Molecular Motion in Liquids: On the Prevalence of Large-Size Rotational and Translational Diffusion Steps

It is shown experimentally (Fourier inversion of vibrational band contours) that the molecular orientational motion in some representative common liquids [methylene chloride (CH2Cl2), methyl iodide (CH3I), chloroform (CHCl3), cyclohexane (C6H12)] appears to consist of angular jumps of the order of 20° to 60°. During a jump, which is completed in times of the order of 0.4 × 10−12 sec, the molecules are observed to rotate as if they were in their vapor phase, regardless of whether or not the orientational motion involves the tilting of a permanent dipole moment or of the size of the molecule. The infrared data are compared with results from nuclear magnetic relaxation studies. It is seen that the rotational motion in liquids, as it is observed by these techniques, is not sensitive to molecular association, weak hydrogen bonding, etc. On the basis of the extended Torrey model of intermolecular spin–lattice relaxation, it appears that molecular translational diffusion in a large variety of liquids, some of them strongly associated, occurs by a jump of the order of a molecular diameter after a trapping time of the order of 10−11–10−10 sec. As far as can be predicted, liquids which obey a classical diffusion model of very small diffusion steps would be the exception rather than the rule. The possibility is discussed that a dimer is the diffusing entity in acetic acid.