Scattering of Thermal Neutrons by Liquid Solutions of Ortho- and Paradeuterium

The structures of two samples of liquid deuterium, one enriched in paradeuterium (85% para-D2) and the other enriched in orthodeuterium (the 20°K equilibrium concentration 96.9% ortho-D2), have been investigated by diffraction of 1-Å neutrons. The scattering data have been interpreted both as an assemblage of atom scatterers and, somewhat more exactly, as molecular scatterers. Fourier inversion of the intermolecular scattering yields a radial distribution interpretable for both samples as exhibiting approximately 10.3 nearest-neighbor molecules (cf. 12 for close packing of spheres). In the case of the converted sample (96.9% J = 0, nonrotating D2) the nearest-neighbor molecules are at a mean distance of 3.85 Å from the reference molecule; in the enriched sample (85% J = 1) the nearest-neighbor distance is 3.72 Å corresponding to a smaller molar volume attributable to the angular anisotropy of the intermolecular interactions when the rotations are hindered. Small-angle scattering also indicates a higher compressibility for the 96.9% para-D2 (J = 0) sample.