Isotope effects, dissociation lifetime

The lifetimes of hypervalent radicals have been found to depend rather dramatically on isotope substitution. For example, dimethyloxonium, (CH3)2OH, dissociates completely on a 1 -ps time scale when formed by collisional reduction of the stable cation (CH3)2OH+. By contrast, (CH3)2OD furnishes an abundant survivor ion in the +NR+ mass spectrum that is evidence that the deuterated hypervalent radical is metastable [178,179]. From the time scale of the NR measurements and the survivor ion relative intensities one can estimate that (CH3)2OH dissociates >5 times faster than (CH3)2OD. Similar isotope effects have been observed for CH3OH [180], C2H5OH [181], and hypervalent ammonium radicals, e.g., CH3NH [182], (CH3)2NH [60], (CH3)3NH [183], and [pyrrolidinium] [184], which are metastable only as deuterated species. 

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