The ''principal" band in the ultraviolet spectrum (due to excitation to a dipolar state) has been measured for nitrobenzene and acetophenone and their para methyl, ethyl, isopropyl and ¿-butyl derivatives in the gas phase and in a wide variety of solvents. The gas phase excitation energies are in the inductive order, with the alkyl groups responding in linear proportion to the change in electron demand in proceeding from the acetophenone to the nitrobenzene series. The effect of solvent, particularly basic solvents, is to tend to invert the order of excitation energies. Quantitative treatment of the data indicates that this effect is consistent with the operation of steric hindrance to solvation.
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