SOME STERIC EFFECTS OF METHYL IN MASS SPECTRAL FRAGMENTATIONS

Substituent effects of methyl groups ortho to resonance donors were studied for the loss of NO by substituted nitrobenzene ions in the mass spectrometer. The results were found to be consistent with an interpretation as simple steric effects at high voltages. o-Methyl groups seem to prevent coplanarity of the dimethylamino group in this system, but not of the methoxy group. Both peak intensity comparisons and metastable ion characteristics support this conclusion. At low voltage an additional complication, possibly associated with ring expansion, appears. Methyl is not completely inert as a hindering group for this reaction, especially at low voltage, and it may be preferable to use halogen substituents for studies of steric effects. © 1969, American Chemical Society. All rights reserved.