THE UNIMOLECULAR CHEMISTRY OF QUATERNARY AMMONIUM-IONS AND THEIR NEUTRAL COUNTERPARTS

The quaternary ammonium ions (CH3)4N+ (1+), (CH3CH2)4N+ (2+), C6H5N+(C H3)3 (3+), C6H5CH2N+(CH3)3 (4+) and C6H5CH2N+(CH2CH3)3 (5+) and the co rresponding ammonium radicals 1-5 have been studied in the gas phase b y metastable ion (MI) characteristics, collisionally activated dissoci ation (CAD) and neutralization-reionization mass spectrometry (NRMS). The unimolecular chemistry of the ammonium ions is influenced by the n ature of their substituents (R). Based on cations 3+-5+, which carry d ifferent types of N+-R bonds, preferential dissociation at N+-benzyl > N+-alkyl > N+-phenyl takes place. The two major fragmentation channel s 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) f ormation of an immonium ion by loss of the hydrocarbon RH (from all ca tions) or RCH3 (only from 2+ and 5+). These decompositions proceed aft er initial, rate-determining isomerization of 1+-5+ to ion/dipole comp lexes R3N.+/.R or, in case of benzyl substitution, to R+/NR3. The NRMS experiments reveal that the hypervalent radicals 1-5 are unstable, de caying within much less than 0.2 mus by rupture of one of the N-R bond s. 3-5 mainly cleave the N-R bond which yields the thermodynamically m ost stable products.