ELECTRON-DIFFRACTION STUDY OF THE MOLECULAR-STRUCTURE OF 1,3-DIPHENYLPROPANE-1,2,3-TRIONE (DIPHENYL TRIKETONE)

The molecular structure of 1,3-diphenylpropane-1,2,3-trione (diphenyl triketone) has been determined by gas-phase electron diffraction at 130-degrees-C nozzle temperature. It has been found that the phenyl rings are nearly coplanar with the adjacent carbonyl groups, and the two C(ph)-C(O)-C(O) dihedral angles are -129.5 +/- 0.8 and -107.1 +/- 0.8-degrees (0-degrees corresponds to the syn form). Bond distances (r(g), angstrom) and bond angles (deg) with estimated total errors are C-H 1.105 +/- 0.006, O = C 1.213 +/- 0.003, <C(ph)-C(ph)> 1.404 +/- 0.003, C(ph)-C(O) 1.478 +/- 0.005, C(O)-C(O) 1.558 +/- 0.004, C(ph)-C(O)-C(O) 120.0 +/- 0.3, C(ph)-C(O) = O 120.8 +/- 0.4, C(O)-C(O)-C(O) 117.3 +/- 0.4, C(ph)-C(ph)(CO)-C(ph) 119.8 +/- 0.6. Although there is a general agreement between the gas-phase and crystal molecular structures there are some notable differences. One of the benzene rings is slightly turned about the C(ph)-C(O) axis in the opposite direction in the gas molecule as compared to the crystal. This causes a marked shortening of the O...H intramolecular contact (to 2.18 angstrom) between the central oxygen and an orthohydrogen.