Substituent effects in mass spectra of monosubstituted benzils

The mass spectra of a series of thirteen m ‐ and p ‐substituted benzils have been determined at several ionising voltages below 20 eV and at 70 eV. At ionising voltages up to 5 eV above the ionisation potentials the benzil molecular ions decompose entirely by two pathways to give substituted and unsubstituted benzoyl ions. Fractional intensities of the molecular ion (F M ), substituted (F X ) and unsubstituted (F H ) benzoyl ions were obtained for each benzil as a function of energy from measured ionisation efficiency curves, and ionisation and appearance potentials for all major ions determined from the ionisation efficiency curves by a semilogarithmic method. Various correlations of ion intensity and energy parameters with δ + and δ constants are examined; these are generally poor. Fair correlations are obtained between log (F X /F H ) or (AP   (XC   6 H   4 CO)   +– AP   (C   6 H   5 CO)   +) and δ or δ + , 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 (F X /F H ) and AP   (XC   6 H   4 CO)   +· AP   (XC   6 H   5 CO)   +; this suggests that substituents affect F X /F H mainly by changing the activation energies for the competing decompositions of the molecular ions. The competitive shift has a marked effect on these appearance potentials so that in this system AP – IP is not a good measure of the activation energy for the primary decompositions.