A new polymorph of N,N'-dimethylurea characterized by X-ray diffraction and first-principles lattice dynamics calculations

In this study we present a combined crystallographic and computational study of a new polymorph of N,N'-dimethylurea (DMU) with P2₁2₁2 space group symmetry, along with a revised theoretical study of the previously known phase in its corrected space group (Fdd2). X-ray diffraction studies show crystal structures that are very similar, differing only in the relative orientation of the hydrogen-bonded molecular chains that are common to both phases. The vibrational spectra were obtained from B3LYP hybrid functional lattice dynamics calculations and compared with the experimental data for the known phase. The free-energy difference between the forms is derived from the Γ-point optical mode frequencies, and amounts to less than 1 kJ mol^-1 across the temperature range of interest. The electronic densities-of-states of both phases are also computed, yielding only marginal differences in valence and conduction band compositions and band gap widths. Taken together, the results highlight the small but important differences separating the two crystal lattices.