Charge-transfer complexes. Part V. Nature of the interaction of halogenomethanes and aromatic hydrocarbons

The ultraviolet absorption spectra of n-hexane solutions of mixtures of halogenomethanes and polycyclic aromatic hydrocarbons have been examined. At high hydrocarbon concentrations it appears that a new absorption band is occurring under the low-energy tail of the most intense hydrocarbon absorption band. When concentrations are used which enable all the absorption bands of the hydrocarbon to be studied, the effect of the halogenomethane appears to be a broadening of the most intense hydrocarbon absorption band, the total integrated intensity remaining constant. Under these conditions no new bands could be resolved. This effect is interpreted as being due to perturbation of the hydrocarbon absorption band by a halogenomethane molecule in the solvation shell. Such a species is held to be a possible precursor to the 1 : 1 complex formation which appears to occur in the solid phase. The effects on the hydrocarbon spectrum are discussed in relation to previous observations of similar effects which have been ascribed to ‘contact’ charge-transfer. Finally, photochemical reactions of aromatic hydrocarbons with carbon tetrachloride are discussed in relation to charge-transfer interaction.