoxide anion at m/z 93, due to the ortho-effect. The collision-induced dissociation (CID) experiments of all the anionic precursor molecules formed from either ESI or APCI produced comparable mass spectra as those observed in the ESI-ISD or the APCI-ISD experiments. The radical anions at m/z 92 reacted with CO2 and O2 to form the CO2 adduct and the oxygen atom abstraction product, respectively, revealing the dual-character of the distonic radical anions, the phenide ion and the phenyl radical. Computational studies support the results of the ion-molecule reactions.

Figure optionsDownload high-quality image (123 K)Download as PowerPoint slideResearch highlights▶ Distonic phenoxide radical anions at m/z 92 are generated via ESI or APCI process. ▶ Substituted aromatic compounds are suitable precursor molecules. ▶ CID experiments and ion-molecule reactions are performed. ▶ The radical anions exhibit dual characteristics, phenide ion and phenoxyl radical. ▶ Certain ortho-substituted compounds produce the phenoxide anion at m/z 93.