Shape-Dominated Ordering in Nematic Solvents. A Deuterium NMR Study of Cycloalkane Solutes

Abstract

We examine a series of closely related, relatively inert, cyclic aliphatic solutescyclohexane, methylcyclohexane, 1,1-dimethylcyclohexane, 1,4-trans-dimethylcyclohexane, 1,4-cis-dimethylcyclohexane, trans-decalin, and cis-decalinin a nematic solvent and demonstrate that their orientational ordering can be accurately described in terms of purely hard-body interactions with the solvent molecules. These interactions are treated explicitly in the context of a statistical mechanical approximation wherein the orientational correlations among the solvent molecules are not taken into account directly. We test the theory by predicting the observed order parameters of the aliphatic solutes as determined with deuterium nuclear magnetic resonance. The utility and limitations of a description of solute ordering derived exclusively from hard-body interactions is discussed.

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

“…The second case concerns solutes with small electrostatic moments such as molecular quadrupole moments, the so-called "magic solutes." 5 The alkanes are a case in point. For this class of solutes, the orientational order is predominantly governed by the size-and-shape mechanism.…”

Evolutionary algorithms to solve complicated NMR spectra

“…However, in the case of alkanes these additional mechanisms are considered to be of minor importance and can be neglected. 22 Thus the moniker "magic solute" is often used for alkanes. Hence, size and shape models ͑with the interaction constant adjusted to account for changes in liquid-crystal solvent and temperature͒ are probably also appropriate to describe the orientational order of pentane in 1132.…”

Multiple Quantum and High-Resolution NMR, Molecular Structure, and Order Parameters of Partially Oriented ortho and meta Dimethyl-, Dichloro-, and Chloromethylbenzenes Codissolved in Nematic Liquid Crystals

“…The orientational order parameters are related to the anisotropic intermolecular forces and thus can be used to examine statistical theories of liquid crystals (5)(6)(7)(8)(9)(10)(11)(12). Instead of analyzing the liquid-crystal molecules themselves, it is common to dissolve small solutes which probe these anisotropic forces.…”

“…Instead of analyzing the liquid-crystal molecules themselves, it is common to dissolve small solutes which probe these anisotropic forces. Solutes can be chosen so as to emphasize specific anisotropic interactions (5,8,9,13). For example, in this study the three ortho (and the three meta) solutes have similar sizes and shapes but have different multipole moments.…”

“…Ideally all solutes should be codissolved in the same sample tube but, due to overlap of spectral lines, extracting information from the resultant proton NMR spectrum may be impractical. It is common to dissolve solutes in different sample tubes and then to scale the results to account for variation in the solvent orientational order that results from different sample conditions (5,(13)(14)(15)(16)(17). In an effort to alleviate the problem of scaling, in this study three or four fully protonated solutes are codissolved in the same sample tube.…”