Giorgio Celebre

Graphical Interactive Strategy for the Analysis of NMR Spectra in Liquid Crystalline Phases

Spectral analysis of lH NMR spectra of molecules dissolved in liquid crystalline phases to obtain spectral parameters (chemical shifts, Jv indirect and Di, direct couplings) is usually a difficult and time-consuming task due to the peculiar characteristic of this kind of spectra. A procedure that links together a simulation/ iteration program with graphic routines has been developed to be run on a Vax cluster. The procedure has proved to be very useful for the analysis of spectra due to molecules containing up to 11 interacting nuclei, needing reasonably low CPU times for the simulation/iteration step and providing an interactive, friendly graphic environment for such tasks as spectrum display and line assignment, whose importance increases quickly with the number of interacting nuclei.

The conformational distribution in diphenylmethane determined by nuclear magnetic resonance spectroscopy of a sample dissolved in a nematic liquid crystalline solvent

The deuterium decoupled, proton nuclear magnetic resonance spectrum of a sample of diphenylmethane-d3 dissolved in a nematic liquid crystalline solvent has been analyzed to yield a set of dipolar couplings, Dij. These have been used to test models for the conformational distribution generated by rotation about the two ring-CH2 bonds through angles τ1 and τ2. Conformational distributions, particularly when obtained from a quantum chemistry calculation, are usually described in terms of the potential energy surface, V(τ1,τ2), which is then used to define a probability density distribution, P(τ1,τ2). It is shown here that when attempting to obtain P(τ1,τ2) from experimental data it can be an advantage to do this directly without going through the intermediate step of trying to characterize V(τ1,τ2). When applied to diphenylmethane this method shows that the dipolar couplings are consistent with a conformational distribution centered on τ1=τ2=56.5±0.5°, which is close to the values calculated for an isolated ...

The angle of twist between the two phenyl rings in the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl

Abstract The NMR spectrum of the protons in the biphenyl group of 4-n-pentyl-4′-cyanobiphenyl (5CB), was obtained by Sinton et al. (1984, Molec, Phys., 53, 333) and analysed by them to give an angle of twist, φmin, between the normals to the two phenyl rings of 30 ± 2°. Their analysis made the assumption that V(φ), the potential for rotation about the inter ring bond, is such that only the structures with the minimum energy need be considered when calculating averaged dipolar couplings. Re-analysis of their data by a method which allows for the whole of V(φ) to be sampled when averaging the dipolar couplings yields a value for φmin of 38.4 ± 0.1°.

A comparison of the structure, flexibility and mesogenic properties of 4-methoxy-4′-cyanobiphenyl and the α,α,α-trifluorinated derivative

Abstract The compound 4-cyano-4′-(α,α,α-trifluoromethoxy)biphenyl (1OCBF3) has been synthesized. Unlike the fully protonated analogue, 4-cyano-4′-methoxybiphenyl (1OCB), it does not show a liquid crystalline phase on cooling from the melting point (51°C) to room temperature. The transition temperature to a monotropic nematic phase was obtained as approximately 0°C by determining the transition temperatures of mixtures with 1OCB. The structures, conformational properties and orientational ordering of both 1OCB and 1OCBF3 as solutes in a nematic solvent ZLI 1132 have been investigated via the 17 dipolar couplings obtained by analysing the proton and fluorine NMR spectra of these solutions. It is concluded that the major difference between the two molecules lies in the potential, V(φ2), governing rotation about the ring–oxygen bonds. In 1OCB the potential has the same form as in anisole, with a minimum when the C–O bond is in the plane of the attached ring (φ2 = 0°), and a maximum of about 15 kJ mol−1 when φ...

The Structure and Orientational Ordering of 4−Methoxy−4'−Cyanobiphenyl in the Nematic Mesophase

Abstract The proton N.M.R. spectra of two partially deuteriated samples of the mesogen 4-methoxy-4′-cyanobiphenyl (1-OCB) have been obtained. The spectra were simplified by decoupling the interactions between protons and deuterons and analysed to yield sets of dipolar couplings, [Dtilde]ij . The [Dtilde]ij are used to investigate the angle φ between the two ring normals and the conformation of the methoxy group relative to the attached phenyl ring. If the potential governing rotation, V(φ), is assumed to have a deep minimum such that only structures with the minimum value of φ are appreciably populated, then φmin is determined to be 30·4° ± 0·4°. However, adopting a continuous form for V(φ) changes φmin to 36°. For the methoxy group the data are consistent with the potential governing rotation about the ring-oxygen bond being zero when the carbon atom is in the ring plane and infinite otherwise; in addition, we have investigated the consequences for the structure and ordering of adopting more realistic fo...

Exploiting the information content of dipolar couplings: determination of the temperature dependence of the inter-ring twist angle of biphenyl dissolved in uniaxial mesophases

In the present work, the quite intriguing question of the temperature dependence of the inter-ring angle of biphenyl in fluid condensed phases has been investigated by the proton liquid crystal nuclear magnetic resonance technique. The spectra of the molecule dissolved in three different thermotropic uniaxial solvents (one of which shows also a smectic A phase) at different values of temperature have been analysed by standard procedures, and the resulting temperature-dependent dipolar coupling sets have been rationalised by the additive potential for the treatment of the ordering interactions method, combined with the direct probability description of the torsional curve, in order to obtain the distribution of the twist angle φ for each temperature. The results emphasise a very slight but unequivocal and systematic increase of φM (the most probable value of the dihedral angle) with temperature, so qualitatively confirming what was previously found by a cruder hybrid approach (based on experimental quadrupolar splittings of perdeuteriated biphenyl combined with computer-simulated order parameters) carried out by two of the present authors (Celebre, G.; De Luca, G.; Mazzone, G. J. Mol. Struct. (THEOCHEM) 2005, 728, 209–214).

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(*).

On the anisotropic intermolecular potential of biaxial apolar solutes in nematic solvents: Monte Carlo predictions and experimental data

Recently, a new formulation has been proposed about a strictly short-range anisotropic potential acting on biaxial apolar particles dissolved in a uniaxial medium [Chem. Phys. Lett. 342, 375 (2001)], where the solute-solvent interactions are treated at a molecular level and the solute order parameters are calculated by making use of the Monte Carlo–Metropolis sampling scheme. In the present paper the cited model has been used for the study of 1,4-difluorobenzene, 1,4-dichlorobenzene, and 1,4-dibromobenzene molecules and the simulated order parameters have been compared with the 1H-NMR experimental data for the solutes in the nematic solvents ZLI1132 (a Merck commercial eutectic mixture of alkylcyclohexylcyanobenzenes and alkylcyclohexylcyanobiphenyl), EBBA [the N-(4-ethoxybenzylidene)-4′-n-butylaniline], and in the zero average electric field gradient nematic mixture 55 wt% ZLI1132+EBBA (the so-called “magic” mixture). The orientations predicted by the model match almost perfectly the experimental Saupe m...

Short- and long-range contributions to the ordering of rigid planar solutes dissolved in a 55wt% ZLI1132+ EBBA nematic mixture

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...