ON THE DISSOCIATION AND CONFORMATION OF GAS-PHASE METHONIUM IONS

The dissociation pathways of both doubly and singly charged methonium ions, (CH3N+-(CH2)(n)-N-+(CH3)(3) . X(-) (n = 6, 10; X = Br, I, and OA c), are measured using blackbody infrared radiative dissociation (BIRD ) and SORI-CAD in a Fourier transform mass spectrometer. SORI-CAD of t he doubly charged decamethonium ions results primarily in the formatio n of even electron ions by hydrogen rearrangements. In contrast, homol ytic bond cleavage to form two odd electron ions is highly favored in the hexamethonium ion, presumably due to increased Coulomb repulsion i n this ion. For BIRD of the singly charged salts, ions are mass select ed and dissociated by heating the vacuum chamber to elevated temperatu res. Under the low pressure conditions of our experiment, energy is tr ansferred from the chamber walls to the ions by the absorption of blac kbody radiation. From the temperature dependence of the unimolecular r ate constants for dissociation, Arrhenius activation energies in the z ero-pressure limit are obtained. The primary dissociation pathways cor respond to counterion substitution reactions which result in loss of N (CH3)(3) and CH(3)X. For hexamethonium and decamethonium with X = Br o r I, the branching ratios for these pathways differ dramatically; the ratio of loss of N(CH3)(3) and CH3Br is 3.8 and 0.4 for hexamethonium and decamethonium bromide, respectively. The hexamethonium acetate sal t has a branching ratio of 0.1. The Arrhenius activation energies for hexamethonium (Br or I) and decamethonium (Br or I) are 0.9 and 1.0 eV , respectively. This value for hexamethonium acetate is 0.6 eV. Molecu lar dynamics simulations and Monte Carlo conformation searching are us ed to obtain the lowest energy structures of hexamethonium and decamet honium bromide. These calculations indicate that the methonium ion fol ds around the counterion to form a cyclic salt-bridge structure in whi ch both quaternary nitrogens interact with the oppositely charged coun terion. The significantly different branching ratios observed for thes e ions is attributed to the large change in orientation of the counter ion with respect to the ammonium centers as the number of methylene gr oups in these ions increases. Similar ion conformational differences a ppear to explain the fragmentation for the OAc counter ion as well.