Tunneling states and nuclear spin conversion in solid CH3D

Abstract

Total neutron cross section measurements have been made on condensed CH3D over the temperature region 0.75<T<100 K. The neutron wavelength was sufficiently long (4.7 Å) so that, in the low temperature region, the cross section was proportional to 〈I (I+1) 〉, the mean squared proton nuclear angular momentum per molecule, where I = total nuclear spin. 〈I (I+1) 〉 was determined to an accuracy of 1% or better and was only found to change significantly in the region T<10 K where conversion between nuclear spin symmetry species becomes appreciable. In contrast to the example of CH4, the rate of conversion appears to be rather insensitive to the amount of O2 impurity. For pure CH3D, the rate of conversion is rapid and is shown to correspond closely to the rate of thermal relaxation observed in calorimetric measurements on the solid at low temperatures. Estimates of low-lying tunneling states are made from the complementary calorimetric and neutron cross section results. There is a slight indication that the energy levels dilate in the region T<1.6 K, as has been predicted to occur in solid CH4. It is concluded that the unexpectedly rapid conversion between nuclear spin symmetry species in CH3D is a consequence of an accidental degeneracy between an A and an E tunneling level in each of two manifolds. Analysis of the thermodynamic data shows that the lowest temperature phase (phase III) of solid methane is not fully orientationally ordered.