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

David M S Martins, Christopher K Spanswick, Derek S Middlemiss, Nasir Abbas, Colin R Pulham, Carole A Morrison

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish
Pages (from-to)5998-6003
Number of pages6
JournalThe Journal of Physical Chemistry A
Volume113
Issue number20
DOIs
Publication statusPublished - 21 May 2009

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