Analysis of the infrared and Raman spectra of the symmetrically substituted 1,3-diphenylurea and 1,3-diphenylacetone (dibenzyl ketone)

The structural stability of 1,3-diphenylurea and 1,3-diphenylacetone was investigated by the DFT-B3LYP and ab initio MP2 calculations using the 6-311G basis set. From full energy optimization at the MP2 level of theory the near-planar cis-trans form of 1,3-diphenylurea was predicted to be about 1.5 kcal/mol lower in energy than the X-ray near-planar cis-cis conformer. At the DFT-B3LYP level of theory the near-planar cis-trans and cis-cis forms were predicted to have a very comparable relative stability. The dibenzyl ketone was predicted by both levels to exist predominantly in a gauche-gauche conformation. On comparison the DFT near-planar structures of each of the two molecules were found to be consistent with the reported X-ray structures. The analysis of the infrared and Raman spectra of 1,3-diphenylurea suggests the presence of a second form at room temperature. The vibrational frequencies of the conformers most consistent with X-ray data of diphenylurea and dibenzyl ketone were computed at the B3LYP level and tentative vibrational assignments of their normal modes were provided on the basis of combined experimental and computed data.