A nuclear magnetic resonance study of some solid alkylamines and their clathrate deuterates

Abstract

A proton second-moment and spin-lattice relaxation data are reported on solid N,N-d 2 -isopropylamine, N,N-d 2 -isopropylamine clathrate deuterate, solid trimethylamine, and trimethylamine clathrate deuterate. The second-moment data on isopropylamine indicate that a nearly rigid structure exists at 77 K, and that a hindered rotation of both the methyl groups takes place around 165 K. The barrier hindering the methyl group rotation, as estimated from nonexponential spin-lattice relaxation data, is found to be 3.50 kcal mole −1 with τ c = (1.78 ± 0.22) × 10 −13, exp (3.50 ± 0.07)/ RT. In N-N-d 2 -isopropylamine deuterate there is, in addition to the methyl group rotation, a rotation of the molecule arond its pseudo threefold axis near 212 K. Measurements of T 1 on this deuterate exhibited a single broad minimum of 45 ± 1 msec in the temperature range of 170 to 178 K which we assign as a rotation of the isopropylamine molecule around its pseudo threefold axis. The activation energy associated with the pseudo-threefold-axis rotation was found to be > 1.63 ± 0.03 kcal mole −1. In solid trimethylamine there was found, in addition to methyl group reorientation, a reorientation of the molecule around its molecular threefold axis from ∼145 to 157 K. The barrier associated with methyl group reorientation in its upper limit was found from T 1 measurements to be 5.75 ± 0.14 kcal mole −1. The second-moment data on the trimethylamine clathrate deuterate gave a second moment of 8.46 ± 0.76 G 2 at 77 K (all three methyl groups rotating), 1.42 ± 0.02 G 2 (methyl plus rotation around the molecular threefold axis), and a plateau value of 0.6 G 2. Only a single broad minimum was noticed in the T 1 measurements, and this was assigned to rotation of methyl groups around the threefold axis of the whole trimethylamine molecule. The activation energy for this rotation was found to be ⩾2.90 ± 0.13 kcal mole −1.