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In liquid crystals, while the second and fourth rank orientational order parameters characterizing a nematic phase can be experimentally determined via several techniques, there is no straightforward experiment rendering the positional order parameters characterizing a smectic A phase. This work illustrates a novel method to estimate the positional order parameters of a smectogenic liquid crystal solvent from knowledge of the orientational order parameters of a number of solutes dissolved therein. The latter order parameters can be experimentally determined via liquid crystal NMR spectroscopy. These data can be then analyzed with a statistical-thermodynamic density functional theory, whose basic ingredient is a model for solute-solvent intermolecular interactions. Its parametrization and the subsequent fitting procedure eventually permit one to obtain the positional order parameters of the solvent besides the positional-orientational distribution function of the solutes. The method is applied to the smectogen 4,4′-di-n-heptyl-azoxybenzene, in which the solutes 1,4-dichlorobenzene and naphthalene have been dissolved. With the help of this exploratory practical example, pros and cons of the method are pointed out and further developments prospected. [ABSTRACT FROM AUTHOR]

The ionic liquid crystal 1-dodecyl-3-methylimidazolium tetrafluoroborate slightly doped with water is presented as a promising NMR alignment medium for the measurement of residual dipolar couplings for polar molecules dissolved therein. [ABSTRACT FROM AUTHOR]

A weakly ordering organic-based lyotropic phase and a strongly ordering nematic liquid crystal have been used as NMR media for a parallel study of the conformation and orientational order of the same solute, trans-stilbene. A careful comparison allows to figure out some interesting conclusions on accessibility and precision of the experimental data and reliability and accuracy of the conformational descriptions into these two kinds of mesophases. [ABSTRACT FROM PUBLISHER]

Abstract: The mechanisms conferring to the trans-stilbene derivatives their well known important biological properties are not yet completely clear; then, it is reasonable to suppose that their conformational equilibrium could play some kind of role (as found for many other molecules of pharmacological interest). In this scenario, the 4,4′-dichloro-trans-stilbene (DCS) can be considered as the prototype of a series of stilbenes substituted in 4,4′ positions in order to enhance their properties. Following this starting idea, we decided to study the conformational distribution of DCS in solution by Liquid Crystal NMR Spectroscopy (LXNMR), one of the best techniques to obtain this kind of information in the liquid state of matter. Our results show that the molecule is basically not planar: the most populated conformations we found are (a) the couple of symmetry-related, propeller-like structures, belonging to the C 2 point group and characterized by the phenyl rings disrotated of about 18.7° with respect to the vinyl plane and (b) the couple of symmetry-related Ci structures (where the rings are parallel, like the pedals of a cycle) having this time the two rings conrotated of an angle of ∼18.7° with respect to the vinyl fragment. [Copyright &y& Elsevier]

In the present paper, the peculiar orientational behavior, studied by liquid crystal NMR (LXNMR) spectroscopy, of the D2dsymmetry quasi-spherical molecule of tetramethylallene (TMA) dissolved in the nematic solvent I52 is exploited to attempt a quantitative experimental assessment of the correlation between molecular vibrations and overall rotations in weakly oriented molecules. The analysis of the very small DHHand 1D13C–Hdipolar couplings, available from the natural abundance LXNMR spectra of TMA at different temperatures, allows for a derivation leading (by making a few approximations) to the quantification of the vibration–reorientation (also called nonrigid) contribution affecting the observed direct 1D13C–Hdipolar coupling. The obtained results show that, under the particular conditions of the studied system (very weak orientational ordering of a highly symmetric molecule), this contribution is particularly important, in order to reproduce the whole value of the “observed” dipolar coupling. This issue is discussed and commented on at length in the work, also, by making reference to the analogy with perfectly symmetric molecules (such as methane and analogues) dissolved in liquid crystalline phases. [ABSTRACT FROM AUTHOR]

Very accurate bond distances can be derived from dipolar couplings, Dij, obtained by analysis of the NMR spectra of samples dissolved in anisotropic solvents (LXNMR). To do this, however, the couplings must be corrected for the averaging produced by vibrational motion. The vibrational corrections require a knowledge of the force field, which may be obtained by fitting experimental vibrational frequencies or, much more easily, from quantum-mechanical computations. In the present study, the reliability of the methods used to calculate the force field and the effects on the accuracy of the vibrational corrections to the Dij is tested on benzene for which a very accurate structural study is available. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we suggest a new methodological approach to get in a very simple way a crude (but meaningful) conformational analysis of flexible molecules in fluid condensed phases (a very interesting matter where inter- and intra-molecular interactions are deeply involved). The test case we have chosen is the never-studied-before temperature dependence of inter-ring angle of biphenyl in a nematic solution. Undemanding 2H NMR experiments were carried out on perdeuterated biphenyl dissolved in the mesophase at different temperatures, to obtain deuterium quadrupolar splittings. Since the experimental data alone are not sufficient to allow the determination of the inter-ring angle for each temperature, they were supplemented with simulated orientational information: this ‘hybrid’ set of data was enough to obtain the searched conformational distribution versus T. The results, showing a very slight tendency of the inter-ring angle to increase with the temperature, lend themselves to immediate interesting speculations and encourages the accomplishment of more sophisticated and expensive experiments to investigate more in detail the phenomenology. In our opinion the suggested methodology, appealing for its easiness, could be useful, for example, for coarse grained conformational investigations of large flexible systems (as, e.g. biomolecules) in liquid media. [Copyright &y& Elsevier]

The induction of a cholesteric phase by doping an achiral nematic liquid crystal with an enantiopure solute is a phenomenon that, as in all general supramolecular phenomena of chiral amplification, depends in a subtle way on intermolecular interactions. The micrometric helical deformation of the phase director in the cholesteric phase is generated by the interplay of anisotropy and chirality of probe-medium interactions. In the case of a flexible chiral dopant, the solvent can influence the twisting power in two ways, difficult to disentangle: it is responsible for the solute orientational order, an essential ingredient for the emergence of phase chirality; but also it can affect the dopant conformational distribution and then the chirality of the structures present in the solution. In this work we have investigated methyl phenyl sulfoxide, a flexible, chiral molecule that, when dissolved in different nematics, can produce cholesteric phases of opposite handedness. This peculiar, intriguing sensitivity to the environment makes MPS a suitable probe for a thorough investigation of the effects of solute-solvent interactions on chiral induction in liquid crystals. NMR experiments in various nematic solvents have been performed in addition to twisting power measurements. From the analysis of partially averaged ¹H-¹H and 13C-¹H dipolar couplings, the effects of solvent on solute conformation and orientational order are disentangled, and this information is combined with the modeling of the chirality of intermolecular interactions, within a molecular field theory. The integration of different techniques allows an unprecedented insight into the role of solvent in mediating the chirality transfer from molecule to phase. [ABSTRACT FROM AUTHOR]

The differences between experimental and calculated order parameters of polar solutes dissolved in zero electric field gradient nematic liquid crystal mixtures (magic mixtures) could be interpreted, in the context of a recently proposed model [1,2], as an evidence of small but detectable effects of solute polarity in the orientational mechanisms. [ABSTRACT FROM AUTHOR]

We present a theoretical study of the potentialities of the Inverse Laplace Transform when applied to Pulsed Gradient Spin Echo NMR data from water in lyotropic liquid crystalline mesophases. This mathematical application should be able to produce structural information about the studied systems by the determination of the self-diffusion coefficient distribution, giving in this way a significant fingerprint of the molecular organisation in lamellar and reverse hexagonal phases. The critical point of finding the best algorithm to treat the data has been faced: several numerical methods have been tested comparing the performances of the different algorithms with results predictable by theory. [ABSTRACT FROM AUTHOR]

For spin systems with N[sup I] and N[sup S] interacting nuclei of magnetogyric ratio γ[sub I] and γ[sub S] respectively, it is possible to obtain multiple quantum spectra characterized by changes in the magnetic numbers mX by N[sup I], (N[sup I]—1)…, 0 with X = I, S. The use of such heteronuclear MQ spectra as an aid in the automatic analysis of NMR spectra of both rigid and flexible molecules dissolved in liquid crystalline phases, is discussed. It has been found that when I = [sup 1]H and S = [sup 19]F, the decoupling efficiency of the standard multipulse sequences, developed and optimized for heteronuclear decoupling in isotropic liquids, depends on the spin system and also on the overall orientation of the molecule when orienting mesophases are involved. These facts impose limitations on the applicability of a procedure which, when the decoupled heteronuclear MQ spectrum is obtained, significantly reduces the computing requirements of the automated spectral analysis. [ABSTRACT FROM AUTHOR]

In this Letter, a general expression is derived for the conformational distribution function of a molecule dissolved in an anisotropic condensed fluid medium by combining an a priori model with the maximum entropy principle applied to treatment of liquid crystalline-NMR data. The recently proposed additive potential maximum entropy (APME) method is recovered as a special case, when the AP is chosen as the a priori model and the orientational order is low. [ABSTRACT FROM AUTHOR]

A correction related to the article "Publisher's Note: Rigid probe solutes in a smectic-A liquid crystal: An unconventional route to the latter's positional order parameters," that was published in the December 13, 2011 issue is presented.

• LXNMR is used to study chirality-dependent conformations and order parameters of a chiral solute. • Two different R and S conformational distributions of a flexible case test molecule have been found. • Different R , S order parameter distributions of flexible chiral case test molecule have been found. The unique power of Liquid Crystal NMR (LXNMR) in obtaining information about the conformational equilibrium of small solutes dissolved in orienting media can be effectively used to study chirality-related phenomena. In this work we have been able, for the first time in our knowledge, to fully resolve the two different conformational distributions for the R and S enantiomers of a case test molecule (the chiral flexible 4FMBOH: 4-F-α-methyl benzyl alcohol), dissolved in a typical chiral non-racemic aligning solvent, made of PBLG (poly- γ -benzyl- L -glutamate, a helical chiral polypeptide) co-dissolved in CDCl 3. The two enantiomers show significant differences in their conformational behaviors, due to the different solute–solvent orientational molecular interactions experienced by the R and S species. In our opinion, fundamental studies of this kind could pave the way for a better understanding of chirality-dependent molecular interactions, often involved in a number of important chemical processes (concerning pharmaceuticals, biochemistry, advanced materials, etc.) [ABSTRACT FROM AUTHOR]