Intramolecular Dipole Reorientation of Amino Compounds

The dielectric absorption of dilute solutions of aniline, methyl substituted anilines, chlorine substituted anilines, N,N,N',N'-tetramethyl-p-phenylene diamine and 4-aminobiphenyl in various solvents has been measured between 0.3 and 135 GHz. The measurements have been carried out at 20 °C and for some mesitylene solutions also at —30 °C and 60 °C. The absorption curves have been resolved into multiple absorption regions. A far infrared (FIR) term had to be included in each analysis. Its contribution to the total dipole reorientation depends on the solvent and on the mobility of the substituted groups in the phenyl ring. The obtained group relaxation times are longer than those reported previously and also their weight factors are smaller. Differences in the mobility of the amino group are observed in methyl substituted anilines. Methyl substituents in the amino group itself strongly reduce the mobility. The decrease in the group mobility parallels the increasing mesomeric interaction between the lone pair electrons in the nitrogen and the ^-electrons of the phenyl ring. In addition to the reorientation of the amino group of 4-aminobiphenyl another internal reorientation process is found which is attributed to a mesomeric charge shift. Indications are obtained for weak association between some amino compounds and mesitylene.