Pyridine N ‐oxide and pyridine‐ d 5 N ‐oxide: an electrospray/tandem mass spectrometric study carried out at high mass resolution

A mass spectrometric study of pyridine N ‐oxide and pyridine‐ d 5 N ‐oxide was carried out with a hybrid quadrupole/time‐of‐flight (TOF) mass spectrometer coupled with an electrospray (ES) source. In addition to the observation of protonated, sodiated, and proton‐bound dimers of pyridine N ‐oxide and pyridine‐ d 5 N ‐oxide, mass scans revealed the presence of several doubly‐charged ion species. Doubly‐charged ions of m/z 191 were identified as diprotonated tetramers of pyridine N ‐oxide; a structure has been proposed for the diprotonated tetramer and its energy relative to that of protonated pyridine N ‐oxide has been obtained from geometry optimizations. The principal ion species observed were subjected to collision‐induced dissociation; accurate mass measurements were made of each fragment ion so as to determine its elemental composition. On the basis of mass spectrometric evidence, it is suggested that dissociation of pyridine N ‐oxide may occur during the ES process and the resulting fragments become embedded in doubly‐charged ions. The proton affinity for both pyridine N ‐oxide and pyridine‐ d 5 N ‐oxide was calculated; the difference between these proton affinities was compared with an experimentally determined difference between the proton affinities of pyridine N ‐oxide and pyridine‐ d 5 N ‐oxide. Copyright © 2005 John Wiley & Sons, Ltd.