Molecular motions in polymorphic forms of ethanol as studied by nuclear magnetic resonance

Abstract

Measurements were made of the temperature dependence of the N.M.R. lineshape between 32 and 159 K and the spin-lattice relaxation time T 1 between 56 and 190 K at 10 MHz to study the molecular motion in various states of ethanol and ethanol-D. The methyl group reorientation about C 3 axis is the main relaxation mechanism in the stable crystalline phase, having an activation energy of 12·6 kJ mol-1 and τc 0 = 3·3 × 10-13 s. The molecular tumbling together with rapid methyl rotation is responsible for the dominant relaxation mechanism in the undercooled liquid and the metastable plastic phase. On the other hand, T 1 in the glassy liquid and the glassy crystal may be described by a quantum picture, i.e. the tunnelling rotation of the methyl group. The slope of the T 1 curve for these glassy states corresponds to the separation between the ground and the first excited torsional levels, which is 1·9 kJ mol-1. The tunnelling splitting was deduced to be 15 ± 5 MHz and the barrier height 8 ± 1 kJ mol-1.