Diffuse X-ray scattering from benzil, C14H10O2: analysis via automatic refinement of a Monte Carlo model

A recently developed method for fitting a Monte Carlo computer-simulation m odel to observed single-crystal diffuse X-ray scattering has been used to s tudy the diffuse scattering in benzil, diphenylethanedione, C6H5-CO-CO-C6H5 . A model involving 13 parameters consisting of 11 intermolecular force con stants, a single intramolecular torsional force constant and a local Debye- Waller factor was refined to give an agreement factor, R = [Sigma omega(Del taI)(2) / Sigma omegaI(obs)(2)](1/2), of 14.5% for 101 324 data points. The model was purely thermal in nature. The analysis has shown that the diffus e lines, which feature so prominently in the observed diffraction patterns, are due to strong longitudinal displacement correlations. These are transm itted from molecule to molecule via a network of contacts involving hydroge n bonding of an O atom on one molecule and the para H atom of the phenyl ri ng of a neighbouring molecule. The analysis also allowed the determination of a torsional force constant for rotations about the single bonds in the m olecule. This is the first diffuse scattering study in which measurement of such internal molecular torsion forces has been attempted.