Dipartimento di Chimica, Università della Calabria, v. P. Bucci, I-87036 Rende (CS), Italy.
J Phys Chem B. 2010 Jan 14;114(1):235-41. doi: 10.1021/jp907311k.
In the previous paper of this issue, [Celebre, G.; Ionescu, A. J. Phys. Chem. B doi: 10.1021/jp907310g], following a generalized reaction field approach in the linear response approximation, we were successful in obtaining an analytical compact expression for the mean-field anisotropic orientational potential U(Q-EFG) theoretically experienced by a highly idealized nonionic and apolar solute, considered as a point quadrupole immersed in a uniaxial polarizable continuum medium (model of a nematic solvent comprised of dipolar mesogenic molecules). The term U(Q-EFG) describes the electrostatic interaction between the electric quadrupole of the solute and the electric field gradient induced at the solute by the surrounding medium polarized by the distribution of electric charges representing the quadrupolar solute itself. In the present paper, the obtained potential has been considered as an additional orientational interaction contributing to the solute ordering, besides the well-recognized and very effective "short-range" (size-and-shape-dictated) mechanisms. Since in our theory the solvent is characterized by its dielectric tensor, the model has been widely tested by taking as references the experimental order parameters of several uniaxial and biaxial different small rigid probe molecules (H(2), N(2), acetylene, allene, propyne, benzene, hexafluorobenzene, 1,4-difluorobenzene, and norbornadiene) dissolved in the nematic solvents ZLI1132 (Deltaepsilon >> 0) and EBBA (Deltaepsilon < 0); moreover, the order parameters of the same solutes in the so-called nematic "magic mixture" (45 wt % EBBA + 55 wt % ZLI1132), where the short-range orientational effects are commonly believed to be very dominant, have been conventionally assumed as reference of the absence of electrostatic orientational effects. The experimental order parameters of the treated solutes, obtained in the past by liquid crystal NMR and available from literature, have been then compared with those theoretically predicted by our theoretical approach in order to obtain useful hints about two basic points, (a) the real physical nature of the interactions (other than the "size-and-shape") involved in the orientational mechanisms and (b) the conceptual effectiveness of the suggested mean-field approach in describing this kind of phenomena. Successes and failures of the approach in the predictions are discussed at length, along with their possible reasons and implications.
在本期论文的前一篇文章中,[Celebre, G.; Ionescu, A. J. Phys. Chem. B doi: 10.1021/jp907310g],我们在线性响应近似下采用广义反应场方法,成功地获得了一个高度理想化的非离子和非极性溶剂溶质在各向异性取向势 U(Q-EFG)方面的理论解析紧凑表达式,该溶质被视为沉浸在单轴极化连续介质中的点四极子(由偶极介晶分子组成的向列溶剂模型)。术语 U(Q-EFG)描述了溶质的电四极矩与由周围介质诱导的电场梯度之间的静电相互作用,该周围介质通过代表四极子溶质本身的电荷分布而极化。在本文中,除了众所周知且非常有效的“短程”(大小和形状决定)机制之外,所获得的势被认为是对溶质有序性的另一种取向相互作用。由于在我们的理论中,溶剂由介电张量表征,因此该模型已通过参考几种单轴和双轴不同刚性探针分子(H(2)、N(2)、乙炔、丙二烯、丙炔、苯、六氟苯、1,4-二氟苯和降冰片二烯)在向列溶剂 ZLI1132(Deltaepsilon >> 0)和 EBBA(Deltaepsilon < 0)中的实验有序参数进行了广泛测试;此外,相同溶质在所谓的向列“魔术混合物”(45wt%EBBA+55wt%ZLI1132)中的有序参数通常被认为具有很强的短程取向效应,被传统上视为不存在静电取向效应的参考。过去通过液晶 NMR 获得并可从文献中获得的处理溶质的实验有序参数,然后与我们理论方法预测的理论有序参数进行比较,以获得关于两个基本点的有用提示,(a)参与取向机制的相互作用(除“大小和形状”之外)的实际物理性质,以及(b)建议的平均场方法在描述这种现象方面的概念有效性。详细讨论了该方法在预测中的成功和失败,并讨论了其可能的原因和影响。