Substituent effects on ion abundances and energetics in substituted acetophenones

Based on the quasi‐equilibrium theory of mass spectra it is shown that the intensity ratio [A]+/[M]+, where [A]+is a fragment ion and [M]+is the molecular ion, is given by [A]+/[M]+= f′ (k1/kt) ((1/f) − 1), where f is the fraction of molecular ions with insufficient energy to fragment, f′ is the fraction of [A]+ions with insufficient energy to fragment, and k1/ktis the fraction of fragmenting molecular ions which form [A]+. For substituted acetophenones it is shown that f depends on the substituent present and that f′ k1/ktis also substituent dependent for formation of both [CH3CO]+and [YC6H4CO]+. It is also shown that no direct information concerning the effect of a substituent on the rate of a particular fragmentation reaction can be obtained from intensity studies. The ionization potentials of the parent molecules and the appearance potentials of the [YC6H4CO]+fragment ions have been measured for fifteen substituted acetophenones and the correlations with substituent constants are discussed.