Application of molecular orbital theory to the interpretation of mass spectra study of substituent effect on a series of acetophenones

Semi-empirical molecular orbital calculations have been used to study the substituent effect on the intensity of the m/e 43 peak (COCH₃]+) in the mass spectra of a series of 25 substituted acetophenones. The two most relevant molecular properties calculated appear to be the ionization potential and the bond density of the fragmenting bond. A semi-logarithmic plot of relative peak intensitiesthe ratio of m/e (43/M]⁺)] versus the ratio of ionization potential to bond density for these compounds produced a linear relationship with a correlation coefficient of 0.90 from the INDO method and 0.93 from the IEHT method. The use of two different semi-empirical methods to calculate ionization potentials and bond densities, together with the qualitative agreement between the two methods and with available experimental data, substantiates the correlations obtained. In addition, the molecular orbital calculations were used to understand the anomalous behavior of the teritary butyl and para-phenyl substitutes.