A COMPUTATIONAL INVESTIGATION OF THE DYNAMICS OF UREA MOLECULES IN SOLIDS

A computational investigation of the dynamic properties of urea molecu les in the pure crystalline phase of urea and in the urea tunnel struc ture (which is found in the conventional urea inclusion compounds) is reported. Both of these crystalline solids are extensively hydrogen bo nded structures, and in both cases it has been shown experimentally th at the urea molecules undergo 180 degrees jumps about their C=O bonds at sufficiently high temperature. The computational investigations rep orted here have probed aspects of the potential energy barriers for th is reorientational motion, and the possibility of correlations between the motions of different urea molecules. Various structural models re presenting 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 properti es of the clusters were investigated as a function of rotation of a re ference urea molecule, leading to new insights concerning the degree o f correlation between the rotational motions of different urea molecul es. A critical assessment is presented of the extent to which the resu lts from these computational investigations can be compared with exper imental results on the dynamic properties of the urea molecules.