Search

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.

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.

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 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.

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.

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 (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 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 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.

An NMR lineshape study of pyridine dissolved in nematic PCH mesophase has been carried out. Measurements of spectral frequencies and intensities enabled us to determine the order parameters defining the molecular orientation. Lineshape analysis indicates that the highly selective broadening of lines is mainly due to the coupling of protons with the rapidly relaxing 14N quadrupolar nucleus. A further contribution to the line width comes from the dipolar intermolecular relaxation mechanism. The quadrupolar relaxation time of the 14N nucleus and the correlation time of the intermolecular interaction were calculated.

The ring mobility of some 1,5-benzodiazepine-2,4-diones and 1,4-benzodiazepin-2-one has been investigated by the temperature dependence of their 1H NMR spectra. The activation parameters have been obtained, and the influence of N-1 substitution on the ring mobility is discussed, taking into account previously reported data for similar compounds.

Spectra of benzyl chloride were analysed in two nematic phases (PCH and ZLI). The existence of the molecule as conformer with the CCl bond perpendicular or parallel to the benzene plane can be safely excluded. The data are consistent with a conformer with the CCl bond forming an angle of 60° with the benzene plane. A full discussion of the conformational problem was not possible owing to the small number of independent Dij couplings related to the internal motion. In addition, when dealing with low potential barriers, the LCNMR technique does not give rigorous answers.

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.

Line width in high resolution NMR spectra can be affected by several relaxation mechanisms.

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.

A new strategy is described for analyzing the NMR spectra of molecules dissolved in liquid-crystalline phases. The multiple-quantum spectra which involve a change in the magnetic quantum number m by N − 1, where N is the maximum possible, are recorded and analyzed automatically starting from a parameter set having all the spectral parameters set to zero. The results of such an analysis are then used as the starting parameters in an analysis of the single-quantum spectrum using an automatic analysis procedure. The method is illustrated by analyses of the proton spectra of bromobenzene (five spins) and ethynylbenzene and naphthaquinone (six spins) dissolved in nematic solvents.

The 1 H and 13 C NMR spectra of anisole-methyl- 13 C dissolved in a nematic solvent ZLI 1115 have been analyzed. The set of dipolar couplings obtained is used to test various models for the internal rotations about the ring-O and O-CH 3 bonds. It is shown that the data are not consistent with the simplest models for this motion: a 2-fold jump about the ring-O bond between planar structures or a four-site jump between nonplanar structures, in both cases with a three-site jump model for motion about the O-CH 3 bond

The n.m.r. spectra of 3-(1), 4-(2), and 5-phenylisoxazole (3) partially oriented in the nematic phase of a liquid crystalline solvent have been studied. A twisted conformation around the bond connecting the two rings is preferred, with a twist angle of ca. 18° for compounds (1) and (3) and ca. 20° for compound (2). Reorientation of these molecules is correlated with the internal rotation, while the direction of the principal order axis, with respect to a reference axis system fixed on the phenyl ring, depends on molecular shape.

Proton and proton–{deuterium} spectra of 4-[2H3]acetylpyridine have been recorded on nematic solutions and analysed to yield dipolar coupling constants, Dij. The deuterium spectrum yields the quardrupole splitting, Δν, which together with the Dij values was used to predict all the dipolar couplings in the spectrum of the fully protonated compound. This procedure enabled an analysis of the complex proton spectrum of 4-acetylpyridine in the same nematic solutions. It is demonstrated that the data from analysis of either the CD3 labelled, or from the fully protonated compound can be used to investigate the structure of the pyridine ring and the conformational preference of the acetyl group.

The proton spectrum of p-dimethoxybenzene containing deuterated methyl groups and dissolved in a liquid-crystal solvent has been analysed. The analysis was aided by recording a proton spin-echo spectrum, which gives the proton–proton dipolar couplings, and a two-dimensional autocorrelation spectrum, which identifies the sub-spectra in the proton spectrum. The quadrupolar splitting and deuteron–deuteron dipolar coupling within each CD3 group were obtained from a deuteron spin-echo spectrum. These data are consistent with a planar arrangement for the carbons and oxygens in the lowest-energy conformation.

The 100 MHz n.m.r. spectra of 3-phenyl-1,2,5-oxa-(1), -thia-(2), and -selena-diazole (3) partially oriented in the nematic phase have been obtained and analysed. The spectral analysis affords examples of conformational situation in molecules containing two aromatic rings of six and five members respectively. A preferred twisted conformation around the bond between the two rings has been found, the twisting angle θ ranging from 18 to 21°. The conformation is little influenced by the heteroatom in position 2 and by the solvent. The results are discussed in connection with theoretical EHT calculations.