On the Orientation Mechanism of Small Molecules in Liquid‐Crystalline Environments

Abstract

A general molecular theory for the description of the orientation of small molecules in anisotropic environments is presented. Assuming a second-order tensorial interaction between some solute property p,, and the anisotropy in a liquid-crystal "field" F,,, rigid and non-rigid contributions to the dipolar and quadrupolar couplings observable by NMR are evaluated. The behaviour of the solutes hydrogen, methane and their deuterated analogues in nematic phases can be described by special cases of this general theory and excellent agreement between observed and calculated anisotropic couplings is obtained. The present results suggest that the coupling between the solute molecular quadrupole moment and the anisotropy in the liquid-crystal electric field gradient plays a significant role in the orienting process in the case of hydrogen.

Molecules with Large-Amplitude Torsional Motion Partially Oriented in a Nematic Liquid Crystal: Ethane and Isotopomers

“…7,12,13 Our current results lend strong support to the notion that average external electric field gradients present in liquid-crystal solvents should not be ignored in the interpretation of measured quadrupolar couplings.…”

“…Spectra of 13 C isotopomers, obtained simultaneously with the 12 C spectrum from the 13 C satellites in the proton spectrum, were scaled using the same factor as for the 12 C isotopomer. To ensure identical conditions, a deuteron spectrum was obtained immediately following the proton spectrum without removing the sample from the NMR probe.…”

“…To ensure identical conditions, a deuteron spectrum was obtained immediately following the proton spectrum without removing the sample from the NMR probe. Because of signal-to-noise considerations, 13 C satellites were not observed in the deuteron spectra and hence 13 CD couplings were not obtained.…”

“…It will be shown that in representative cases (e.g., 13 CH 3 I) inclusion of the reorientation-vibration coupling leads to a situation where all the observations can be explained on the basis of a single gas-phase structure, thus removing the need for more elusive explanations.…”

“…2,3 Seminal papers in this field have employed molecular hydrogen [4][5][6] and its deuterated [7][8][9] and tritiated 10 isotopomers, as well as methane and its isotopomers. [11][12][13] From such studies, it has become apparent that in liquid-crystal solvents both short-and long-range interactions generally contribute to the orientational order. Under favorable conditions, these contributions can be obtained separately when different liquid crystals and so-called "magic mixtures" are employed.…”