Molecular reorientation of carbon monoxide dissolved in dense simple fluids

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Abstract
Extensive infrared measurements of the carbon monoxide fundamental vibration band for CO−Ne, CO−Ar, CO−Kr, CO−Xe, CO−N2, and CO−O2 systems have been carried out in the dense fluid region. We have investigated both the whole liquid range from the triple to the critical point along the saturation line and the continuous change from gas to liquid along an isobar well above the critical pressure. In the case of O2 and N2, we have also studied the liquid−solid transition. The rotational correlation functions Φu(t) and the orientational correlation time τu have been determined from band profile. An order of magnitude of the intermolecular mean square torques has been obtained from band moments analysis. We discuss the ability of ’’m’’ and ’’j’’ diffusion models to describe experimental dipolar momentum correlation functions when the solvent density increases continuously from gas to liquid. These models appear to be unsatisfactory in the liquid phase except for N2 solutions where the ’’j’’ diffusion model gives a rough description of the reorientational process. This has led us to use another description based on the memory function formalism [B. J. Berne and G. D. Harp, Adv. Chem. Phys. 17, 63 (1970); P. Desplanques, E. Constant, and R. Fauquembergue, Molecular Motions in Liquids (Reidel, Dordrecht, Holland, 1974), p. 133; C. R. Acad. Sci. B. 272, 1354 (1971); 274, 611 (1972)]. We discuss the temperature and density dependence of the two involved parameters: the mean square torque 〈 (OV)2〉 and its mean square temporal derivative 〈 (?V)2〉. These parameters have been obtained by fitting the approximate angular momentum memory function: KJ(t) = [〈 (OV)2〉/〈J2〉] exp{−[〈 (?V)2〉/〈 (OV)2〉 − 〈 (OV)2〉/〈J2〉]t2/2}. We show the important role on rotational motion of the local force field anisotropy (which decreases when the solidlike order is appearing in liquid).