title: Thermally induced intramolecular oxygen migration of N-oxides in atmospheric pressure chemical ionization mass spectrometry.
abstract: 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.
authors.affiliation: ACDS-Supply Analytical Sciences, Merck & Co., Inc., 1011 Morris Avenue, Union, NJ 07083, USA.
authors.forename: Xin
authors.initials: X
authors.lastname: Wang
authors.forename: Min
authors.initials: M
authors.lastname: Li
authors.forename: Abu M
authors.initials: AM
authors.lastname: Rustum
mesh_terms: D001274:Atmospheric Pressure; D006358:Hot Temperature; D013058:Mass Spectrometry / Q000379:methods*; D008968:Molecular Conformation; D009589:Nitrogen Oxides / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D010546:Perphenazine / Q000031:analogs & derivatives / Q000737:chemistry
chemical_list: D009589:Nitrogen Oxides; D010546:Perphenazine; D010100:Oxygen