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

Abstract

The crystal structure of benzoylacetone (1-phenyl-1,3-butanedione, C(10)H(10)O(2); P2(1)/c, Z = 4) has been determined at 300, 160 (both Mo Kalpha X-ray diffraction, XRD), 20 (lambda = 1.012 Å neutron diffraction, ND) and 8 K (Ag Kalpha XRD), to which should be added earlier structure determinations at 300 (Mo Kalpha XRD and ND, lambda = 0.983 Å) and 143 K (Mo Kalpha 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 range. The atomic displacement parameters have been analyzed using the thermal motion analysis program THMA11. The most marked change in the molecular structure is in the disposition of the methyl group, which has a librational amplitude of approximately 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 differ 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 single enol hydrogen trough (rather than two half H atoms), approximately centered 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-minimum potential well between the O atoms, with a libration amplitude of approximately 0.30 Å at 8 K. These results suggest that at 8 K the cis-enol ring in benzoylacetone has quasi-aromatic character, in agreement with the results of high-level ab initio calculations made for benzoylacetone [Schiøtt et al. (1998). J. Am. Chem. Soc. 120, 12117-12124]. Application [in a related paper by Madsen et al. (1998). J. Am. Chem. Soc. 120, 10040-10045] of multipolar analysis and topological methods to the charge density obtained from the combined lowest temperature X-ray and neutron data provides evidence for an intramolecular hydrogen bond with partly electrostatic and partly covalent character, and large p-delocalization in the cis-enol ring. This is in good agreement with what is expected from the observed bond lengths. Analysis of the total available (through the Cambridge Structural Database, CSD) population of cis-enol ring geometries confirms earlier reports of correlation 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 that the only reliable method of determining whether the enol hydrogen is found in a single or double potential well is by low-temperature X-ray or (preferably) neutron diffraction.

Acetylcholinesterase and Cytotoxic Activity of Chemical Constituents of Clutia lanceolata Leaves and its Molecular Docking Study

“…The situation was quite different for compound 4 . In the experimental geometry, there was a high degree of chemical symmetrization in the enol ring, defined as near equality of the C–C and C–O bond lengths compared pairwise, according to Herbstein et al [25]. The theoretical geometry did not display this symmetrization.…”

The nature and energy characteristics of intramolecular hydrogen bonds in crystals

“…7.8•10 -4 and 6.5•10 -4 Å 2 for the phenyl ring, see Ref. 26). At 20 K, the maximum ∆ value for the keto enol ring is as large as 40•10 -4 Å 2 .…”

“…23 How ever, analysis of the ∆ values for the benzoylacetone crys tal in the temperature range 8-300 K showed that the Hirshfeld test is met only at 8 K (∆ = 7.0•10 -4 Å 2 on the average; the maximum ∆ value for one of the two C-O bonds is 10•10 -4 Å 2 ). 26 The ∆ values obtained for the bonds in the keto enol fragment at 20 K (neutron diffrac tion study) and at 143 K (X ray diffraction study) are respectively 13.6•10 -4 and 31.4•10 -4 Å 2 , (cf. 7.8•10 -4 and 6.5•10 -4 Å 2 for the phenyl ring, see Ref.…”

Single-crystal neutron diffraction studies of hydrogen-bonded systems: Two recent examples from IPNS

“…The long axis of the H1 anisotropic displacement ellipsoid lies along the O1?O2 bond, as is typical for compounds containing a short, strong hydrogen bond [1]. The corresponding C-O distances also change significantly on cooling: the O2-C3 distance increases from 1.285(4) to 1.309(1) Å , while the O1-C1 distance decreases from 1.284 (4) It appears curious that 2, a symmetric b-diketone enol, exhibits significant asymmetry in its short hydrogen bond, even at ambient temperature, while benzoylacetone [13,14], which is asymmetric, exhibits a more nearly symmetric hydrogen bond. The crystal structure of 2 exhibits three short intermolecular C-H?O interactions, two on one side of the O-H?O hydrogen bond and one on the other.…”