Substituent effects in the mass spectra of substituted benzils

Abstract

The mass spectra of a series of m- and p-substituted benzils have been determined at several ionizing voltages below 20 eV, and at 70 eV. The fractional intensities of the molecule-ion (FM), substituted benzoyl ion (FX) and unsubstituted benzoyl ion (FH) were obtained as a function of energy from measured ionization efficiency (IE) curves. Ionization potentials (IP), and appearance potentials for the substituted (APX) and unsubstituted (APH) benzoyl ions were determined from the IE curves by a semi-logarithmic method. Various correlations of ion intensity and energy parameters with substituent constants (σ+,σ) are examined; by the best standards of solution chemistry these are generally poor. Fair correlations are obtained between log (FX/FH) or APX-APH and σ+(σ )and these are interpreted in terms of the expected effect of substituents on the stabilities of the product ions in the decompositions. A good correlation is observed between log (FX/FH) and APX-APH; it suggests that substituents mainly affect FX/FH by changinq the activation energies for the competing decompositions of the benzil molecule-ions. The competitive shift has a marked effect on these appearance potentials, and in this system AP - IP is therefore not a good measure of the activation energy for the primary decompositions.