CHEMICAL IONIZATION MASS SPECTROMETRY .X. TEMPERATURE STUDIES WITH SUBSTITUTED BENZYL ACETATES

Isobutane chemical ionization mass spectra have been determined for five para-substituted benzyl acetates as a function of the temperature of the mass spectrometer ionization chamber: p-methoxy, fluoro, methyl, chloro, and nitrobenzyl acetates. The spectra are strongly temperature dependent. For all compounds except p-nitrobenzyl acetate the dominant ions at low temperature are (M + 1)+, but at high temperatures the dominant ions are fragment ions formed by decomposition of (M + 1)+. Temperature-dependent equilibrium reactions are also observed. Rate constants for the decomposition of (M + 1)+ are deduced from the observed ion intensities, and the rate constants at different temperatures follow the Arrhenius relationship. Activation energies and frequency factors are given. The activation energies depend on the nature of the para substituent, decreasing as the electron-releasing propensities of the substituents increase. Hammett plots of the rate constants against σ+ show a satisfactory correlation except for p-nitrobenzyl acetate, for which the rate constant is much too slow. It is concluded that the activation energies obtained from chemical ionization rate determinations are meaningfully related to the relative energies of the ions produced, although there is reason to think that the differences in activation energy are not exactly equal to the differences in the ion energies. Ions with general formula (2M + 1)+ and (M + 39) + are formed by reactions which appear to be equilibrium reactions. Equilibrium constants are calculated, and from the temperature variation of the equilibrium constants enthalpies and entropies for the reactions are obtained. As was found in a previous study the surprising result was obtained that the entropy changes for these reactions are positive. © 1969, American Chemical Society. All rights reserved.