The unimolecular chemistry of quaternary ammonium ions and their neutral counterparts

Abstract

The quaternary ammonium ions (CH 3) 4N + ( 1 +), (CH 3CH 2) 4N + ( 2 +), C 6H 5N + (CH 3) 3( 3 +), C 6H 5CH 2N + (CH 3) 3 ( 4 +) and C 6H 5CH 2N + (CH 2CH 3) 3 ( 5 +) and the corresponding ammonium radicals 1–5 have been studied in the gas phase by metastable ion (MI) characteristics, collisionally activated dissociation (CAD) and neutralization—reionization mass spectrometry (NRMS). The unimolecular chemistry of the ammonium ions is influenced by the nature of their substituents (R). Based on cations 3 +– 5 +, which carry different types of N +R bonds, preferential dissociation at N +—benzyl > N +—alkyl > N +—phenyl takes place. The two major fragmentation channels observed for all ions are (i) direct cleavage of the N +R bond with charge retention on the species of lowest ionization energy and (ii) formation of an immonium ion by loss of the hydrocarbon RH (from all cations) or RCH 3 (only from 2 + and 5 +). These decompositions proceed after initial, rate-determining isomerization of 1 +–5 + to ion/dipole complexes R 3N + / R or, in case of benzyl substitution, to R +/NR 3. The NRMS experiments reveal that the hypervalent radicals 1–5 are unstable, decaying within ⪡ 0.2 μs by rupture of one of the NR bonds. 3–5 mainly cleave the NR bond which yields the thermodynamically most stable products.