MIXED-SITE VS CHARGE-SITE-REMOTE FRAGMENTATION REACTIONS OF LONG-CHAIN QUATERNARY AMMONIUM-IONS

The mixed-site fragmentation reactions discovered by Tuinman, Cook and Magid (J. Am. Soc. Mass Spectrom., 1 (1989) 85), as reaction channels competitive with charge-site-remote fragmentations, have been further investigated. Two complementary deuterium-labelled tetra-alkylammoniu m ions, (CH3)(3)N+C14D29 and (CD3)(3)N+C14H29, were investigated as pr ecursor ions for MS/MS/MS experiments using a sector/time-of-flight hy brid instrument employing orthogonal acceleration into the time-of-fli ght analyzer. By comparing the second-generation fragment ion spectra originating from the two precursors, it was possible unambiguously to assign atomic compositions and also to make plausible structural assig nments. The fragment spectra thus obtained for the first-generation pr oduct ions of the charge-site-remote reactions are consistent with pre vious conclusions concerning their structures. The first-generation pr oduct ions from the mixed-site fragmentations, for example (CH3)(2)N+( CD2)(m), are competitive with their charge-site-remote counterparts fo r m = 4, 5 and 6 in a manner consistent with the anticipated stabiliti es of cyclic transition states. However, interpretation of the second- generation fragment spectra of the (CH3)(2)N+(CD2)(m) ions and their d euterium-labelled complements was best accomplished in terms of a post ulated diradical ion intermediate. Crown copyright (C) 1997 Elsevier S cience B.V.