Kinetic studies in mass spectrometry—IX: Competing [M NO2] and [M NO] reactions in substituted nitrobenzenes. Approximate activation energies from ionization and appearance potentials

Abstract

Ionization (IP) and appearance potentials (AP) have been secured for apparently competing [M  NO2] cleavage and [M  NO] rearrangement reactions in a series of m-and p-X substituted nitrobenzene molecular ions. Approximate activation energies AP–IP were within the experimental reproducibility for most substituents for the [M  NO2] reaction of m-and p-X isomer pairs of compounds, but outside it for the [M  NO] process. This is interpreted as either effective substituent randomization in the reacting molecular ions or as fortuitously similar activation energies for the [M  NO2] cleavage. Assuming the latter, AP–IP for both reactions gave acceptably linear correlations of negative slope when plotted against σ or σ+, which has been interpreted mechanistically in terms of a degree of positive charge dissipation in the rate limiting transition states. These findings are in direct contrast to the energetics of the [M  CH3] and [M  CH2O] reactions recently studied in substituted anisoles, where plots of AP–IP vs. σ+ displayed positive slopes. The advantages of AP–IP over peak relative abundance as a mechanistic probe are discussed, and corresponding plots of logZ/Z0 vs. σ(σ+) and log [A0]/[M0] (1 – f) vs. σ(σ+) for the same reactions are presented and discussed. The disadvantages and limitations of IP and AP measurements are also emphasized.