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The alignment of biphenyl (2P) in the liquid-crystal phases of 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-octyl-4'-cyanobiphenyl (8CB) is investigated by using a combination of predictive atomistic molecular dynamics (MD) simulations and (1)H liquid-crystal nuclear magnetic resonance (LXNMR) residual dipolar coupling measurements. A detailed comparison and validation of the MD results with LXNMR is provided, showing a good agreement between the simulated and experimental dipolar couplings at the same reduced temperature. MD is then used to examine the location of 2P in the smectic phase, which is unavailable to LXNMR, and 2P is found to be rather uniformly distributed. The combination of MD and NMR spectroscopy provides detailed information about the order, interconnection between orientation and conformation, local positional order, and interactions with the liquid-crystalline solvent.

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 quadrup...

Many discotic mesogens are molecules with a central aromatic ring with adjacent alkylcarboxylate substituents. The simplest such molecule, 1,2‐dihydroxydiacetylbenzene, which is not mesogenic, is studied by NMR spectroscopy as a solute in a nematic solvent. The spectra are analysed to give sets of residual dipolar couplings, Dij , which are then used to test models for the conformation adopted by the acetate side groups. The conformations and geometry of an isolated molecule are calculated by the ab initio MP2/6‐311G method and also by the DFT approach using the B3LYP functional with the 6‐311++G** basis set. The quantum chemical calculations find that the minimum energy conformer has the acetate groups rotated in opposite directions out of the ring plane, and this kind of structure is also consistent with the NMR data.

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.

The proton NMR spectrum of the doubly enriched acetophenone-carbonyl,methyl-13C2 isotopomer dissolved in a liquid-crystalline solvent (LXNMR) was analyzed to yield a data set of 19 dipolar couplings. The presence of so many couplings, and in particular the dependence of some of them on the acetyl carbons enabled the investigation of the structure of the acetyl moiety and of possible cooperative motions about the aryl-carbonyl and carbonyl-methyl bonds. Methodological aspects, and approximations relating to the application of the vibrational correction procedure in the presence of large-amplitude torsional motions, are discussed. Results show that it is possible to discriminate between a continuous and a discrete conformer distribution about the angle phi(1) but not among a few proposed continuous shapes of U(iso)({phi}). In this study, the use of dipolar couplings with a non-negligible contribution from the indirect spin-spin coupling tensor J, (D(C8C9) in our case), for structural determination is extended from rigid to flexible molecules. The 1/2J(aniso)(C8C9) contribution was derived theoretically using the density functional theory linear response (DFT-LR) first-principles calculation of the J(C8C9) spin-spin coupling tensor.

The proton NMR spectra of samples of 2-thiophenecarboxalde-hyde dissolved in a nematic liquid crystalline solvent, including those from all five singly labelled C-13 isotopomers, have been obtained. These have been analysed to yield sets of partially averaged dipolar couplings which have been used to determine the structure and the relative amounts of the cis and trans forms, which are the two minimum-energy structures generated by rotation about the ring-aldehyde bond. A procedure for applying vibrational corrections to the dipolar couplings in the presence of large amplitude motions is discussed.

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.

We have analysed a set of NMR dipolar couplings of ethylbenzene (Ph–CH2–CH3) as solute in the liquid crystalline solvent ZLI1132. The analysis has been carried out by means of the maximum entropy (ME) method in the unconstrained formulation. The unconstrained ME technique confirms recent ab initio results for ethylbenzene: besides the well established absolute maximum of the probability density in relation to the ethyl group perpendicular to the phenyl ring, we have found a relative shallow maximum when the ethyl group is in the plane of the aromatic ring. This contrasts with other previously reported studies which considered this conformation as the transition state. The similarity between ab initio and ME results has prompted the proposal of a constrained maximum entropy method as a tentative one for combining the information from the liquid crystal NMR spectroscopy (LXNMR) technique with knowledge of the conformations of the isolated molecule coming from ab initio calculations found in the literature. Due to lack of a parameterised function describing the torsional motion of the ethyl group around the phenyl ring, we have tested the method for the case of the benzyl chloride dissolved in ZLI1132. We can see that, whereas the unconstrained ME method strongly depends on the degree of the orientational order of the molecule, in agreement with the general principle of maximum entropy, the constrained ME method gives a peaked distribution function even if that order is low. Detailed theoretical calculations of the entire potential energy surface of the crucial molecule of ethylbenzene seem to be highly requested in order to be used as an input in the constrained ME method. The latter could be useful in the study of solvent effects.

The orientational order parameters of four aromatic solutes in nematic solvents, derived from proton dipolar couplings in NMR spectra, show a correlation with the dielectric anisotropy of the solvents. Explanation of the experimental behaviour is provided by a molecular theory based on the reaction field approximation.

The orientational data of four rigid solutes dissolved in the nematic solvents ZLI1132, EBBA and in a 55wt% ZLI1132 + EBBA mixture have been obtained from proton NMR spectra at different temperatures and used to find a relationship between the solute order in the two nematics and in their mixture. A crossed comparison of the data shows that a simple linear relation, where the normalized weights of the combination are coincident with the molar fractions of ZLI1132 and EBBA in the mixture, gives excellent agreement independently of the solutes and temperatures. Consideration is given to the possibility of a generalization of the results to any mixture of two nematics.

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.

Previous attempts to analyze the proton spectrum of ethylbenzene as a solute in nematic liquid crystalline solvents failed, but a successful strategy has now been devised and is described here. The proton spectra of samples of ethylbenzene dissolved in four different liquid crystals have been analyzed to yield sets of the partially-averaged dipolar couplings, Dij. The couplings are then used to test models for the structure and conformation of this molecule.

It is demonstrated that the NMR spectra of liquid crystalline samples can be simplified by using multiple quantum filtering. In a system of N spin- 1 2 nuclei, the N or (N-1)-multiple quantum filtered spectra (NQF or (N-1)QF) contain lines which originate only from transitions among the eigenstates belonging to the highest symmetry class of the spin permutation group. In addition the NQF spectra are divided further into two sets of lines which differ in phase by 180°. A method for simulating and analysing multiple quantum filtered spectra is described, with examples from molecules with up to eight interacting spins.

A partially-deuteriated sample of the nematogen 4-n-pentyl-4′-cyanobiphenyl has been synthesized which contains six protons, four in an aromatic ring and two on the attached methylene group. The pr...

The conformation of the ethyl group relative to the phenyl ring plane in 4-chloroethylbenzene has been investigated by recording and analysing the proton NMR spectrum of a sample dissolved in a nematic liquid crystalline solvent. The dipolar couplings obtained are compared with values calculated by the additive potential (AP) and maximum entropy (ME) molecular mean field theoretical models. In both cases good agreement between observed and calculated couplings could be obtained only by adopting a model for rotation about the ring-C bond which allows for geometry relaxation. The ME method produces a conformational distribution with an absolute maximum at 90° and a secondary maximum at 0, which is also the shape found to give the best fit to the data by the AP method. However, constraining the shape of the conformational distribution to that found by the molecular orbital calculations and using the AP method yields an acceptable, if not the best, fit to the dipolar couplings, and so is the preferred solutio...

The 1H, 2H and 13C NMR spectra of phenyl acetate, phenyl acetate-[13CO] and phenyl acetate-[C2H3] dissolved in a nematic liquid crystalline solvent have been analysed to yield dipolar coupling, D ij. These have been interpreted using the additive potential model to provide information on the molecular conformation, resulting in three possible shapes for V(φ), the potential energy for rotation about the ring-oxygen bond. A comparison with the results of molecular orbital calculations leads to the conclusion that a potential with a minimum at 54.4° ± 0.1° is the most probable.

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...

Arenediazonium salts [p-YC6H4N2][BF4] (YH, CH3, OCH3, F, NO2) react with the A-frame precursor complexes [Pd2X2(dppm)2] (1) (XCl, Br, I; dppmbis(diphenylphosphino)methane) affording 1:1 adducts. 1H and 31P NMR spectroscopic data support the formulation of the new complexes as A-frame molecules [Pd2X2(dppm)2(μ-N2-p-C6H4Y)][BF4] (2) containing a symmetrically bridging arenediazenido ligand. The crystal structures of [Pd2I2(dppm)2(μ-N2-p-C6H4CH3)][BF4] (2d) and [Pd2Cl2(dppm)2(μ-N2-p-C6H4F)][BF4] (2f) have been determined by X-ray crystallography. 2d crystallizes in the orthorhombic space group P212121 with Z=4 in a unit cell of dimensions a=14.642(4), b=19.166(5), c=21.338(6) A. The structure was refined to R=0.068 and Rw=0.073 on the basis of 4325 observed reflections with I>3σ(I). 2f crystallizes in the tetragonal space group P43212 with a=14.886(1), c=25.303(8) A and Z=4. The structure was refined to R=0.039 and Rw=0.043 using 4681 unique reflections and 353 parameters. The formation of [PtPdCl2(dppm)2(μ-N2-p-C6H4F)][BF4] (4) and its solution structure is also described.

Dipolar couplings obtained previously by analyzing the proton NMR spectra of samples dissolved in nematic solvents of thioanisole and 4-chloroseleno anisole were interpreted in terms of jumps between symmetry-related forms for the motion about the ring-X and X-CH3 bonds. Two models were found to fit the data: jumps between four positions of minimum energy having the X-CH3 bond rotated through ϕ about 40° out of the ring plane, or jumps between planar and orthogonal structures with unequal weightings. The proton and fluorine dipolar couplings for 2,3,5,6-tetrafluoro anisole were interpreted previously in terms of free rotation about the ring—O bond, but with the assumption that the orientational order parameters are independent of the rotational angle ϕ. The data for all three molecules is re-interpreted here, plus new data for thioanisole dissolved in a second nematic solvent, using a theoretical model which allows for the dependence of the orientational ordering on ϕ, thus permitting continuous potentials to be tested, and which also involves fewer variable parameters. This constraining of the model leads to a rejection of the non-planar positions for the minimum in the rotational potential, but leaves two other possible solutions: a two-fold continuous potential for rotation about the ring-X bond with a minimum when the C-X bond is in the ring plane and a maximum when it is at 90°, or a four fold continuous potential with a secondary minimum at 90°. In the case of the tetrafluoro anisole the two-fold potential corresponds to virtually free rotation.

The experimental linewidth of the hydrogen NMR spectrum of benzene dissolved in a nematic solvent (Merck PCH) was analyzed. Experiments performed at different time intervals show that a contribution to spectral linewidth, increasing with spectral frequency, disappears from sample preparation after some months. The residual linewidth can be very well explained in terms of an intermolecular relaxation mechanism.

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°.

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 forms for this potential. The 24 structures of equal, minimum energy, which are generated by rotation about the inter ring bond, the phenyl-O bond and the O—CH3 bond, all have the same shape, and hence each of these conformations has the same ordering matrix, with principal elements Szz and Sxx — Syy . The magnitudes of Sxx and Sxx — Syy have been obtained, together with the orientation of x, y and z with respect to a frame fixed in the methoxylated ring, with the assumption that only these minimum energy structures are populated. The magnitudes of these principal order parameters are used to test the form of a potential of mean torque.

The conformational equilibrium of 1,3-butadiene in a condensed fluid phase is investigated by liquid-crystal NMR spectroscopy. The full set of D HH and D CH dipolar couplings is determined from the analysis of the 1 H spectra of the three 1,3-butadiene most-abundant isotopomers (i.e. the all 12 C and the two single-labeled 13 C isotopomers) for a total of 21 independent dipolar couplings. A very good starting set of spectral parameters for the analysis of the 1 H spectrum is determined in a semiautomated way by the analysis of the (N-1) (specifically, N = 6, the number of 1/2 spin nuclei in the spin system) quantum refocused (5QR), and not (5Q), spectra. As an alternative approach, a Monte Carlo (MC) numerical simulation, capable of predicting the solute ordering, is tested to simulate the 5QR spectrum. The set of D ij couplings is very good, proving that the MC method can represent a novel, valid alternative to the existing spectral simplification procedures. The experimentally determined dipolar-coupling data set is fully compatible with the 1,3-butadiene conformational distribution reported in the literature for isolated molecules, indicating the presence of about 99% of s-trans conformer. With regards to the remaining 1 %, in spite of the direct and very strong dependence of the observables on the molecular structure, it was not possible to discriminate between the planar s-cis and s-gauche forms, both of which produce a very good fit of the dipolar couplings. Vibrational corrections, up to the anharmonic term, were applied; the calculated geometrical parameters are in good - although not exact - agreement with those reported in the literature from experimental and theoretical investigations. This result can be considered as supporting the methodology used for obtaining the structure and conformational distribution of a flexible molecule in a liquid phase.

Partially averaged dipolar couplings (also referred to as residual dipolar couplings) D-ij 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 $^{13}C$ nuclei. The geometrical parameters and the rotational potential obtained are compared with the results from a density-functional theory method.

The (1)H NMR spectrum of a sample of acrolein dissolved in the nematic liquid crystal phase I52 has been analysed to yield 18 dipolar couplings between all the magnetic nuclei in the molecule; moreover, the (13)C and (13)C{(1)H} NMR spectra of a sample of acrolein in CDCl(3) were recorded and analysed to determine the indirect J(ij) couplings. The data were used to obtain the relative positions of the carbon and hydrogen atoms, assuming that these are independent of the conformations generated by rotation around the C--C bond through an angle phi, and to obtain a probability distribution P(phi). It has been found that in the liquid phase, the distribution is a maximum at the trans form whereas the abundance of the cis form is significantly smaller compared with that found by microwave spectroscopy or high level quantum mechanical calculations. Such calculations produced also a suitable force field needed to develop suitable strategies for vibrational correction procedure in the case of flexible molecules.

We have obtained a very large data set of spectral parameters from the analysis of (1)H NMR and (13)C satellite spectra of 2,2'-bithiophene dissolved in anisotropic, partially orienting mesophases. In particular, this parameter set includes 33 dipolar couplings, which are directly related to the interatomic distances, the dihedral angle phi between the two thiophenic rings, and the anisotropic solute-solvent interaction potential. This allows an exhaustive investigation of the conformational equilibrium of 2,2'-bithiophene in a liquidlike phase. Comparison with the predictions of high-level theoretical calculations for the isolated molecule provides evidence of a strong flattening as well as the sharpening effect of the medium on the conformer population. The approximations needed to apply vibrational corrections to flexible molecules are discussed in detail and some general conclusions concerning their effect on structure and conformational equilibria are proposed.

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 study of the conformational distribution of “flexible” molecules (i.e., molecules displaying large-amplitude, low-frequency torsional motions) in the liquid phase represents a fundamental subject, from the point of view of both theory (e.g., molecular modeling) and applications (e.g., therapeutic properties). In this context, NMR spectroscopy of molecules dissolved in liquid-crystalline solvents is unique, since no other technique can give such a large amount of experimental information.

The deuterium decoupled, proton nuclear magnetic resonance spectrum of a sample of diphenylmethane-d(3) dissolved in a nematic liquid crystalline solvent has been analyzed to yield a set of dipolar couplings, D-ij. These have been used to test models for the conformational distribution generated by rotation about the two ring-CH2 bonds through angles tau(1) and tau(2). Conformational distributions, particularly when obtained from a quantum chemistry calculation, are usually described in terms of the potential energy surface, V(tau(1),tau(2)), which is then used to define a probability density distribution, P(tau(1),tau(2)). It is shown here that when attempting to obtain P(tau(1),tau(2)) from experimental data it can be an advantage to do this directly without going through the intermediate step of trying to characterize V(tau(1,)tau(2)). When applied to diphenylmethane this method shows that the dipolar couplings are consistent with a conformational distribution centered on tau(1)=tau(2)=56.5+/-0.5degrees, which is close to the values calculated for an isolated molecule of 57.0degrees, and significantly different from the asymmetric structure found in the crystalline state.

For spin systems with N I and N S interacting nuclei of magnetogyric ratio γI and γS respectively, it is possible to obtain multiple quantum spectra characterized by changes in the magnetic numbers mX by N I, (N 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 = 1H and S = 19F, 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.

The NMR spectra of C6H5CH2X (X=Cl, Br, I) dissolved in three different nematic liquid crystal solvents were analysed to investigate, using the AP-ELS model, the internal and external potentials acting on the solutes. The nematics [ZLI1132, I35 and a zero electric field gradient mixture (55% ZLI1132:EBBA)] were selected to compare the effects depending on the nature of the solvents on both potentials. The internal potential for the rotation around the exocyclic C—C bond shows minima located at γ4=±90° (C—X bond lies in a plane perpendicular to the ring) and torsional barrier increasing with the steric hindrance of the halide (from X=Cl to X=I) and with the polarity of the solvent (I35

The maximum entropy (ME) and additive potential (AP) methods of determining the angular distribution functions, p(ω,χ), from the partially-averaged dipolar couplings obtained from the NMR spectra of liquid crystalline samples are compared. Here ω represents the orientation of the mesophase director in a molecular frame, and χ represents bond rotational motion. It is emphasized that these two methods are fundamentally different. Thus, the model-independent ME analysis can determine only p LC (ω,χ) and p LC (χ), which are dependent on the potential of mean torque, U ext (ω,χ), and the subscript LC denotes that these distributions are for the liquid-crystalline phase

The (1)H, (19)F and (13)C spectra have been obtained of a sample of peri-difluoronaphthalene dissolved in the nematic liquid crystalline solvent ZLI 1695. The (13)C satellite spectra from the six, single-(13)C isotopomers at natural abundance in both the (1)H and (19)F spectra were identified and analysed to yield a set of residual total, anisotropic spin-spin couplings, T(ij). This was achieved by first obtaining residual (13)C-(19)F and (13)C-(1)H couplings from a proton-encoded, (13)C detected, local field 2D spectrum. The 45 values of T(HH), T(HF) and T(CH) were used to obtain the structure of the molecule, and then to estimate whether there is a significant contribution from the component along the magnetic field, J, of the anisotropic, electron-mediated, spin-spin coupling tensors for (13)C-(19)F and (19)F-(19)F pairs. It is found that there is strong evidence for a significant contribution of J to T(FF) but not for the (13)C-(19)F pairs.

The structure of 4-cyanobiphenyl when dissolved in nematic liquid-crystalline solvents has been investigated. This has been achieved by recording and analysing the proton and deuterium NMR spectra of samples dissolved in the nematic solvents. The dipolar couplings obtained are compared with values calculated by averaging over the bond rotational motion, and the conclusion is reached that the minimum-energy structure has an angle between the two ring normals of 37.0 ± 0.1° in each of three solvents, compared with a value for biphenyl, obtained previously by the same method, of 37.4 ± 0.2°.

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.

The proton NMR spectra of selenophene together with the 77Se and 13C satellites have been recorded both in isotropic CDCl3 solution and in a thermotropic nematic phase. Analysis of the spectra yielded HH, 13CH and 77SeH direct dipolar couplings which have been corrected for the effect of molecular vibrations according to the procedure proposed by Lucas and used to obtain a complete rα molecular structure. A comparison of the NMR interatomic distance ratios with those from microwave (mw) measurements is made. The significant distortion of the molecule in the nematic liquid crystalline phase, with respect to the mw structure, is discussed.

Different models for conformational equilibrium of 2,2′-biselenophene have been examined in order to reproduce the D H H and D Se H direct dipolar couplings obtained by NMR spectra using liquid crystal solvents. The results allow the rigorous exclusion of various conformational possibilities and confine the conformation to an equilibrium between two twisted conformers, the transoid one being the more abundant.

1 H N.m.r. spectra of [14N]- and [15N]-pyridine partially oriented in nematic phase IV in the presence of iodine show dipolar couplings (Dij) increasing linearly as a function of iodine concentration. The spectral changes induced by iodine can be rationalized, to a first approximation, by assuming that the pyridine molecule is in equilibrium, in the oriented phase, with the pyridine–iodine charge transfer complex so that a considerable variation in the mean orientation of the pyridine moiety occurs. Attempts to reproduce the measured dipolar couplings of the exchanging complex on the basis of an average geometry and orientation afforded only dramatic distortion effects on the apparent geometry. Looking, moreover, at the geometry of [15N]pyridine in phase IV and in other liquid crystals geometrical distortions were also found, probably due to solute–solvent interactions, except for the ZLI 1167 phase, where an rα structure in close agreement with gas-phase microwave measurements was observed. The results suggest that, for pyridine in the nematic phase in the presence of iodine, at least a three-site mechanism is operating, one of the sites being the complexed pyridine.

The proton N.M.R. spectra of samples of ethoxy-d 3-benzene and 4-fluoro-ethoxy-d 3-benzene dissolved in the nematic solvent I52 have been recorded and analysed to yield sets of dipolar coupling [Dtilde]ij. The dipolar couplings are used together with the theoretical model of Emsley, Luckhurst and Stockley to obtain V(φ) the potential governing rotation about the O-CH2 bond, and the dependence on φ of the orientational order parameters S αβ.

The dipolar couplings obtained previously for samples of benzyl chloride dissolved in liquid crystal solvents are re-interpreted in terms of a theoretical model which allows for the dependence of orientational ordering on molecular shape. The shape of the potential function V(θ) governing rotation about the phenyl-C bond is derived and found to be similar in form to those obtained by electron diffraction and ab initio molecular orbital calculations.

The NMR spectra of 3-phenylthiophene as solute in liquid-crystal solvents were analyzed and the direct couplings constants, D ij obtained were used to test different models of the potential energy function V (φ) for intramolecular rotation of the phenyl and thiophene units. The data are consistent with a V(φ) having a minimum at about 24°. The LCNMR technique is highly discriminant among different models of V(φ) provided that enough D ij couplings are available. Ab initio calculations are also reported.

The proton N.M.R. spectrum of a sample of 3,5-dibromoethylbenzene dissolved in a liquid-crystalline solvent has been recorded and analysed to yield a set of partially-averaged dipolar couplings, Dij . The Dij are used to test models for the nature of the rotational potential V(ϕ) of the ethyl group about the ring-C bond. It is concluded that V(ϕ) has a minimum at 60°, which contrasts with the predictions of an ab initio molecular orbital calculation for ethyl benzene. The MO calculations find a minimum in V(ϕ) at 90°, which corresponds to the ethyl C-C bond being in a plane orthogonal to the plane of the phenyl ring.

The proton and fluorine spectra of samples of benzyl fluoride dissolved in two nematic solvents (ZLI 1132 and I52) have been analysed to yield sets of dipolar couplings [Dtilde]ij between the interacting nuclei. These were compared with values calculated by a theoretical model which allows for the dependence of orientational order on the angle that the C-F bond makes with the plane of the phenyl ring. The observed and calculated [Dtilde]ij are in excellent agreement, and the potential for rotation about the C-CH2F bond is found to have a minimum when the C-F bond is in the plane at 90° to the phenyl ring, but the barrier height changes from 0·3 ± 0·02 kJ mol-1 in I52 to 1·5 ± 0·08 kJ mol-1 for ZLI 1132 as solvent.

The complete r α structure of tellurophene has been determined from proton magnetic resonance spectra, including 13C and 125Te satellites, obtained in three thermotropic nematic mesophases. The molecular structures observed in the different solvents have been compared both to each other and with microwave data, and discussed with respect to possible solute-solvent interactions. Isotropic P.M.R. spectra and the relative heteronuclear sub-spectra have also been analysed in order to obtain all the Jij indirect coupling constants. Absolute signs were determined for 2 J TeH and 3 J TeH. The anisotropic contribution to the Te-H indirect couplings was found to be negligible.

The conformation of ethylene glycol (EG) has been investigated by means of liquid crystal nuclear magnetic resonance spectroscopy (LICRY-N.M.R.). The proton and 13C N.M.R. spectra were obtained from a quarternary solution of potassium laurate (KL), decanol, water and EG. These spectra were analysed to get the proton-proton and proton-carbon direct dipolar couplings. Spectra obtained from an aqueous solution have also been analysed. The experimental results were then used to investigate the conformational equilibrium of the molecule. It has been found that in the lyotropic liquid crystalline solution EG exists in a gauche conformation only, and that the value of the OC-CO dihedral angle is 72°.