Studies of mixed-site and charge-site-remote fragmentations of quaternary ammonium ions. II: Effects of chain length

The objective of the present work was to investigate further the charge-sit e-remote (CSR) and mixed-site-fragmentation (MSF) mechanisms through the us e of high-energy tandem mass spectrometry (MS/MS) and MS/MS/MS experiments on the specifically deuterated tetra-alkyl ammonium ions (C2D5)(3)N+(C14H29 ) (1), (C2D5)(2)N+(C14H29)(2) (2a) and (CD3)(2)N+(C14H29)(2) (2b). The work reported here shows unequivocally that the MSF process proceeds only at ca rbon atoms directly bonded to the nitrogen charge site. Both the short and long alkyl chains could participate in the MSF process, which is best ratio nalized in terms of diradical intermediate species, necessitating the uptak e of large amounts of internal energy. MS/MS experiments on first-generatio n CSR ions produced from 2a showed a second series of CSR and MSF ions aris ing from the intact long chain, whereas first-generation MSF ions showed qu ite different behaviour under the same experimental conditions. A remarkabl y diverse set of molecular processes is triggered by the input of substanti al amounts of energy into these closed-shell ions. (Int J Mass Spectrom 188 (1999) 27-38 Crown copyright (C) 1999.