Far infrared solution spectra. Volume of rotation and effective torque

Abstract

Far infrared absorption bands have been measured for about thirty-five dipolar solutes in liquid and glassy decalin. The results are interpreted in terms of a general relation between the volume of molecular rotation and mean square torque. The band frequency maxima (ν_max) are dependent at constant temperature only on the root mean square torque divided by the reduced moment of inertia (I_r) so that the data are conveniently classified in terms of the “torque product”T_q= 10³⁸I_rν²_max. Denoting by V the volume swept out on molecular rotation the rule T_q∝V² is obeyed generally both in the liquid and in the glass with a proportionality constant P= 1.38 × 10¹⁹ N m^–2, the mean force per unit normal area of the rotating molecule. Although P(0) is independent of the temperature it is solvent dependent. For molecules such as C₆H₅Cl, C₆H₅Br, C₆H₅NO₂, α-chloro- and α-bromo-naphthalene there are indications that the librational relaxation is strongly coupled to the translation of the centres of mass.