Proton magnetic relaxation in ethylene oxide, tetrahydrofuran, and triethylene diamine as clathrate deuterates

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Abstract
Proton spin-lattice relaxation times (T 1) have been measured for triethylene diamine, ethylene oxide, and tetrahydrofuran as clathrate deuterates. The results are interpreted in terms of anisotropic rotation of the guest molecules. Triethylene diamine is thought to be undergoing rotation about its C 3 axis with a correlation time given by τc/s = 4·87 × 10-14 exp (1680 K/T) at temperatures between 120 K and the decomposition point (308 K). Between 77 K and 120 K, T 1 is dominated by conformational distortions of the guest molecule. Ethylene oxide and tetrahydrofuran rotate about at least two axes in the deuterate at rates sufficient to produce some motional narrowing. At high temperatures the relaxation is caused in both cases by rotation about an axis perpendicular to the C 2 axis, and at lower temperatures by rotation about the C 2 axis itself. The correlation times are for ethylene oxide τc/s = 6·76 × 10-14 exp (450 K/T), T < 160 K; and for tetrahydrofuran τc/s = 4·79 × 10-14 exp (470 K/T), T < 140 K. The free induction decay shapes indicate that, in each case, low frequency motion is occurring about all axes throughout the temperature range studied (77 K to the decomposition temperature in each case). From the lack of an observable signal from the clathrate deuterates of hexamethylene tetramine and dioxan, it is deduced that there is no reorientational motion of these guests at frequencies greater than their rigid-lattice linewidths.