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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >NMR of short-chain hydrocarbons in nematic and smectic a liquid crystals
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NMR of short-chain hydrocarbons in nematic and smectic a liquid crystals

机译：向列和近晶液晶中短链烃的NMR

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摘要

The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered in two liquid-crystal solvents that exhibit both nematic (N) and smectic A (SmA) phases (one of which also has a reentrant nematic (RN) phase) are analyzed using CMA-ES (covariance-matrix adaptation evolution strategy). To deal with problems arising from the broad liquid-crystal background signal, a smoothed experimental spectrum is fitted to a smoothed calculated spectrum. The ethane and propane dipolar couplings and anisotropic energy parameters scale with each other, and the n-butane couplings are assumed to behave likewise. This restriction on the relative values of the n-butane energy parameters facilitates a fit to the temperature dependence of the n-butane dipolar couplings, in which the six n-butane energy parameters (three for trans and three for gauche), the isotropic trans-gauche energy difference E_(tg), and its temperature coefficient Etg′, and the methyl CCH angle decrease are obtained. Unlike earlier studies, the fit does not employ a model for the anisotropic intermolecular potential. The nematic potential in the SmA phase of the liquid-crystal mixture 6OCB/8OCB is estimated by interpolated values obtained from the ethane spectra for the N and RN regions. Analysis of results for the SmA phase involves adding a nematic-smectic coupling prefactor and a smectic prefactor. Spectra obtained in the liquid-crystal 8OCB are calculated with no adjustable parameters, scaling the potentials using the ethane values, and the agreement between experiment and calculation is outstanding. Conformer populations are affected by the environment, and the isotropic solute-solvent interactions result in increased gauche populations, whereas anisotropic interactions increase the trans probability. The trans probability in the SmA phase is slightly lower than expected from the nematic results.
机译：乙烷，丙烷和正丁烷在两种液晶溶剂中共溶和取向有序的质子NMR光谱，既显示向列相（N）相又显示近晶A（SmA）相（其中一个也具有折返向列相（RN）相）使用CMA-ES（协方差矩阵自适应演化策略）进行分析。为了处理由宽液晶背景信号引起的问题，将平滑的实验光谱拟合到平滑的计算光谱。乙烷和丙烷的偶极耦合和各向异性的能量参数彼此成比例，并且假定正丁烷耦合的行为相同。对正丁烷能量参数相对值的这种限制促进了对正丁烷偶极耦合的温度依赖性的拟合，其中六个正丁烷能量参数（三个为反式，三个为脂乳状），各向同性反式获得了-gauche能量差E_（tg），其温度系数Etg'和甲基CCH角减小。与早期的研究不同，拟合没有采用各向异性分子间势的模型。液晶混合物6OCB / 8OCB的SmA相中的向列电势是根据从N和RN区域的乙烷光谱获得的内插值来估计的。 SmA阶段的结果分析涉及添加向列-近层耦合前因子和近列前因子。在没有可调参数的情况下计算液晶8OCB中获得的光谱，使用乙烷值缩放电势，并且实验和计算之间的一致性非常出色。适形种群受环境影响，各向同性溶质与溶剂的相互作用导致薄纱种群的增加，而各向异性的相互作用则增加了反式概率。 SmA阶段的反穿概率略低于向列结果的预期。

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