Metastable Ions in the Mass Spectra of CD3CH3 and C2H6

Abstract

The mass spectra of CD₃CH₃ and C₂H₆ were searched for metastable transitions using a mass spectrometer with high sensitivity. With a detection limit of about 3 × 10⁻⁸ of the total ionization, 54 metastable transitions in CD₃CH₃, and 19 in C₂H₆ were found. All of these were checked for possible collision‐induced contributions by admitting krypton to the analyzer tube. The CH₃CD₃ decompositions are classified as (a) C–C ruptures, (b) H₂, HD, and D₂ eliminations, and (c) loss of an H or D atom. The first category provides information about D–H scrambling. The rate constant for D–H scrambling prior to C–C rupture is 5 × 10⁴ sec⁻¹ for parent ions, but very much larger in fragment ions, with the possible exception of CH₂CD₂⁺. The second category gives information about the isotope effect between H₂, HD, and D₂ loss. Compared to the loss of H₂, the elimination of HD is diminished about fivefold and that of D₂ by a factor of about 150. In the parent ion much less (if any) D–H scrambling occurs prior to the HD elimination than prior to the C–C rupture, in accord with the expected dependence of the scrambling rate on internal energy. Practically no H₂ and D₂ loss from the parent ion is found, indicating that the mass‐28 ion of ethane has the ethylene structure. The H and D losses provide only a few, but very convincing, examples of the strong isotope effect which suppresses D loss in the presence of H by a factor of at least 600. Furthermore, evidence is seen for a secondary isotope effect which slows down the loss of H from CH₂CD₂⁺ as compared to C₂H₄⁺.