NEUTRALIZATION-REIONIZATION OF ALKENYLAMMONIUM CATIONS - AN EXPERIMENTAL AND AB-INITIO STUDY OF INTRAMOLECULAR N-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-C=C INTERACTIONS IN CATIONS AND HYPERVALENT AMMONIUM RADICALS

A series of isomeric hexenylammonium and hexenyldimethylammonium catio ns were neutralized by collisional electron transfer in the gas phase in an attempt to generate hypervalent ammonium radicals. The radicals dissociated completely on the 4.8-5.4 mu s time scale. Radicals in whi ch the hexene double bond was in the 3-, 4-, and 5-positions dissociat ed by competitive N-H and N-C bond cleavages. Allylic 2-hexen-1-ylammo nium and 2-hexen-1-yldimethylammonium radicals underwent predominant c leavages of allylic N-C bonds. Deuterium labeling experiments revealed no intramolecular hydrogen transfer from the hypervalent ammonium gro up to the hexene double bond. Ab initio and density functional theory calculations showed that alkenylammonium and alkenylmethyloxonium ions preferred hydrogen bonded structures in the gas phase. The stabilizat ion through intramolecular H bonding in 3-buten-1-ylammonium and 3-but en-1-yl methyloxonium ions was calculated by B3LYP/6-311G(2d,p) at 26 and 18 kJ mol(-1), respectively. No intramolecular hydrogen bonding wa s found for the allylammonium ion. The hypervalent 3-buten-1-yl-methyl oxonium radical was calculated to be unbound and predicted to dissocia te exothermically by O-H bond cleavage. This dissociation may provide kinetic energy for the hydrogen atom to overcome a small energy barrie r for exothermic addition to the double bond. The 3-buten-1-ylammonium and allylammonium radicals were found to be bound and preferred gauch e conformations without intramolecular hydrogen bonding. Vertical neut ralization of alkenylammonium ions was accompanied by small Franck-Con don effects. The failure to detect stable or metastable hypervalent al kenylammonium radicals was ascribed to the low activation barriers to exothermic dissociations by N-H and N-C bond cleavages. (C) 1997 Ameri can Society for Mass Spectrometry.