Geppi M, Marini A, Veracini C A, Urban S, Czub J, Kuczyński W, Dabrowski R
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Risorgimento 35, 56126 Pisa, Italy.
J Phys Chem B. 2008 Aug 14;112(32):9663-76. doi: 10.1021/jp800378g. Epub 2008 Jul 23.
Structural and orientational order properties of 3Cy2CyBF2 and of 5CyCy2BF2 have been investigated by means of (13)C-NMR, optical, and dielectric spectroscopy methods. In the case of NMR, order parameters have been independently obtained from the analysis of either (13)C-(19)F dipolar couplings or (13)C chemical shift anisotropies, both measured from (13)C-{(1)H} NMR static spectra. The assignment of the (13)C resonances has been carried out thanks to the comparison with solution state spectra and DFT calculations, and the relevant geometrical parameters and (13)C chemical shift tensors needed to derive orientational order parameters have been calculated by DFT methods. In the analysis of (13)C-(19)F dipolar couplings, empirical corrections for vibrations and anisotropic scalar couplings have been included. Dielectric measurements have been performed over a broad frequency range for two orientations of the nematic director with respect to the measuring field. At low frequencies (static case) a positive dielectric anisotropy has been determined, which has enabled the calculation of the order parameters according to a well-tested procedure. At high frequencies the dielectric anisotropy changes its sign, a property which can be useful in designing a dual addressing display. The nematic order parameter determined from optical, dielectric, and NMR methods have been compared: their trends with temperature are very similar, apart from some slight shifts, and were analyzed by Haller and Chirtoc models. The differences among the results obtained by the four methods have been discussed in detail, also with reference to the assumptions and approximations used in each case, and to the results recently reported for similar fluorinated nematogens. The presence of a non-negligible order biaxiality has been related to the presence of a CH2CH2 bridging group, linking one cyclohexylic unit with either the other cyclohexyl or the phenyl ring.
通过（13）C - NMR、光学和介电谱方法研究了3Cy2CyBF2和5CyCy2BF2的结构和取向有序性质。在NMR的情况下，有序参数是通过对（13）C - （19）F偶极耦合或（13）C化学位移各向异性的分析独立获得的，这两者均从（13）C - {（1）H} NMR静态谱测量得到。通过与溶液态谱和DFT计算进行比较，完成了（13）C共振的归属，并且通过DFT方法计算了推导取向有序参数所需的相关几何参数和（13）C化学位移张量。在分析（13）C - （19）F偶极耦合时，纳入了对振动和各向异性标量耦合的经验校正。针对向列型指向矢相对于测量场的两个取向，在很宽的频率范围内进行了介电测量。在低频（静态情况）下确定了正的介电各向异性，这使得能够根据一个经过充分验证的程序计算有序参数。在高频下，介电各向异性改变其符号，这一性质在设计双寻址显示器时可能有用。比较了由光学、介电和NMR方法确定的向列型有序参数：除了一些微小的偏移外，它们随温度的变化趋势非常相似，并通过Haller和Chirtoc模型进行了分析。详细讨论了这四种方法所得结果之间的差异，还参考了每种情况下所使用的假设和近似，以及最近报道的类似氟化向列相液晶的结果。不可忽略的有序双轴性的存在与连接一个环己基单元与另一个环己基或苯环的CH2CH2桥连基团的存在有关。
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