Étude du Comportement D'Isomères Géométriques sous L'Impact Électronique. VI. Les Cis‐ et Trans‐1, 2‐Di‐Terbutyl‐Éthylènes

Mass spectrometry investigation of cis and trans di-t-butylethylenes has been carried out by the study of the effect of electron energy on mass spectra, measurement of ionization and appearance potentials and examination of metastable transitions. It is shown that the geometrical isomerism strongly influences the behavior of these compounds under electron impact: (1) relative abundance of the molecular ion from cis isomer is much smaller than that of trans, especially for low energy electrons; (2) the ionization potentials of both isomers are the same, but the appearance potentials of main fragment ions are smaller in the cis isomer, indicating that less energy is required to break down the ionized cis molecule; and (3) for several processes, the probability of decomposition of metastable ions is larger in cis isomer. This difference of behavior of cis and trans di-t-butyl-ethylenes under electron impact has to be interpreted as a result of an effect of conformational energy (10 kcal/mole) in the cis isomer, which is nonexistant in the trans. Molecular ions of di-t-butyl-ethylenes, either cis or trans, undergo rearrangement by H migration due to the double bond and the quaternary carbon atoms. Calculation of the heat of formation of main fragment ions leads to the same values in both isomers, indicating that a common ionic intermediate (or a set of intermediates) is involved in the dissociation process. It can also be shown from such data that main fragment ions have a rearranged structure.