Molecular Motion in Liquids: Rotational and Translational Diffusion in Weakly Associated Systems

Detailed characteristics of the rotational motion (jump angles and intermolecular torques) and of the translational motion (trapping times between molecular jumps) in the system chloroform–benzene have been obtained from infrared correlation functions and proton nuclear magnetic relaxation measurements at room temperature. For this, the band contour of a far-infrared parallel band of CDCl3 (362 cm−1) and a mid-infrared perpendicular band of CHCl3 (1219 cm−1) in benzene solutions were measured and Fourier-inverted. The nuclear magnetic experiments involved proton spin–lattice relaxation of benzene in CDCl3 as well as spin–echo self-diffusion measurements, evaluated on the basis of the Torrey model of intermolecular spin–lattice relaxation. The results indicate that the molecular moieties forming the weak complex do not perform librational motions (torsional oscillations) about some type of intermolecular bond, but behave essentially as “random rotators,” performing gaslike orientational jumps through average angles of 0.4–0.7 rad around their inertial axes. Likewise, the translational motion appears to be carried mainly by large-distance jumps (about 5 Å) of the molecular moieties and not by translational jumps of the complex as a whole. It is suggested that this picture holds generally for weak complexes and weakly nonideal liquids and solutions.