X-ray and neutron diffraction study of benzoylacetone in the temperature range 8-300 K: comparison with other cis-enol molecules

The crystal structure of benzoylacetone (1-phenyl-1,3-butanedione, C10H10O2 ; P2(1)/c, Z = 4) has been determined at 300, 160 (both Mo K alpha X-ray di ffraction, XRD), 20 (lambda = 1.012 Angstrom neutron diffraction, ND) and 8 K (Ag K alpha XRD), to which should be added earlier structure determinati ons at 300 (Mo K alpha XRD and ND, lambda = 0.983 Angstrom) and 143 K (Mo K alpha XRD). Cell dimensions have been measured over the temperature range 8-300 K; a first- or second-order phase change does not occur within this r ange. The atomic displacement parameters have been analyzed using the therm al motion analysis program THMA11. The most marked change in the molecular structure is in the disposition of the methyl group, which has a Librationa l amplitude of similar to 20 degrees at 20 K and is rotationally disordered at 300 K. The lengths of the two C-O bonds in the cis-enol ring do not dif fer significantly, nor do those of the two C-C bonds, nor do these lengths change between 8 and 300 K. An ND difference synthesis (20 K) shows a singl e enol hydrogen trough (rather than two half H atoms), approximately center ed between the O atoms; analogous results were obtained by XRD (8 K). It is inferred that the enol hydrogen is in a broad, flat-bottomed single-minimu m potential well between the O atoms, with a libration amplitude of similar to 0.30 Angstrom at 8 K. These results suggest that at 8 K the cis-enol ri ng in benzoylacetone has quasi-aromatic character, in agreement with the re sults of high-level ab initio calculations made for benzoylacetone [Schiott et at. (1998). J. Am. Chem. Sec. 120, 12117-12124]. Application [in a rela ted paper by Madsen er al. (1998). J. Am. Chem. Sec. 120, 10040-10045] of m ultipolar analysis and topological methods to the charge density obtained f rom the combined lowest temperature X-ray and neutron data provides evidenc e for an intramolecular hydrogen bond with partly electrostatic and partly covalent character, and large p-delocalization in the cis-enol ring. This i s in good agreement with what is expected from the observed bond lengths. A nalysis of the total available (through the Cambridge Structural Database, CSD) population of cis-enol ring geometries confirms earlier reports of cor relation between the degree of bond localization in the pairs of C-C and C- O bonds, but does not show the dependence of bond localization on d(O ... O ) that was reported earlier for a more restricted sample. It is suggested t hat the only reliable method of determining whether the enol hydrogen is fo und in a single or double potential well is by low-temperature X-ray or (pr eferably) neutron diffraction.