A Computational Investigation of the Dynamics of Urea Molecules in Solids

Abstract

A computational investigation of the dynamic properties of urea molecules in the pure crystalline phase of urea and in the urea tunnel structure (which is found in the conventional urea inclusion compounds) is reported. Both of these crystalline solids are extensively hydrogen bonded structures, and in both cases it has been shown experimentally that the urea molecules undergo 180° jumps about their C=O bonds at sufficiently high temperature. The computational investigations reported here have probed aspects of the potential energy barriers for this reorientational motion, and the possibility of correlations between the motions of different urea molecules. Various structural models representing clusters of urea molecules in these solids were considered, with two different potential energy parameterizations used to compute the energies of these clusters. The energetic and structural properties of the clusters were investigated as a function of rotation of a reference urea molecule, leading to new insights concerning the degree of correlation between the rotational motions of different urea molecules. A critical assessment is presented of the extent to which the results from these computational investigations can be compared with experimental results on the dynamic properties of the urea molecules.

Key Words:

• Urea
• reorientational motions
• clusters