Molecular reorientation of CD₄in gas‐phase mixtures

Abstract

Spin-lattice relaxation times were measured for the deuterons in CD(4) in pure gas and in mixtures with the following buffer gases: Ar, Kr, Xe, HCl, N(2), CO, CO(2), CF(4), and SF(6). Effective collision cross sections sigma(theta, 2) for the molecular reorientation of CD(4) in collisions with these ten molecules are obtained as a function of temperature. These cross sections are compared with the corresponding cross sections sigma(J) obtained from (1)H spin-rotation relaxation in mixtures of CH(4) with the same set of buffer gases. Various classical reorientation models typically applied in liquids predict different ratios of the reduced correlation times for the reorientation of spherical tops. The Langevin model comes closest to predicting the magnitude of the sigma(theta, 2)/sigma(J) ratio that we obtain for CD(4).

Collisional cross-section of water molecules in vapour studied by means of 1H relaxation in NMR

“…2). We fixed τ C to values predicted by the Ivanov model (τ C  = 5/4τ J ), extended diffusion model (τ C  = 1/4τ J ) or Langevin model (τ C  = 1/24.4τ J )1415. Our experimental data are compatible with a negligible dipole-dipole contribution or with the Langevin model (τ C  ≪ τ J ), according to which significant contributions of only occur at low pressures p  < 10 kPa.…”

“…For dilute gases (τ J → ∞), the ratio of correlation times τ C /τ J varies from 5/4 (Ivanov model) to 1/4 (extended diffusion model) or 1/24.4 (Langevin model)1415.…”

“…Experimentally determined cross-sections can be used to refine intermolecular potentials111213. In methane, isotopic substitution141516 affects relaxation rates associated with the different isotopomers such as CH 4 , CH 3 D, CH 2 D 2 , and CHD 3 . In supercritical water171819, spin-rotation significantly contributes to NMR relaxation despite the high density.…”

Thermodynamic state dependence of orientational order and rotational relaxation in carbon tetrachloride

“…It seems fair to assume that the case of carbon tetrachloride should be most similar. However, a recent study 62 on the molecular reorientation of CD 4 in gas-phase mixtures has produced results that contradict the J-diffusion model and are closer to the Langevin model prediction. Therefore, the purpose of this work is twofold: first, relaxation times are computed along several phases of carbon tetrachloride in order to obtain an extended data set for a realistic model; second, the ability of theoretical models to describe the resulting 2 versus J relation is tested.…”

“…Similar results were also found in recent experimental measures of CD 4 in gas-phase mixtures. 62 While theories fail most notably in the gas phase, in the high density limit non-negligible deviations occur as well. In consequence, the present analysis of a realistic model shows that rotational relaxation is not understood even in this case of weakly associated, almost spherically symmetric molecules.…”