Can transacylation reactions occur via S(N)2 pathways in the gas phase? Insights via ion-molecule reactions of N-acylpyridinium ions and ab initio calculations

[reaction: see text]The gas-phase identity exchange reactions of N-acylpyridinium ions with pyridine have been examined experimentally in an ion trap mass spectrometer through the use of isotope labeling experiments. The nature of the acyl group plays a crucial role, with the bimolecular rates following the order acetyl > benzoyl > N,N-dimethylaminocarbamyl. The experimental results correlate with ab initio calculations on the simple model system RC(O)NH3+ + NH3, which also demonstrates that these are "SN2 like" processes.