Solvent effects in the spectra of Benzene, Toluene, and Chlorobenzene at 2600 and 2000 Ǻ

Abstract

The ultraviolet spectra of benzene, toluene, and chlorobenzene at 2600 and 2000 Ǻ have been measured in carbon tetrachloride, chloroform, cyclohexane, 1%-hexane, ethanol, and water. Compared with the gases the solution spectra are all displaced to the red by amounts that agree qualitatively with the predicted effect of the solvent refractive index and the transition intensity according to the theory of Bayliss (1950). Quantitative agreement with this theory can be obtained only by assuming the effective cavity occupied by the solute molecule to be considerably smaller than the actual molecular size. The significance of this effect is discussed. The intensities of the solution spectra vary with the solvent refractive index, but in a way that is incompatible with the classical theory of Chako (1934). A marked increase in the intensity (particularly in toluene) is found where the solute absorption is close to an absorption band of the solvent, that is, for the 2600 Ǻ transitions in carbon tetrachloride and to a less extent in chloroform. In the 2600 Ǻ transition of benzene, a band appears in water, chloroform, and carbon tetrachloride that is very close to the position of the (0,0) band that is forbidden in the gas spectrum. The nature of this band is discussed.