Giuseppina De Luca

A comparison of proton‐detected 13C local field experiments with deuterium NMR at natural abundance for studying liquid crystals

A comparison is made between the information which can be derived on structure, conformation and orientational order of the molecules in a nematic liquid crystal 4‐pentyl‐4′‐cyanobiphenyl (5CB) from the NMR spectra of the deuterium atoms at natural abundance (NAD NMR), and the two‐dimensional proton‐detected 13C local field experiment (PDLF). The nine residual quadrupolar splittings, Δν k , obtained experimentally have been compared with quadrupolar tensors and a geometry and conformational potentials calculated by the DFT method B3LYP/6‐311G**. The PDLF experiment yielded 42 scaled 13C–1H residual dipolar couplings, kD CiHj . The scaling factor, k, is determined experimentally by comparing unscaled and scaled residual dipolar couplings in a sample of fluorobenzene dissolved in a nematic liquid crystalline solvent. The corrected residual dipolar couplings, D CiHj , are used to investigate the structure and rotational potentials about each bond in the molecule.

Structure of biphenyl in a nematic liquid-crystalline solvent

The proton NMR spectra of biphenyl dissolved in two nematic liquid-crystalline solvents have been analysed to yield 12 independent dipolar coupling constants, Dij. The Dij have been used to obtain the probability distribution, Pnem(ϕ), for the angle ϕ between the ring normals. Two methods of relating Dij to Pnem(ϕ) have been used: the ME method, which invokes the maximum-entropy principle, and the AP method, which is based on the construction of a potential of mean torque from an addition of contributions from the rigid subunits in a molecule. The value of ϕ when Pnem(ϕ) is a maximum is found to be essentially independent of the solvent, and is determined to be 34 ± 1° by the ME method, and 37.2 ± 0.1° by the AP method.

Is styrene planar in liquid phases

The proton NMR spectra of two (13)C-labeled isotopomers of styrene dissolved in two liquid crystalline solvents have been obtained and analyzed to yield four sets each of 24 dipolar couplings. These couplings were then used to investigate the structure of the ring and the ene fragments of the molecule, and the position of the maximum, phi(0), in the ring-ene bond rotational probability distribution. To do this, the effect on the dipolar couplings of small-amplitude vibrational motion was taken into account using vibrational wave functions calculated by molecular orbital and density functional methods. It is concluded that the NMR data are consistent with the ring fragment, averaged over the ring-ene rotation, planar, while the ene fragment is not. The value of phi(0) is found to be 18.0 degrees +/-0.2 degrees for the two solutions, compared with a value of 27 degrees calculated by the molecular method MP2/6-31G(*).

The orientational behaviour of 1,4-difluorobenzene and p-benzoquinone in ZLI1132 and EBBA nematic solvents

Abstract The non-polar rigid molecules 1,4-difluorobenzene and p -benzoquinone dissolved in ZLI1132 and EBBA nematic solvents show a bizarre ‘ double face ’ rod-like or disk-like orientational behaviour, depending on the used solvent. In the present Letter it is shown that the observed effect, unpredictable on the basis of solely short-range orientating mechanisms, can be qualitatively accounted when also electrostatic long-range interactions are taken into account (in particular, the coupling between the solute quadrupole moment and the solvent electric field gradient). The obtained result confirms the non-negligible role played by electrostatic interactions, even in the case of non-polar molecules.

The conformation and orientational order of a 1,2-disubstituted ethane nematogenic molecule (I22) in liquid crystalline and isotropic phases studied by NMR spectroscopy

The structure, conformation and orientational order of the mesogen I22 have been studied by proton, carbon-13 and deuterium 1D and 2D-NMR spectroscopies at natural abundance and at various magnetic fields when in the nematic phase, the isotropic phase close to the nematic-isotropic phase transition, and as a solute in the chiral nematic solution comprised of the polypeptide PBLG dissolved in chloroform. It is concluded that 95% of conformers have a trans arrangement about the central C-C bond of the ethane fragment in all phases.

The temperature dependence of biaxiality of solutes dissolved in nematic solvents

The predictions of several models present in the literature, using the common second rank two-terms spherical harmonics expansion to express the mean-torque potential experienced by molecules dissolved in nematic solvents, are often contradicted by experimental evidence of temperature dependence of the solute orientational biaxiality (related to the ratio between the coefficients of the expansion). In this paper, from orientational data obtained by proton NMR technique for four rigid solutes dissolved in six nematic solvents, a linear dependence of the biaxiality parameter against the reduced temperature is observed in a central region of temperatures (quite far from phase transitions). The results are rationalized relating the slopes of the curves to the sign and magnitude of the dielectric anisotropies of the solvents.

The conformational behaviour of naproxen and flurbiprofen in solution by NMR spectroscopy

Naproxen and flurbiprofen are among the most popular and widely used drugs for treating pain and inflammation and new different therapeutic uses are still being proposed. Their pharmacological activity comes from the specific binding to target proteins and is evidently driven by the spatial arrangement they adopt at the active sites. A detailed conformational elucidation analysis in solution able to probe equilibria between more possible conformers is then crucial for understanding and rationalizing their biological role and possibly for designing improved analogues. Here we combine NMR data obtained in weakly ordered media and suitable theoretical models to look for the energetically admissible conformers of these two anti-inflammatory drugs in a liquid medium. This approach leads to rather complex probability distributions characterized for both molecules by an equilibrium of different global and local minimum energy structures.

Smectic order parameters via liquid crystal NMR spectroscopy: Application to a partial bilayer smectic A phase

AbstractSolute molecules were dissolved in the liquid crystal 4-cyano-4′-n-octyloxybiphenyl (8OCB), known to form a partial bilayer smectic-A phase. Through measurement of solutes’ and solvent’s orientational order parameters via nuclear magnetic resonance spectroscopy, and their analysis via a statistical thermodynamic density functional theory, values of the solvent’s positional order parameters and solutes’ positional-orientational distribution functions were obtained. Near to the transition to the nematic phase, the main positional order parameter of the smectic liquid crystal turned out to be comprised in the interval 0.4–0.6, though the quality of the fittings assuming the phase as nematic all across the temperature range investigated was only slightly worse. This may be ascribed to the looseness of the partial bilayer smectic structure. Solutes were found to preferentially lie in those regions where liquid crystal molecule terminal chains are located.

Calculated Versus “Experimental” Force Fields: The Influence in the Structure Determination of Benzene by NMR Spectroscopy in Liquid Crystal Solvents

Very accurate bond distances can be derived from dipolar couplings, D ij , 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 D ij is tested on benzene for which a very accurate structural study is available.

Alignment of druglike compounds in lipid bilayers analyzed by solid-state (19)F-NMR and molecular dynamics, based on dipolar couplings of adjacent CF3 groups.

Solid-state (19)F-NMR spectroscopy is frequently used to analyze the structure and dynamics of lipophilic drugs and peptides embedded in biomembranes. The homonuclear dipolar couplings of trifluoromethyl (CF3) labels can provide valuable parameters such as orientational constraints and/or distances. To characterize the complex dipolar patterns of multiple (19)F spin interactions, three different model compounds carrying two CF3 groups in meta-position on a phenyl ring were incorporated in macroscopically aligned DMPC bilayers. The dipolar patterns obtained with the CPMG (Carr-Purcell-Meiboom-Gill) multipulse sequence were analyzed to yield simultaneously the intra-CF3 and intergroup dipolar coupling values. The fluorine-fluorine distances were predicted by a density functional calculation, and the alignment of the labeled molecular segment could be determined from these distances and the dipolar coupling values. The different compounds were found to align in the lipid bilayer according to their amphiphilic properties, though with a weak anisotropic preference that is typical of solutes in liquid crystals. The residual dipolar couplings were used to calculate Saupe order parameters. For the least complex molecule, (CF3)2-BA, an orientational probability function for the solute in the lipid matrix could be derived. The overall description of how (CF3)2-BA is embedded in the bilayer was independently assessed by molecular dynamics simulations, and compared in structural and dynamical terms with the results of the NMR experiments.