Thermally induced intramolecular oxygen migration of N-oxides in atmospheric pressure chemical ionization mass spectrometry.

N-Oxides are known to undergo three main thermal degradation reactions, namely deoxygenation, Cope elimination (for N-oxides containing a β-hydrogen) and Meisenheimer rearrangement, in atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The ions corresponding to these thermal degradants observed in the ensuing APCI mass spectra have been used to identify N-oxides as well as to determine the N-oxidation site when the analyte contains multiple tertiary amine groups. In this paper, we report a thermally induced oxygen migration from one N-oxide amine to another tert-amine group present in the same molecule through a six-membered ring transition state during APCI-MS analysis. The observed intramolecular oxygen migration resulted in the formation of a new isomeric N-oxide, rendering the results of the APCI-MS analysis more difficult to interpret and potentially misleading. In addition, we observed novel degradation behavior that happened after the Meisenheimer rearrangement of the newly formed N-oxide: a homolytic cleavage of the N-O bond instead of elimination of an aldehyde or a ketone that usually follows the rearrangement. Understanding of these unusual degradation pathways, which have not been reported previously, should facilitate structural elucidation of N-oxides using APCI-MS analysis.