Metastable transitions in the mass spectra of ketals and ketones. Competition between loss of small and large radicals

An investigation of competing metastable transitions in the mass spectra of ethylene ketals R_SR_LC(OCH₂)₂ (where R_L is a larger n-alkyl group than R_S) has established that in most cases R_S is lost with a lower activation energy than R_L. This technique has also been applied to ketones R_SR_LC?O, to show again that R_S is usually lost with the lower activation energy (thus supporting earlier data based on relative daughter ion abundances at the threshold). In the classes of compounds so far investigated, although M⁺ ? R_S] ions are formed with lower activation energies than M⁺ ? R_L] ions, the ion yield of M⁺ ? R_S] ions is anomalously low from ions of high internal energy. Factors which may influence the M⁺ ? R_S]/M⁺ ? R_L] ratio of daughter ion intensities are examined. It is suggested that at the threshold M⁺ ? R_S] and M⁺ ? R_L] ions may be formed with rearrangement, or from an electronic state that cannot be effectively populated from molecular ions of high internal energies.