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The isothermal crystallization kinetics of a set of bio-nanocomposites produced by in situ catalytic step growth polycondensation of adipic acid and 1,4-butanediol in the presence of Moroccan clay beidellite (BDT) organo-modified with hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide, CTA) was investigated and compared with that of the parent poly(butylene adipate) (PBA) matrices from which the clay had been extracted. In situ bio-nanocomposites had different contents (0-5 wt %) of CTA/BDT nanofillers characterized by different extents of organo-modification (CTA/BDT equivalent ratios from 0 to 5). The isothermal crystallization rates of the bio-nanocomposites and of the parent PBA matrices were investigated by differential scanning calorimetry (DSC) at 45, 40, and 37 °C and analyzed according to the Avrami model. The bionanocomposites with an intermediate (2 wt %) concentration of organoclays with a higher CTA/BDT ratio (3 and 5) showed the highest exfoliation degree, along with an increase in the crystallization rates, compared to those of the parent PBA matrices, which was larger than that in the other nanocomposites. The lack of a simple correlation between the nanoclay content/composition and crystallization kinetics was ascribed to the molecular mass, an additional variable for in situ bio-nanocomposites as compared to nanocomposites prepared by simple physical blending of nanoclays with a single polymer matrix. The specific contribution of the molecular mass to the crystallization kinetics was untangled from those of the organoclay content and CTA/BDT ratio by comparing each bio-nanocomposite with its parent polymer matrix. The crystallization rate of the nanocomposites was always found to reach a maximum within an intermediate range of molecular weights of the polymer matrix, a behavior previously reported only for pure polymers. Such differences in the crystallization rate of in situ bio-nanocomposites may affect the crystalline phase morphology and, in polymorphs such as in PBA, phase composition, with consequent effects on properties that may be of interest for specific applications.

Orientational order properties of two fluorinated nematogens, exhibiting a wide nematic range, have been investigated by means of optical methods, dielectric spectroscopy, and 13C NMR. 13C NMR spectra have been recorded by 1H-SPINAL decoupling CP techniques under both static and magic angle spinning conditions. The order parameters of the fluorinated aromatic fragments have been calculated by analyzing the 13C−19F dipolar couplings at different levels of approximations by means of a least-squares fitting procedure using geometrical parameters determined by DFT methods, eventually including empirical corrections for vibrations and anisotropic scalar couplings. The nematic order parameters determined from optical birefringence data, dielectric anisotropy, and NMR have been compared: their trends with temperature, analyzed by the Haller model and by more sophisticated ones, are very similar, even if the order parameters from optical birefringence are shifted to slightly higher values. The differences among ...

In this work, two selectively deuterium-labeled isotopomers of the (S)-2-methylbutyl- [4'-(4' '-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')-benzoyl)-propionyl)]-propionate (ZLL 7/), one labeled on the phenyl ring (ZLL 7/-phe-D2) and the other one on the biphenyl fragment (ZLL 7/-biphe-D2), have been investigated by deuterium NMR (DNMR) spectroscopy and other experimental techniques. These compounds possess the paraelectric SmA, the ferroelectric SmC, the antiferroelectric SmC(A), the re-entrant ferroelectric SmC(re), and the ferroelectric hexatic phases down to room temperature. The orientational ordering properties of the two labeled fragments have been determined by means of DNMR, and the mesophase behavior at two magnetic fields is discussed. In particular, the effect of the magnetic field on the supramolecular structure of the SmC and SmC(re) phases is commented. This study revealed to be useful to understand the structural and conformational properties of the ferroelectric/antiferroelectric/re-entrant/hexatic smectic phases. Mesomorphic properties, spontaneous tilt angle, polarization, and layer spacing have been studied for the labeled materials and compared with those obtained for the nonlabeled compound. The two self-consistent set of data, from optical and DNMR measurements and X-ray results, allow us to associate at the transition from the SmC to the SmC(A) phase a change of the molecular conformation.

The structure of the silk from cocoons of two common spiders, Araneus diadematus (family Araneidae) and Achaearanea tepidariorum (family Theridiidae) was investigated by means of (13)C solid state NMR and FT-IR spectroscopies. The combined use of these two techniques allowed us to highlight differences in the two samples. The cocoon silk of Achaearanea tepidariorum is essentially constituted by helical and beta-sheet structures, whereas that of Araneus diadematus shows a more complex structure, containing also beta-strands and beta-turns. Moreover, the former silk is essentially crystalline while the latter contains more mobile domains. The structural differences of the two cocoon silks are ascribed to the different habitat of the two species.

It is shown that the addition of 2 to 6 wt.% of a chiral dopant (CD) to a series of nematic copolymers (Pn) induces blue and cholesteric mesophases. The mesomorphic behaviour of various CD+Pn mixtures is investigated by polarized optical microscopy and the phase transition temperatures are determined by DSC measurements. Deuterium NMR spectra are recorded as a function of temperature within the mesophases and the spectral line shapes characteristic of different local orientational order parameters and orientational ordering processes of molecules in a magnetic field are observed depending on temperature, heating or cooling process, chiral dopant concentration, copolymer composition, and magnetic field strength.

In this work, a new procedure for the synthesis of the selectively deuterium-labeled chiral liquid crystal (+)-(S)-4-[4'-(1-methylheptyloxy)] biphenyl 4-(10-undecenyloxy)benzoate (11EB1M7) has been developed, aimed to thestudy of its structure, orientational ordering, and dynamic behavior by means of 2 H NMR. 11EB1M7 was investigated throughout its mesophasic range, formed by a very rich variety of chiral liquid crystalline phases including blue, cholesteric, TGBA*, and smectic C*. Two isotopomers were synthesized by labeling 11EB1M7 on either the phenyl or the biphenyl moiety of the mesogenic core. This allowed structural information (such as the tilt angle of the rigid fragments of the molecule and the angle between their para axes), orientational order parameters, and individual diffusional coefficients for the various overall and internal motions to be obtained from 2 H NMR spectra and relaxation measurements by applying suitable data analyses based on several theoretical models.

2 H NMR spectroscopy is employed to investi- gate the orientational order, molecular structure and phase transitions of the chiral smectic liquid crystal (S)-[4-(2- methylbutyl)phenyl]-4'-octylbiphenyl carboxylate (8BEF5), partially deuterated in both the chiral chain and the phenyl moiety. 8BEF5 shows a rich polymorphism; the following phases have been observed: Blue phase, N*, SmA, SmC*, SmI*, SmF*, SmJ* and SmG* phases. The tilt angle and the orientational order of the phenyl fragment has been investigated throughout the mesophasic range; the additive potential method has been applied in the SmA phase in order to obtain the order parameter of the average conformer of the whole molecule.

2H-NMR in three different magnetic fields and small angle X-ray scattering (SAXS) are employed to investigate the orientational order, molecular organisation and phase transitions of the chiral smectic liquid crystal (−)-(S)-[4-(2-methylbutyloxycarbonyl)phenyl] 4-n-heptyl-biphenyl-carboxylate (MBHB) suitably deuterated for 2H-NMR measurements. Unwinding of the ferroelectric chiral helix by high magnetic fields is observed and discussed. Tilt angle values are obtained with the two different techniques and compared. The results show the importance of molecular conformation for the structural properties of this mesogen.

2H-NMR spectroscopy is employed to investigate the orientational order, tilt angle and layers organization of the chiral smectic liquid crystal (-)-(S)-[4-(2-methylbutyloxy carbonyl) phenyl] 4-n-heptyl-biphenyl carboxylate-d8 (MBHB-d8) in the magnetic field. The electrooptical behaviour of this material has also been investigated. The results from these two different techniques show the importance of molecular conformation for the electric and structural properties of this mesogen.

2H NMR spectroscopy is employed to investigate the orientational order, molecular structure and phase transitions of the chiral smectic liquid crystal 1-methylheptyl 4'-(4-n-decyloxybenzoyloxy)biphenyl-4-carboxylate (10B1M7), showing smectic A, C* and J phases, as well as several sub-smectic C* phases. Two optically pure, differently deuteriated isotopomers have been purposely synthesized and studied.

2H-NMR spectroscopy is employed to investigate the orientational order in the neinutic and smectic A, B, and G phases formed by 4-(2-methylbutyl)phenyl-4′n-heptylbiphenyl-4-car-boxylate (7BEF5). The dynamics of the aromatic molecular core is studied in a temperature interval including the SA—SB transition, by the analysis of the deuterium Zeeman and Quad-rupo'. ar relaxation times with the rotational diffusion model.

A new method is described of deriving a distribution, P(x), in a set of variables, x, which is consistent with a set of data, A(x), and which is the least biased modification (maximizes entropy) of a constraining distribution, r(x). The method is demonstrated using nuclear spin dipolar couplings obtained for n‐alkanes dissolved in a liquid crystalline solvent. The constraining distributions are obtained from computer simulations of the molecular dynamics.

2H-NMR spectroscopy is employed to investigate the orientational order in the nematic and smectic A, B, and crystal G phases formed by 4-(2-methylbutyl)phenyl 4′n-heptylbiphenyl-4-carboxylate-d18. Evidence of the lock in the trans-conformation of the first two methylene groups of the n-heptyl chain is found in the Sb and G phases. The dynamics of the aromatic molecular core are studied in a temperature interval including the SA-SB transition, by the analysis of the deuterium Zeeman and Quadrupolar relaxation times using the rotational diffusion model. Hence the diffusion constants related to the molecular spinning and tumbling motions are evaluated.

The Pd complex Azpac, synthesised from the 4,4′-bis(hexyloxy) azoxybenzene mesogen, shows a nematic phase between 90° and 105°C. The present work concerns the study of the orientational ordering of the ligand and complex molecules in their respective nematic phases, by 2H NMR spectroscopy. To this aim, 4,4′-bis(hexyloxy)azoxybenzene, HL, has been specifically deuterated in the positions 3 and 5 of both the rings and the complex Azpac-d4 synthesised from it. Complex Azpac-d26 was also synthesised starting from the ligand fully deuterated in the alkoxy chains. The morphism of Azpac-d4 and Azpac-d26 was investigated by optical microscopy and differential scanning calorimetry. 2H NMR spectra were recorded on samples of HL-d4 , Azpac-d4 and Azpac-d26 in their mesophases as a function of temperature. By a procedure of best fitting of the experimental quadrupolar and dipolar splittings the order parameters and some structural data of the metallomesogen molecules were determined. The results are discusse...

A model of a mesogenic molecule, built up as a rigid anisotropic Gay–Berne site mimicking the aromatic core, connected to an array of isotropic sites mimicking a flexible chain, is proposed and tested in molecular dynamics calculations. Simulations have been performed on a system composed of 256 molecules with three different numbers of methylenic units in the chain, in order to explore the effect of chain length on static and dynamic properties. The systems are all at the same mass density and temperature and result in nematic liquid crystalline phases. The order parameters for various molecular fragments and the T1z nuclear magnetic resonance (NMR) relaxation times of deuterons are in agreement with previous molecular dynamics simulations on atomistic systems and, at least qualitatively, with 2H‐NMR experimental results. The intermolecular interactions are always dominated by the anisotropic site simulating the molecular core. The influence of the phase order on the chain static and dynamic properties is put in evidence. Extensions of the model are suggested in order to have a better reproduction of the dynamical features of such systems.A model of a mesogenic molecule, built up as a rigid anisotropic Gay–Berne site mimicking the aromatic core, connected to an array of isotropic sites mimicking a flexible chain, is proposed and tested in molecular dynamics calculations. Simulations have been performed on a system composed of 256 molecules with three different numbers of methylenic units in the chain, in order to explore the effect of chain length on static and dynamic properties. The systems are all at the same mass density and temperature and result in nematic liquid crystalline phases. The order parameters for various molecular fragments and the T1z nuclear magnetic resonance (NMR) relaxation times of deuterons are in agreement with previous molecular dynamics simulations on atomistic systems and, at least qualitatively, with 2H‐NMR experimental results. The intermolecular interactions are always dominated by the anisotropic site simulating the molecular core. The influence of the phase order on the chain static and dynamic properties i...

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 [1.9] sigmatropic rearrangement has been studied by Dynamic NMR spectroscopy for two 2-acyloxytropone mesogens and for 2-acetyloxytropone. The proton spectra of the compounds in CDCI3 have been recorded in the range 213-328 K and the rate constants for the dynamic process determined by line-shape analysis. Evidences of the same rearrangement were found in the 2H NMR spectra of the three molecules in the nematic phase E63 between 293 and 350 K. Values of the rate constants were yielded also by 2D-exchange spectra in the slow rate limit (300-320 K). The trends of the kinetic constants against temperature, analyzed by the Eyring equation, give the activation parameters of the rearrangement. The activation energies in the nematic solvent are slightly higher than the corresponding ones in CDCI3. By extrapolating these results, we can infer a range of values for the frequency of the process in the liquid-crystalline phases of compounds I and II.

The orientational order parameters, Szz and Biax, of UB-d4 [1,4-bis(undec-10-en-1-yloxy)benzene-d4 ] in the ZLI1167 nematic mixture are studied by means of 2H NMR at variable temperature. The Szz(T) trend fairly follows the Maier–Saupe theoretical curve; therefore, UB-d4 can be considered an excellent probe to monitor the ordering of the nematic phase itself, provided that Biax is not neglected and the complete analysis of quadrupolar and dipolar splittings is performed. UB-d4 is a typical crosslinker in liquid crystal elastomers (LCEs): the order parameters here collected help us to discuss the peculiarities of the crosslinker as a probe for the study of internal order in LCEs. From the analysis of our data and of data in the literature, we infer that the order parameter Biax of the crosslinker in the LCE matrix is low enough to be neglected without severely perturbing the determination of . The crosslinker UB-d4 is a reliable probe to monitor the nematic order degree inside the LSCE-UB-d4 sample.

Deuterium decoupled proton spectra of the chain region of a partially deuteriated liquid crystal have been taken. The liquid crystal is a trimethylene chain with perdeuteriated cyanobiphenyloxy groups on either end: 1,3‐bis(4,4’‐cyanobiphenyloxy‐d8) propane (BCBO3‐d16). The spectra have been analyzed to give the dipolar couplings within the propyl chain at various temperatures in the nematic phase of the liquid crystal. These couplings are the most sensitive measurements of molecular flexibility that have yet been taken of an alkyl chain in a pure liquid crystal molecule and give a unique insight into rotational potentials of the flexible component essential to a liquid crystal molecule. The couplings have been analyzed using a maximum entropy method designed for a two‐rotor system; in our case the two C–C bonds in the propyl chain. The analysis was also performed by adding to the data set the deuterium quadrupolar splittings of the chain hydrogen atoms, previously measured on the same molecule but deuteriated in the chain. The method picks out the flattest distribution consistent with the data. This also happens to be the most likely distribution given only the data used in the analysis and no other constraints. The results for BCBO3‐d16 show a probability maximum for a partially eclipsed conformation (twisted approximately 86° in opposite directions about each C–C bond from the all‐trans conformation), a smaller maximum for the all‐trans conformation itself, and no significant probability for any of the gauche conformations. The possible motions, that are suggested by these results, therefore, are concerted librations (±86°) between the all‐trans position and the partially eclipsed conformations, and full (360°) rotations. These results are in stark contrast to the assumptions made in the rotational isomeric state (RIS) model that the probability distribution can be well approximated by constraining the molecules to exist only in trans and/or gauche states with fast isomerization between such states. In fact the analysis rules out a rotational isomeric state distribution altogether, since our data could not be reproduced by restricting the distribution in this way. Instead it is shown that other distributions are not only possible, but also more likely than any kind of constrained trans–gauche model.

In this study we combined QM calculations and NMR measurements to understand at a detailed level the complex interplay of structural/electronic properties with the effects of the solvent in the NLO activity of push-pull systems, quantified in terms of variations of the static hyperpolarizability. Different parameters (bond lengths and bond length alternation, vibrational frequencies, electronic charge distribution) are introduced and tested to rationalize both the solvent sensitivity of three molecular systems (namely, p-nitroaniline, ethyl 4-ammino benzoate, and 5-nitro-1H-indole) and the differences among them. This analysis has finally allowed us to establish a clear correlation between the charge transfer behavior of the systems, their NLO properties, and NMR parameters also validating simplified but effective chemical analyses based on resonance limit forms.

The orientational order and dynamics of perylene in the nematic discotic phase (NdII) of the potassium laurate/KCl/decanol/water system has been investigated by fluorescence depolarization (FD) spectroscopy. The order parameters of perylene-d 12 in the same phase had been previously determined by 2H NMR. FD experiments were performed with the phase-modulation technique in the temperature range between 10°C and 50°C. At each temperature we could determine and for the axis of the transition moments of perylene, together with the rotational correlation time ΘR. All these quantities assume reasonable values and show regular trends as functions of temperature. In particular, is in good agreement with that determined by 2H NMR.

The deuterium Zeeman spin-lattice relaxation times TIZ in 4,4′-di-n-heptylazobenzene (HB), 4,4′-di-n-heptylazoxybenzene (HAB), and 4,4′-di n-octylazoxybenzene (OAB) deuterated on the aromatic rings and on the first methylene group of the chains were measured at 10 and 46.04 MHz by means of the inversion-recovery technique. Measurements were made throughout the mesomorphic range and also in the isotropic phase. The relaxation rates show a discontinuity at the isotropic-nematic transition for all deuterons not ascribable to the onset of orientational order. The trends of InR are linear within the different phases except near phase transitions, and allow to obtain apparent activation energies for the overall motions of the different C—D bonds.

The orientational order of 4-n-hexyloxybenzilidene-4′-fluoroaniline, (FAB-OC6) and its 4-n-heptyloxy analogue, (FAB-OC7), studied by means of 2H NMR and 19F NMR is reported. The entire temperature range of the liquid-crystalline phases (SB, SA and N for FAB-OC6, SB and SA for FAB-OC7) have been investigated. The results are discussed in relation to previous X-ray diffraction measurements. The chemical shift tensor components of the fluorine bond, evaluated from the 19F NMR spectra are compared with previous results.

Following the optimum Rouse-Zimm local dynamics (ORZLD) model, high values of the orientation persistence length, p, for the backbone chain have been determined for both polymers (p= 39 and 42 monomeric units, respectively), while the order continuously and substantially decreases along the lateral chains

Cocoons produced by different strains of Bombyx mori larvae were investigated by a combination of several high- and low-resolution 1H and 13C solid-state NMR techniques in order to characterize and compare their dynamic behavior at a molecular level. A detailed interpretation in terms of molecular motions in these very complex systems was possible thanks to the integrated analysis of different relaxation measurements and high-resolution selective experiments. Untreated cocoons of all strains were found to be mainly constituted by two different types of rigid domains and by a third one, more mobile, due to physisorbed water molecules. Dynamic processes in the MHz and kHz ranges were characterized by means of different 1H and 13C relaxation times. Cocoons arising from different strains exhibit a different content of physisorbed water and also slightly different dynamic behavior, especially in the MHz regime.

In this article we report a dynamics investigation of two chiral smectogens, 1-methylheptyl 4‘-(4‘ ‘-n-decyloxybenzoyloxy)biphenyl-4-carboxylate (10B1M7) and [4-(2-methylbutyloxycarbonyl) phenyl] 4...

The [1,9] sigmatropic rearrangement has been studied by Dynamic NMR spectroscopy for a 2-benzoyloxytropone mesogen. The proton spectra of the compound in CDCI3 and C6D6 as isotropic solvents have been recorded in the ranges 238-328 K and 277-343 K, respectively, and the rate constants for the dynamic process have been determined by line shape analysis. One-dimensional 2H NMR spectra of the compound in its own mesophases and in the nematic phase E63, recorded between 300-330 K and 298-353 K respectively, show that the exchange process is in the slow limit for this kind of spectra and allow the study of orientational order. The values of the rate constants in anisotropic environments were yielded by the analysis of two-dimensional exchange 2H NMR spectra in the range 310-330 K for the pure compound and 328-348 K for the compound in E63. The trends of the kinetic constants against temperature, analyzed by the Eyring equation, give the activation parameters of the rearrangement. The activation enthal...

We have investigated the mesomorphic behaviour of a partially deuteriated side chain LC polymethacrylate, poly[4-[6-methacryloyloxyhexyl-oxy]-4′-methoxyazobenzene], by means of 2H NMR, DSC and optical microscopy. Evidence for a phase transition occurring in the middle of the nematic phase has been found.

The dipolar couplings observed by NMR spectroscopy of solutes in nematic solvents (LX-NMR) are used to build up the maximum entropy (ME) probability distribution function of the variables describing the orientational and internal motion of the molecule. The ME conformational distributions of 2,2′- and 3,3′-dithiophene and 2,2′:5′,2″-terthiophene (α-terthienyl)thus obtained are compared with the results of previous studies. The 2,2′- and 3,3′-dithiophene molecules exhibit equilibria among cisoid and transoid forms; the probability maxima correspond to planar and twisted conformers for 2,2′- or 3,3′-dithiophene, respectively, 2,2′:5′,2″-Terthiophene has two internal degrees of freedom; the ME approach indicates that the trans, trans and cis, trans planar conformations are the most probable. The correlation between the two intramolecular rotations is also discussed.

The analysis of interproton dipolar couplings derived by 1H-NMR spectra of molecules dissolved in partially oriented nematic solvents (LX-NMR) is a powerful tool for accurate structure determination in liquid phase. A characterization of the distribution of the internal variables in non-rigid molecules has been one of the central goals of LX-NMR. This kind of study appears particularly relevant when considering that the mesogens themselves are often composed of molecules with one or several internal degrees of freedom. The application of the principle of Maximum Entropy (ME) has been shown able to give the least biased rotameric distribution for one internal degree of freedom. This method is now extended to the problem of two internal rotations and applied to analyse the dipolar couplings of ethoxybenzene. A bidimensional probability distribution is thus obtained, that is in agreement with preliminary findings of a ab initio calculation on the molecule.

The statistical principle of maximum entropy is applied to the analysis of dipolar couplings from 1H NMR of nonrigid molecules dissolved in liquid‐crystalline phases. A distribution function for the orientational and inter‐ring angles is so obtained. The most probable internal angle φ is determined for 4,4’‐dichlorobiphenyl (φ=34°) in the nematic phase of I52, 4‐pentyl‐4’‐cyanobiphenyl (φ=32°) and 4’‐Br‐4‐Cl‐2,6‐difluorobiphenyl (φ=42°) in EBBA. The physical reliability of the distributions determined is discussed. The maximum‐entropy treatment seems to indicate a limit for the information on the internal motion obtainable from the experimental data.

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.

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 problem of obtaining information on the molecular properties of non-cylindrically symmetric solute molcules in liquid crystals is examined. Nitrobenzene, tetrafluoro-p-benzoquinone and pyridine have been investigated in various nematic solvents at a series of temperatures using proton, fluorine and deuterium NMR where relevant. The ordering matrices for these essentially rigid molecules have been determined as a function of temperature. The predictions of a mean field theory for solutes deviating from cylindrical symmetry are considered. The relative importance of polarizability, steric forces and specific interactions in determining orientational ordering is discussed.

The 1H NMR spectra of isoxazole including 13C satellites in natural abundance have been studied in two nematic solvents: nematic phase IV and ZLI 1167. The molecular structure corrected for harmonic vibrations (rβstructure) has been obtained in both phases and compared with a microwave “best structure”. The apparent geometric deformations observed in the phase IV solvent are discussed.

Deuterium N.M.R. spectroscopy has been used to investigate the orientational ordering of perdeuteriated naphthalene and two perdeuteriated fluorescent probes (pyrene and perylene) dissolved in a discotic nematic micellar phase of potassium laurate/potassium chloride/decanol/water. The results show that the ordering tensors of these probes are not cylindrically symmetric.

The orientational ordering in the three related compounds 4,4-di-n-heptyl-d2-azobenzene-d, (HB-d12), 4,4′-di-n-heptyl-d2-azoxybenzene-d, (HAB-d12) and 4,4-di-n-octyl-d2-azoxybenzene-d8 (OAB-d12) has been monitored using deuteron N.M.R. spectroscopy. In these non-rigid molecules it is possible to obtain only the elements of local order matrices. For the aromatic rings these are found to be axially symmetric within the precision of our measurements. The temperature dependences of the local order parameters for the aromatic rings are compared with those predicted by McMillan's theory of smectic A phases.

The 13C n.m.r. spectrum and the 13C satellites in the 19F n.m.r. spectrum of tetrafluoro-1,4-benzoquinone (TFBQ) dissolved in CDCl3 solution have been recorded and analysed. The partially oriented spectra determine the sign of at least one J19F –19F coupling constant. By assuming that the sign of the one bond J13C –19F coupling is negative, the signs of the remaining J19F –19F and of three of the five J13C –19F have been determined from the isotropic spectra. 19F n.m.r. spectra of TFBQ have been successively obtained in various liquid-crystal solvents: (I)trans-4-heptyl-(4-cyanophenyl) cyclohexane; (II) Nematic Phase IV (Merck); (III) ZLI Nematic Phase (Merck); (IV) 4,4′-bis(heptyl) azoxybenzene; (V) a lyotropic mesophase formed by poly(γ-benzyl-L-glutamate) in CD2Cl2 solution. Some of these solvents [(II) and (IV)] have been shown to form a charge-transfer complex with TFBQ. For solutions (I) and (II) the 13C satellites of the oriented spectra have also been obtained. The dipolar direct couplings, Tij, corrected for the effect of molecular vibrations, were used to obtained the rα structures which have been compared with the electron-diffraction structure and with the solid-state X-ray structures of both complexed and uncomplexed TFBQ. Finally, the chemical shielding anisotropy has been investigated in solutions (II), (IV) and (V). The quantity Δσ=[σaa–½(σbb+σcc)] has been evaluated and compared with the value from solid-state n.m.r. measurements.

The n.m.r. spectra of [13CH3]thioanisole, 4-chloro[13CH3]thioanisole and 4-nitro[13CH3]thioanisole as solutes in liquid crystal solvents have been analysed. The results are used to test models of the potential function V(ϕ) describing the reorientation of the SCH3 group about the phenyl–S bond. It is concluded that the molecules cannot be entirely in the forms with all the heavy atoms coplanar. The data are shown to be consistent with two models for V(ϕ), one with a single minimum between ϕ 0 and 90° and the other having minima of different values at ϕ 0 and 90°. The latter model, a mixture of planar and orthogonal forms, is considered to be the more probable and gives the percentage in the planar form to be between 84 and 50% for thioanisole and between 87 and 78% for 4-nitrothioanisole.

Solute–solvent interactions have been shown to exit by means of NMR of dissolved solutes both in lyotropic and thermotropic liquid crystalline solutions [1, 2]. We mention, for example, the effects of solvent-induced molecular distortions on tetrahedral molecules [3, 4] and specific solute–solvent interactions in acetylene, methyl-fluoride and a variety of other small molecules [5]. As far as the last molecules are concerned, apparent geometrical distortions arise whenever the interacting nuclei are associated with small values of order parameters (Sij) giving small values of direct dipolar couplings (Dij). The observed dramatic geometrical effects have been explained by admitting a two-site (or multi-site) exchange mechanism within the ordered solvent, each site having its own geometry and orientation [2, 5]. We have recently found, however, that this kind of solute–solvent interactions is confined by no means to small molecules and the results concerning partially oriented spectra of two types of medium sized molecules will be discussed: 1. Substituted anilines: 3,5-dichloro and 3,5-dibromoaniline; 2,4,6-trichloroaniline; 2. Heterocyclic compounds: selenophene; tellurophene; isoxazole. In the case of molecules b), in particular, it has been possible, by means of FT techniques, to obtain 13C1H, 77Se1H and125Te1H dipolar couplings which, after correction for the effect of molecular vibrations, allow a satisfactory determination of molecular geometry, so that a meaningful comparison wit Microwave (M.W.) findings can be performed. The geometrical incongruencies invariably found in the accurate treatments of overdetermined systems in liquid crystal mesophases suggest that solvent induced distortions and in particular multisite exchange are quite a common phenomenon in the NMR of thermotropic mesophases.

The 1 H NMR spectra and 13 C satellites of nitrobenzene and nitrobenzene- 15 N in PCH and ZLI nematic phases, respectively, were recorded and analyzed. The dipolar couplings obtained, corrected for the harmonic vibration, yielded two detailed r α structures, coinciding within the experimental error limits ( r CH ortho = 1.087 A , r CH meta = 1.062 A , r CH para = 1.089 A ). The 1 H and 2 H NMR spectra were then recorded on two solutions containing both nitrobenzene and nitrobenzene- 2 H 5 in the PCH and ZLI nematic phases, respectively. The two sets of order parameters for nitrobenzene obtained from the 1 H spectra were assumed to describe also the orientation of nitrobenzene- 2 H 5 in the two solutions. The three pairs of quadrupolar splittings for the three deuterated positions yielded by the deuterium spectra were used, together with the r α structure and with the order parameters, to determine the quadrupolar coupling constants and the asymmetry parameters of the three deuterons ( 2 H ortho : DQCC = 209 ± 2 kHz , η = −0.06 ± 0.01; 2 H meta : DQCC = 183 ± 3 kHz , η = 0.06 ± 0.02; 2 H para : DQCC = 179 ± 4 kHz , η = 0.08 ± 0.04 ). The results for the ortho deuteron deviate strongly from the usual values for aromatic CD bonds. Some possible explanations are suggested.

The proton NMR spectrum of a sample of ω-fluoroacetophenone dissolved in a nematic liquid crystalline solvent has been obtained and analysed to yield a set of 10 independent dipolar couplings. These have been used to obtain a conformational distribution, PLC(ϕ,ψ), for the bond rotational angles ϕ, which is for the C–CF bond, and ψ, which is for the ring–C bond. The distribution has a maximum at ϕ = 0°, which corresponds to a cis arrangement of the C–F and CO bonds, and ψ = 0 or 180°, which refer to the CO bond being in the plane of the phenyl ring. There is a smaller peak in the distribution at ϕ = 180°, corresponding to the trans arrangement of the C–F and CO bonds, and ψ = 0 or 180°. The barrier to rotation about the C–CF bond through ϕ depends strongly on ψ, and vice versa.

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(φ) obtained, where φ is the angle between the two ring normals. This is compared with Pmol(φ), the distribution calculated for an isolated molecule, and it is found that the positions, φmax 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(φ) 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(φ) from the simulated Dij with good precision at all the tempe...