An energy-resolved study of the fragmentation reactions of alkyl ammonium ions

Abstract

,2205 (1994). ' The unimolecular metastable ion and low-energy collision-induced fragmentation reactions of alkyl ammonium ions R,,H,.,,Nf (tz = 2 to4, R = tz-C3H7 and n-C,H9) have been studied and breakdown graphs expressing the % fragment ion abundances as a function of the centre-of-mass collision energy have been established. The dialkyl and trialkyl ammonium ions fragment primarily by olefin ([R -HI) elimination or by formation of the alkyl ion Rf (with rearrangement to the more stable secondary structure). By contrast the tetraalkyl ammonium ions show both elimination of [R -HI and elimination of an alkane, which is C,H12 for the propyl compound C7H,6 for the butyl compound. The results are interpreted in terms of competition between heterolytic bond cleavage to form a [Rf---NH,.,,R,,.,] complex and homolytic bond cleavage to form a [R,,-,H,.,,Nf.---'R] complex. The former complex fragments either to form R+ or undergoes internal proton transfer to form R,,-,H,.,,NH+ + [R -HI, while the latter complex fragments by alkane elimination resulting from attack of R' on the a-CC bond of the alkyl group. For the propyl ammonium ions plausible potential energy profiles are established which show that the former complex is favoured for n = 2, 3 while the two complexes have similar energies for n = 4. [Traduit par la rCdaction]

Homolytic cleavages in pyridinium ions, an excited state process

“…Furthermore, it has been suggested that these reactions occur through a concerted mechanism or are mediated by the formation of radicals [6 -9]. The development of electrospray ionization allowed the formation of various classes of closed shell ions under very mild conditions and hence it has been unambiguously shown that at least in some cases homolytic C™C and C™H cleavages are involved in the gas phase decomposition of cationized organic compounds [10,11].One class of organic cations that are known to undergo homolytic cleavages is quaternary ammonium and pyridium cations [8,[12][13][14][15][16][17]. Fisher and Veith studied the gas phase fragmentation of ammonium and pyridinium ions, generated with field desorption and reported the formation of nitrogen based radical cations under collision induced dissociation (CID) [12,13].…”

“…One class of organic cations that are known to undergo homolytic cleavages is quaternary ammonium and pyridium cations [8,[12][13][14][15][16][17]. Fisher and Veith studied the gas phase fragmentation of ammonium and pyridinium ions, generated with field desorption and reported the formation of nitrogen based radical cations under collision induced dissociation (CID) [12,13].…”

“…Fisher and Veith studied the gas phase fragmentation of ammonium and pyridinium ions, generated with field desorption and reported the formation of nitrogen based radical cations under collision induced dissociation (CID) [12,13]. Harrison showed that the formation of radicals and cations from quaternary ammonium ions is dominated by the relative ionization potential of the appropriate radicals and neutrals, showing that the reaction products are the ones thermodynamically favored [14,15]. Katritzky and coworkers studied homolytic cleavages in pyridium cations that were designed in order to show the substituent effect on the relative stability of radicals.…”

Effects of substrate surfaces in matrix‐assisted laser desorption/ionization mass spectrometry

“…Significant amounts of alkylammonium ions emerged from In and SS substrates, a little less from Au, but no such ions could be observed from Ag. If sufficient energy to induce unimolecular decomposition of gaseous tetraalkylammonium ions (R 4 N + ) is available, most of the abundant fragment ions should be R 2 NCH and R 3 NH + 7. As no such ions were found in any of the spectra, the metal substrates transfer little energy to cause decomposition of the tetraalkylammonium ions.…”

“…Although the substrate material is not excited by the laser, there are obvious differences among the samples; the intensity of the background noise depends on the thickness of the sample layer. The background ions do not seem to be alkylammonium fragments since their typical fragment ions, R 2 NCH and R 3 NH + ,7 are absent in the spectra. The ions in the background might be fragments of the matrix molecule MBBA.…”