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NMR study of methyl group reorientation and relaxation in clathrate hydrates and their guests Khanzada, Abdul Wahab Khan

Abstract

The proton magnetic resonance absorption and spin-lattice relaxation measurements have been carried out on a number of methyl groups containing guest compounds, and their clathrate deuterates in order to study the motional behaviour of methyl groups and the guest molecules. The activation energies associated with these motional processes are reported. The proton second moment data for diethyl amine, and diethyl-amine deuterate over a temperature range of 77 to 270K indicate that in both these materials, the only motion is that of methyl group reorientation. The relaxation data for both compounds exhibit non-exponential behaviour. These data are therefore analysed according to Hilt and Hubbard theory and the activation energies of 2.90 ± 0.03 and 2.34 ± 0.02 kcal/mole are obtained for diethyl-amine and diethyl amine deuterate respectively; these energies represent the barriers hindering methyl group reorientation according to the above theory. The strength of the dipolar interactions in the deuterate as estimated from both the second moment and the maximum in temperature dependence of nuclear relaxation rate is consistent with a C-H bond length of 1.13 Å. The results of proton second moment in acetone reveal that both methyl group rotate around their threefold axis from 77 to 180 K. The relaxation data give an activation energy of 1.33 ± 0.01 kcal/mole for this motion. The acetone molecule in the deuterate hydrate shows isotropic motion in the 16-hedral cavities of the deuterate from ~212 to 260K. The activation energy obtained from the relaxation data in the temperature range of 77 to 120K for acetone deuterate was found to be 0.33 ± 0.01 kcal/mole, which was not assigned. The absorption line measurements of non hydrogen-bonded tert-butylamine deuterate showed a motional behaviour ranging from methyl, tert-butyl, and isotropic rotation of whole molecule in its deuterate cavities in the temperature range of 77 to 272 K. The T₁ measurements of the same compound exhibited a broad distribution of correlation times among these three motions. The activation energies obtained from T₁ measurements in their upper limit were 1.7 kcal/mole (methyl reorientation), and 2.5 ± -1 kcal/mole (tert-butyl group and isotropic rotation of tert-butylamine). In the pure tert-butylamine the second moment data show a direct transition from rigid lattice value (77 K) to a value consistent with all methyl as well as tert-butyl group reorientation (~150 K). The relaxation measurements showed a smaller distribution of correlation times than the deuterate. The activation energy associated with the methyl group reorientation in the upper limit was found to be 3.2 ± -1 kcal/mole in this amine. Isopropylamine, in its pure form showed rigid structure at 77K and motion of both methyl groups at higher temperatures in the proton absorption line measurements. The activation energy for this motion obtained from non-exponential relaxation measurements was 3.50 ± 0.07 kcal/mole. The corresponding isopropyl amine deuterate revealed in addition to methyl reorientation (activation energy less than 1.7 kcal/mole), a rotation around its pseudo threefold axis (activation energy less than 1.6 kcal/mole) from absorption and relaxation measurements. Relaxation measurements of trimethylamine gave an energy barrier of 5.75 kcal/mole for methyl reorientation. The hexagonal deuterate of trimethylamine showed a Lorentzian line shape from 77 to 100 K and non-exponential relaxation in nearly the same range. T₁ data gave an energy barrier of ~0.7 kcal/mole for reorientation around threefold axis and 2.9 ± •2 kcal/mole for translational motion of trimethylamine in the deuterate. Some methylene group containing deuterates of cyclopropane, piperazine, and hexamethylenetetramine were also studied. H¹ resonance spectra of cyclopropane showed isotropic rotation of cyclopropane in its type I. (at high temperatures) and type II structures. No evidence of motion was found in the rest of the two deuterates of piperazine, and hexamethylenetetramine.

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