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Norbornadiene dissolved in three different nematic mesophases are studied by liquid crystal NMR, to contribute to a better understanding of the influence of solvents on the solute's ordering and structure. The ordering is compared with that calculated using a Monte Carlo numerical simulation.

Partially averaged dipolar couplings (also referred to as residual dipolar couplings) Dij can be obtained from the analysis of the NMR spectra of molecules dissolved in liquid-crystalline solvents. Their values for a nonrigid molecule depend upon the bond lengths and angles, the rotational potentials, and the orientational order of the molecules. The molecule studied, 1-chloro-2-bromoethane, is one of the simplest example of a substituted alkane in which the rotational potential has three minimum-energy positions, trans and gauche±conformations, and the present investigation explores the problems inherent in deriving the form of the potential and the molecular geometry from the set of partially averaged couplings between the protons, and between protons and 13C nuclei. The geometrical parameters and the rotational potential obtained are compared with the results from a density-functional theory method. [ABSTRACT FROM AUTHOR]

The deuterium decoupled, proton nuclear magnetic resonance spectrum of a sample of diphenylmethane-d[SUB3] dissolved in a nematic liquid crystalline solvent has been analyzed to yield a set of dipolar couplings, D[SUBij]. These have been used to test models for the conformational distribution generated by rotation about the two ring-CH[SUB2] bonds through angles τ[SUB1] and τ[SUB2]. Conformational distributions, particularly when obtained from a quantum chemistry calculation, are usually described in terms of the potential energy surface, V(τ[SUB1], τ[SUB2]), which is then used to define a probability density distribution, P(τ[SUB1], τ[SUB2]. It is shown here that when attempting to obtain P(τ[SUB1], τ[SUB2] from experimental data it can be an advantage to do this directly without going through the intermediate step of trying to characterize V(τ[SUB1], τ[SUB2]). When applied to diphenylmethane this method shows that the dipolar couplings are consistent with a conformational distribution centered on τ[SUB1] = τ[SUB2] = 56.5 ± 0.5°, which is close to the values calculated for an isolated molecule of 57.0°, and significantly different from the asymmetric structure found in the crystalline state. [ABSTRACT FROM AUTHOR]

A molecular dynamics simulation has been carried out of biphenyl dissolved in a solvent comprised of rigid particles interacting with the Gay–Berne potential. The solution is investigated in isotropic, nematic, and smectic phases, and the probability distribution, PLC([lowercase_phi_synonym]) obtained, where [lowercase_phi_synonym] is the angle between the two ring normals. This is compared with Pmol([lowercase_phi_synonym]), the distribution calculated for an isolated molecule, and it is found that the positions, [lowercase_phi_synonym]max of the maxima of the two distributions differ by about 2°. The molecular dynamics trajectory is used to calculate averaged nuclear spin dipolar couplings, Dij, and these are used to test the maximum entropy (ME) and additive potential (AP) theoretical models which have been used previously to obtain the distribution PLC([lowercase_phi_synonym]) from dipolar couplings obtained on real solutions of biphenyl in liquid crystalline solvents. It is concluded that the AP method is able to recover the true distribution PLC([lowercase_phi_synonym]) from the simulated Dij with good precision at all the temperatures studied, whereas the ME method achieves good precision only when the orientational order is high. The AP method also succeeds in obtaining the correct difference between PLC([lowercase_phi_synonym]) and Piso([lowercase_phi_synonym]), the distribution for an isotropic phase at the same temperature. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Nine well studied ( polar and non-polar) planar solutes have been used as probes to investigate the mechanism for orientational ordering in the nematic phase. Second-rank orientational order parameters have been derived from proton dipolar coupling constants, obtained from the NMR spectra of these molecules dissolved in a 55 wt% ZLI1132 + EBBA nematic mixture. The experimental data are interpreted in terms of an orienting potential experienced by molecules in a liquid crystal environment. A model for the orienting potential is presented which includes, in addition to short-range repulsion interactions described by the surface tensor model, long-range contributions treated according to a generalized reaction field approach. Least-squares fits of solvent dependent parameters to the experimental data have been performed to estimate the weight of the various contributions to the solute orientational order, and the results obtained in this way are discussed. Finally, the results obtained with the surface tensor model are compared with those from the size-shape potential suggested by other workers. [ABSTRACT FROM AUTHOR]

In this work, a sample of pure and certified bergamot essential oil (BEO) was extensively studied for the first time directly by NMR spectroscopy with the aim of investigating its metabolic composition, quantifying the main components of this complex natural matrix and simultaneously assessing whether the NMR technique is able to highlight possible frauds to which this high-cost product may be subjected. Eleven low molecular weight compounds have been identified by using 1D 1 H and 13 C-{ 1 H} NMR experiments, 2D homo- and heteronuclear correlation NMR spectra, and 2D 1 H DOSY experiments; the most abundant of them, i.e., about 90% of the sample analyzed, has been quantified by employing benzoic acid as an internal standard by 1 H NMR spectrum. Moreover, since the commercial fraud of this precious oil is often due to the addition of less expensive oils, we have simulated a possible adulteration through the preparation of BEO samples to which different percentages of orange essential oil (OEO) were added. The results, obtained by combining the 1 H NMR spectra collected on the adulterated samples and on pure BEO, with chemometric analysis, principal component analysis (PCA), indicate that it is possible to distinguish the sample of pure BEO from the adulterated ones and also, among them, to differentiate between the degrees of adulteration., (© 2022. Springer-Verlag GmbH Germany, part of Springer Nature.)

Using Monte Carlo numerical simulation, this work sketches the phase diagram of systems of certain hard C_{2h}-symmetric particles, formed by gluing two aligned and displaced hard spherocylinders with a cylindrical-length-to-diameter ratio realistically, if viewed not only from the lyotropic colloidal liquid-crystal side but also from the thermotropic low-molecular-mass liquid-crystal side, equal to 5, as a function of the displacement. Several distinctive phases are observed, such as a nonperiodic smectic-B-like phase, a nonperiodic smectic-H-like phase, a smectic-C phase, and a short-layer-spacing uniaxial smectic-A phase but no biaxial nematic phase.

The unique power of NMR spectroscopy in anisotropic media (LX-NMR) as a tool to obtain local and bulk structural information, combined with the effectiveness of molecular dynamics simulations at the atomistic level, shows very attractive potentialities for the study of interesting, even though still poorly understood, materials such as Ionic Liquid Crystals (ILCs). In this work, we focused our attention, in particular, on the orientational ordering of two mesophases: 1-dodecyl-3-methylimidazolium chloride, [C12C1im]Cl, and 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C12C1im][BF4]. Both ILCs were studied by a 2H NMR direct investigation of the molecules forming the phases, suitably deuterated, and by 1H NMR spectroscopy, using the small rigid probe-solutes 1,4-dichlorobenzene (DCB), dissolved in [C12C1im][BF4] and [C12C1im]Cl, and 1,4-dibromobenzene (DBB) dissolved in [C12C1im][BF4], to probe the local, internal structure and organization of the mesophases. The experimental results were then compared with the predictions, by atomistic MD simulations, of the structure of the smectic phase of the two salts, at two selected temperatures, containing a single DCB molecule as a probe. The MD simulations show that the DCB solute is distributed only within the hydrophobic layers of the ILC. Orientational order parameters of the imidazolium cations and of the DCB molecule were obtained and compared with the experiments, showing a general good agreement and allowing a deeper understanding of the microscopic structure of the systems.

Detailing the conformational equilibria between global and local minimum energy structures of anti-inflammatory α-arylpropionic acids directly in solution is of the utmost importance for a better understanding of the structure-activity relationships, hence providing valuable clues for rational structure-based drug design studies. Here the conformational preferences of the widely used pharmaceutical ibuprofen were investigated in solution by NMR spectroscopy in weakly ordering phases. A thorough theoretical treatment of the anisotropic interactions that are relevant for NMR spectra led to a conformational model characterized by six pairs of symmetry-related conformers, in particular four couples of gauche structures, with a total probability of 93%, and 2 couples of trans structures, counting for the remaining 7%., (Copyright © 2017 Elsevier B.V. All rights reserved.)

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., (Copyright © 2016 Elsevier Inc. All rights reserved.)

The conformational equilibrium of methyl 4-nitrophenyl sulfoxide (MNPSO) was experimentally investigated in the gas phase by using microwave spectroscopy and in isotropic and nematic liquid-crystal solutions, in which the solvents are nonaqueous and aprotic, by using NMR spectroscopy; moreover, it was theoretically studied in vacuo and in solution at different levels of theory. The overall set of results indicates a significant dependence of the solute conformational distribution on the solvent dielectric permittivity constant: when dissolved in low-polarity media, the most stable conformation of MNPSO proved to be strongly twisted with respect to that in more polar solvents, in which the conformational distribution maximum essentially coincides with that obtained in the gas phase. We discuss a possible explanation of this behavior, which rests on electrostatic solute-solvent interactions and is supported by calculations of the solute electric dipole moment as a function of the torsional angle. This function shows that the least polar conformation of MNPSO is located at a twist angle close to that of the conformational distribution maximum found in less-polar solvents. This fact, associated with a relatively flat torsional potential, can justify the stabilization of the twisted conformation by the less-polar solvents., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The structural and conformational elucidation of flexible bioactive molecules in solution is currently a crucial goal for the scientific community, but it is rarely achievable by available techniques. The anti-inflammatory drug diflunisal is presented here as a model case for supporting the efficiency of NMR spectroscopy combined with the use of weakly ordering media as a promising methodology for the conformational investigation of small bioactive molecules. Starting from NMR anisotropic data (40 independent dipolar couplings), a quite accurate description of its torsional distribution around the inter-ring C-C bond was found, characterized by a pair of two couples of conformers. According to the relative configuration of the carboxylic group and the fluorine atom in the ortho position to the inter-ring C-C bond, the more stable couple of conformers are defined as "trans" type conformers (F opposite to the carboxylic group) whereas the less stable couple are "cis" type conformers (F and carboxylic group on the same side). In order to study the influence of fluorine nuclei on the structure and conformational distribution, the same analytical strategy has been applied to investigate the phenylsalicylic acid, its nonfluorinated analogue.