Fragmentation s. a. Cleavage

Characteristic for the fragmentation of the title compounds (237-243) are a very facile N—S bond cleavage (route A in equation 50 giving rise to 244) as in sulfonylhydrazides (Section V.C) and skeletal rearrangements accompanied by the extrusion of SO2 from the M and [M — 1] ions to the ionized JV-aryliminopyridinium betaines (245, compounds 237-239) or AT-imino-2-benzylpyridinium betaines (247, compounds 240-243 in equation 52) and azacarbazoles (246), respectively . Ion 244 decomposes further by elimination of HCN and via the loss of R N and HCN, as shown in equation (51). The sequence M " (240-243)- 247 ->248 (equation 52) is supported by the fact that AT-imino-a-alkylpyridinium betaines can lose an NH2 radical due to the operation of an ortho effect . 

Subsequent work confirmed this apparently abnormal behaviour. Deuteriation at remote sites (the S- or e-position) induces small inverse secondary isotope effects in a-cleavages occurring in the ion source, but normal isotope effects in the decomposition of metastable ions in the field-free regions94,95. The time dependence of the isotope effect was also studied by field ionization kinetics, which permit the analysis of fragmentations occurring after lifetimes as short as 10 12 s-1. It was found that the inverse isotope effect favouring loss of the deuteriated radical operates at times shorter than 10 9 s95. 

Example Secondary kinetic isotope effects on the a-cleavage of tertiary amine molecular ions occurred after deuterium labeling both adjacent to and remote from the bond cleaved (Chap. 6.2.5). They reduced the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes. [78] With the aid of field ionization kinetic measurements the reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isoto-pomers (Fig. 2.17). [66,76]... 

Finally, the follow-up cleavage can be described as changing from a simple decay, where the unpaired electron is mostly located at the fragmentation site, to an intramolecular ET that now occurs from the aryl-substituent system to the S—S a orbital, particularly with the nitro-substituted compound. The observed change in the ET rate would thus monitor a change in the overall ET-bond fragmentation mechanism. 

Patterson, S.D., Hess, D., Yungwirth,T., and Aeber-sold, R. 1992. High-yield recovery of electrob-lotted proteins and cleavage fragments from a cationic polyvinylidene fluoride-based membrane. Anal. Biochem. 202 193-203. 

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