Secondary hydrogen‐deuterium isotope effects on electron impact induced fragmentations

Secondary hydrogen‐deuterium isotope effects have been observed in the mass spectra of cis‐4‐t‐butylcyclohexyl iodide, 5‐iodononane and 2‐iodopropane. Under conditions which suppress competing and second generation fragmentations, β‐deuterium substitution decreases the intensity ratio {article}{empty}{document}([ M - I]⁺ /[ M]+ _ · ){document} , a result analogous to a normal isotope effect. The decrease is larger in the spectrum of cis‐4‐t‐butylcyclohexyl iodide‐trans‐2‐d than in the spectrum of the cis‐2‐d derivative. Since these effects parallel those in the better understood solvolysis reaction, both effects may have a common origin. In contrast, deuteration of more remote positions in cis‐4‐t‐butylcyclohexyl iodide and 5‐iodononane increases the indicated intensity ratio, an apparent inverse isotope effect. Although similar effects have been observed in solvolysis reactions, the mass spectral effect may be attributable to an increase in the nonfixed energy available for fragmentation. These results suggest that secondary isotope effects can be readily measured in certain cases, and that they may eventually become useful probes into the mechanisms of mass spectral fragmentations.