Electron charge-density distribution in an organic compound: 4,4-dimethylcyanobiphenyl (DMACB)

Abstract

Of particular interest is the relationship between chemical bonding and property for polymer materials since the number of X-ray charge density study was very few due to the difficulty of crystallinity control of single crystal sample. Thus, the charge density study of polymer material is a challenging task. In order to reveal the bonding nature, MEM (Maximum Entropy Method) charge density study of the polyoxymethylene (POM, -[CH 2 O] n -) were carried out by synchrotron radiation (SR) X-ray diffraction experiment using the best quality single crystal at SPring-8 BL02B1.The data with high counting statistics were measured at 123 K by the combination of high brilliance SR and the large cylindrical Imaging Plate (IP) camera. From the fiber pattern of POM recorded on IP, the integrated intensities of each Bragg reflections were obtained and corrected to the observed structure factors. The obtained X-ray pattern represented a typical feature of uniaxially-oriented POM structure on the basis of helical chain molecule. The preliminary structure analysis was done by SHLEX-97 software with the 9/5 helical chain structure model reported by Hengstenberg et al. in 1927[1]. However, the analysis presented an inconclusive answer even for the fundamental structure and the R-factor was 17.1%. This result coincides with the existence of the none-indexed X-ray spots of the present pattern. The possibility of another structure model such as the 29/16 helical chain model by Tashiro et al. [2] should be also examined. The MEM ought to allow visualizing charge density, which is consistent with the observed data. In fact, the MEM charge density based on the 9/5 helical chain model revealed the charge densities based on the mixture of several helical period structures. Even though the examination of bonding nature is still in progress, the existence of disordered and/or random feature of helical POM is uncovered in the charge density level for the first time.[1] Staudinger, H.,The electron-density distribution of coumarin-102, a laser dye has been investigated from high-resolution X-ray diffraction data collected at 100K, and from data based on theoretically calculated structure factors (VASP)[1], using the Hansen & Coppens[2] multipolar atom model. Topological properties of the refined charge density have been determined using the Bader[3] "atoms in molecules" theory. Analysis of deformation electron density peak heights and topological features indicate that the chromen-2-one ring system has a delocalized π electron cloud in resonance with the N amino atom. The molecular electrostatic potential and dipolar moment were estimated from both theoretical and experimental multipolar models, and reveal an asymmetric character of the charge distribution along the molecule. This polarization effect is due to a substantial charge delocalization within the molecule. Moreover, C-H…O contacts are observed in the crystal packing, and are confirmed as true hydrogen bonds by the presence of (3,-1) critical points along H…O paths.