Diffuse X-ray scattering and disorder in p-methyl-N-(p-chlorobenzylidene)aniline, C14H12ClN (ClMe): analysis via automatic refinement of a Monte Carlo model
A recently developed method for fitting a Monte Carlo computer simulation model to observed single-crystal diffuse X-ray scattering has been applied to (h0l) scattering data obtained for the disordered compound p-methyl-N-(p-chlorobenzylidene)aniline, C14H12ClN (ClMe), at 295 K. Each molecular site is occupied by a molecule in one of four basically different orientations defined by combinations of a side-to-side and an end-to-end flip. These different orientations were represented in the model by two sets of binary random variables. In addition to these occupancy variables, the model also allowed local orientational and centre-of-mass translational displacements. The disordered structure was modelled using rigid-body molecules linked by harmonic springs to represent 'effective' intermolecular interactions. The refined model included 4 parameters describing the correlation of neighbouring occupational variables and 28 parameters describing the force-constant and 'size-effect' parameters on seven symmetry-inequivalent types of intermolecular vectors. The model refined to an overall agreement factor, [formula: see text], of 32.8% for 28,500 data points. It was found that the SRO correlation structure observed within the layers of molecules in the ab plane is typical of a frustrated lattice, where nearest- and second-nearest-neighbour effects oppose each other. This appears to be the main difference between this compound and the sister compound, p-chloro-N-(p-methylbenzylidene)aniline (MeCl), studied previously. The local relaxational displacements that result from local 'size-effect' strains operating on different intermolecular vectors generally reflect the tendency for distances involving Cl-Cl interactions to be longer than the observed average distances.
Research School of Chemistry, Australian National University, Canberra, ACT, Australia. welberry@rsc.anu.edu.au
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Welberry