Simultaneous variation of multipole parameters and Gram-Charlier coefficients in a charge-density study of tetrafluoroterephthalonitrile based on X-ray and neutron data.
Simultaneous variation of multipole parameters and Gram-Charlier coefficients in a charge-density study of tetrafluoroterephthalonitrile based on X-ray and neutron data.
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基于 X 射线和中子数据的四氟对苯二甲腈电荷密度研究中多极参数和 Gram-Charlier 系数的同步变化。
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发表时间：
2003
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通讯作者：
S. Larsen
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作者：
H. Sørensen;R. Stewart;G. McIntyre;S. Larsen
通过对122.4 K下用四氟对苯二甲腈测量的精确X射线和中子衍射数据的改进，研究了在模拟氟在有机化合物中的散射时遇到的困难。多极改进导致了氟上高度收缩的八极杆。随后的分析表明，氟不具有价八极，但表现出非简谐的热运动，可以用八极多极参数来模拟。八极子的散射贡献在散射矢量中表现出与原子核位移到三阶的Gram-Charlier展开相同的立方关系。分析还表明，对氟的三阶Gram-Charlier系数的精化要求数据至少达到0.93A(-1)分辨率(Sinthetas/lambda)。扩展到1.27A(-1)的X射线数据具有足够的分辨率，在精化中包括N、F和氰基C原子的三阶Gram-Charlier系数，而中子数据只允许精化氮的三阶Gram-Charlier系数。对中子和X射线衍射数据的改进得到了所有原子的相同的原子位移参数。虽然N原子和F原子的非简谐运动包结提供了最好的模型，但它不影响晶体的电子密度，并且所有的分子内键临界点都具有相同的特征。然而，非谐模型的应用导致了分子间相互作用的微小差异，这可以从N原子附近的静电势来说明。通过晶体电子密度的拓扑分析，阐明了C-F键的性质，也支持了苯环上的喹酮结构。
Difficulties encountered in modelling the scattering of fluorine in organic compounds have been investigated through refinements of accurate X-ray and neutron diffraction data measured on tetrafluoroterephthalonitrile, TFT, at 122.4 K. Multipole refinements led to a highly contracted octopole on fluorine. The subsequent analysis revealed that fluorine does not possess a valence octopole but exhibits anharmonic thermal motion that can be modelled by the octopole multipole parameters. The scattering contribution from the octopole shows the same cubic dependence in the scattering vector as the Gram-Charlier expansion of the nuclear displacements to third order. The analysis also showed that refinement of third-order Gram-Charlier coefficients on fluorine requires data to at least 0.93 A(-1) resolution in sinthetas/lambda. The X-ray data extending to 1.27 A(-1) were of sufficient resolution to include third-order Gram-Charlier coefficients for N, F and the cyano C atoms in the refinement, whereas the neutron data only enabled refinement of the third-order Gram-Charlier coefficients for nitrogen. The refinements of the neutron and X-ray diffraction data yielded identical atomic displacement parameters for all the atoms. Though inclusion of anharmonic motion for N and F atoms provides the best model, it does not affect the crystal electron density, and all intramolecular bond critical points have identical features. Application of the anharmonic model, however, leads to small differences in the intermolecular interactions, which is illustrated by the electrostatic potential adjacent to the N atom. The characteristics of the C-F bond were elucidated by the topological analysis of the crystal electron density, which also supported the proposed quinonoid structure of the benzene ring.