Emsley J W, Lelli M, Joy H, Tamba M-G, Mehl G H
Department of Chemistry, University of Southampton, Southampton SO17 1BJ, UK.
Institut des Sciences Analytiques, Centre RMN à Très Hauts Champs (CRMN), CNRS/ENS Lyon/UCB-Lyon 1, 5 rue de la Doua, 69100 Villeurbanne, France.
Phys Chem Chem Phys. 2016 Apr 14;18(14):9419-30. doi: 10.1039/c5cp07304a.
The order parameter, Szz, where z is the para axis of the difluoroterphenyl groups in DTC5C9, has been obtained from chemical shift anisotropies measured by (13)C - {(1)H} NMR experiments at temperatures throughout the nematic, NU, and twist-bend nematic, NTB, phases shown by this compound. The order parameter temperature profiles are unusual in having a maximum value in the NU phase and then decreasing until the NTB phase is reached. There is a small discontinuity (∼2%) in Szz at TNNTB and then a gradual decrease until a new phase appears. This behaviour is interpreted as revealing a temperature-dependent tilting of local directors in both phases away from the applied magnetic field direction. In the enantiomorphic twist-bend phase this tilt is consistent with the structure of the phase as a helical arrangement of local directors, whilst in the high-temperature non-chiral nematic the tilt must involve a non-chiral arrangement. It is proposed that in both phases the tilting of directors has a common origin in the bent shape of the molecules.
序参量(S_{zz})（其中(z)是DTC5C9中双氟三联苯基团的平行轴）是通过在该化合物所呈现的向列相（N）、近晶相（NU）和扭曲向列相（NTB）的整个温度范围内进行的(^{13}C - {^{1}H})核磁共振实验所测得的化学位移各向异性得到的。序参量随温度的变化曲线不同寻常，在近晶相（NU）中有一个最大值，然后下降，直到达到扭曲向列相（NTB）。在(T_{NNTB})时，(S_{zz})有一个小的不连续（约2%），然后逐渐下降，直到出现一个新相。这种行为被解释为揭示了两个相中局部指向矢相对于外加磁场方向的温度依赖性倾斜。在对映扭曲向列相中，这种倾斜与该相作为局部指向矢的螺旋排列的结构一致，而在高温非手性向列相中，这种倾斜必定涉及一种非手性排列。有人提出，在两个相中，指向矢的倾斜都源于分子的弯曲形状。
