METASTABLE TRANSITIONS IN 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 RSRLC(OCH2)2 (where RL is a larger n‐alkyl group than RS) has established that in most cases RS is lost with a lower activation energy than RL. This technique has also been applied to ketones RSRLCO, to show again that RS 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+ − RS] ions are formed with lower activation energies than [M+ − RL] ions, the ion yield of [M+ − RS] ions is anomalously low from ions of high internal energy. Factors which may influence the [M+ − RS]/[M+ − RL] ratio of daughter ion intensities are examined. It is suggested that at the threshold [M+ − RS] and [M+ − RL] ions may be formed with rearrangement, or from an electronic state that cannot be effectively populated from molecular ions of high internal energies. Copyright © 1969 John Wiley & Sons, Ltd.