The structure and conformation of a mesogenic compound between almost zero and almost complete orientational order

The conformational distributions in molecules that form liquid crystalline phases are predicted to depend strongly on orientational order. Results are presented here to test this hypothesis. The mesogen 4‐hexyloxy‐4′‐cyanobiphenyl (6OCB) has been studied by NMR spectroscopy in the isotropic phase and in the nematic phase. In the isotropic phase the field‐induced orientational ordering produces small dipolar couplings between ¹³C and ¹H nuclei, which were determined from the ¹³C spectra. Couplings between ¹H nuclei were also obtained using 2D selective refocusing experiments. In the nematic phase, both ¹H–¹H dipolar couplings and quadrupolar splittings for deuterium nuclei were measured for partially‐deuterated samples. Both proton and deuterium spectra were also obtained for 6OCB in an equimolar mixture with 4‐(ethoxybenzylidene)‐4′‐butylaniline (EBBA). This mixture exhibits SmA and SmB phases. The data obtained from these experiments has been analysed to yield the probability distribution of the conformations in this molecule generated by rotations about bonds. It is found that there is a substantial influence of the orientational order of the molecules on these distributions.     

An investigation of solute-liquid crystal interaction in hydrogen bonded complexes of palmitic acid and p-octyloxybenzoic acid

The orientational order of a liquid crystalline phase which has a specific solute-liquid crystal interaction was investigated using nuclear magnetic resonance. Three isotopically substituted species of palmitic acid (palmitic acid-d₃₁, 1-¹³C-2.2-H₂-palmitic acid-d₂₉ and 2,2,3,3-H₄-palmitic acid-d₂₇) were dissolved in the liquid crystal p-octyloxybenzoic acid (p-OOBA) and the proton, deuteron and carbon 13 NMR spectra recorded as a function of temperature. ¹H-¹³H dipolar couplings were observed using a spin echo pulse sequence which removes heteronuclear dipolar couplings to the chain deuterons. In the case of the carbon 13 labelled compound, ¹H-¹³C dipolar couplings could be observed by applying an additional refocusing pulse to the ¹³C spins. The dipolar and quadrupolar couplings were used to calculate the complete orientational order matrix of the alpha methylene segment of palmitic acid in p-OOBA. The liquid crystal was shown to largely determine the orientational order of the head group and this was attributed to intermolecular hydrogen bonding. The dipolar and quadrupolar couplings for the rest of the chain were interpreted in terms of a mean field equilibrium statistical model, based on the Samulski Inertial Frame Model. Hydrogen bonding was shown to be of greater importance in the orientational ordering of the solutes in the liquid crystal than are electrostatic interactions in the ordering of the amphiphile in the potassium palmitate/water system.     

The dynamics of the CH3CN molecule in the isotropic phase and in a nematic solution

NMR lineshape studies of acelonitrile in the isotropic and the liquid-crystalline nematic phase of PCH have been performed. The scalar relaxation of the second kind due to the presence of the ¹⁴N quadrupolar nucleus has been confirmed as the most important relaxation mechanism for this molecule in both the isotropic and the anisotropic phase. It has been found largely responsible for the selective broadening on ¹³C and ¹H transitions. A minor contribution arising from intramolecular dipolar relaxation mechanism has also been investigated. Linewidth analysis of the NMR spectra allowed us to determine the quadrupolar relaxation time T_IN of the ¹⁴N nucleus. This is connected to the correlation time for rotational diffusion perpendicular to the molecular symmetry axis. A possible explanation of a residual selective broadeining which effects the ¹³C and ¹H NMR transitions and is not taken into account by this mechanism, is also given.     

Conformational analysis of 1,2-dimethoxyethane and 1,2-diphenyloxyethane incorporated in nematic liquid crystals

The conformations of flexible chain molecules incorporated in a nematic environment have been investigated. The phase behaviours and orientational characteristics of 1,2-dimethoxyethane (DME) dissolved in 4'-methoxybenzylidene-4-n-butylaniline at low solute mol fraction have been reported in our previous paper. In this work, proton-proton and carbon-carbon dipolar coupling constant measurements were attempted in addition to ²H NMR observations of quadrupolar splittings. These conformation-dependent properties were analysed according to the rotational isometric state (RIS) simulation scheme previously proposed. Our treatment rests on the assumption that the molecular axis of the chain should tend to align along the nematic field. Studies were further extended to a mixture of 1,2-diphenyloxyethane (DPE) with a nematic liquid crystal, 4,4'-azoxyanisole. Replacement of the terminal methyl groups in DME by phenyl groups leads to DPE. The results of the analysis indicate that the fraction of elongated conformers such as ttt tends to increase significantly on going from the isotropic to the liquid crystalline solution: DME; 9·4 to 15·6 per cent and DPE; 12·3 to 20·0 per cent. The tgt form (tg⁺ t or tg^-t) is the lowest-energy arrangement of these molecules. The fraction of this conformer also increases in DPE (26·8 to 40·0 per cent), while it decreases in DME (25·7 to 18·8 per cent). The conformational distributions of chain molecules were found to remain invariant over the range of concentration (0·5-6·0 mol per cent) and temperature (22·0-44·5°C) studied. It has been concluded that the observed variation of proton dipolar and deuterium quadrupolar couplings with concentration and temperature mainly arises from the orientational order of the molecular axis, which varies sensitively with the alignment of the surrounding solvent molecules. These results suggest that flexible chains are also participating in the nematic interaction by adjusting their configurations so as to enhance favourable interactions and suppress unfavourable steric repulsions when accommodated in an anisotropic potential field.     

Phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D2O system as determined by 2H and 14N NMR

The high resolution phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D₂O system has been mapped out using ²H and ¹⁴NNMR spectroscopy. The ¹⁴N quadruple splittings are more than an order of magnitude larger than corresponding ²H splittings, while the line widths are only two to three times larger. Thus, ¹⁴NNMR offers an order of magnitude improvement over ²H NMR in the resolution of the spectra from coexisting phases. The ²H spectra of samples in biphasic regions are often complicated by chemical exchange of D₂O molecules between coexisting phases, particularly at low TMAHFN concentrations. Analysis of the ²H line shapes of a TMAHFN/D₂O sample with a weight fraction of TMAHFN of 0.230 obtained at various times following cooling of the sample into the isotropic/nematic biphasic region shows that the mean diameter for the dispersed nematic droplet grows from about 7 to about 26 μm over a period of 2 h. At a mean droplet size of 7 μm the exchange of TMA⁺ ions between the coexisting phases is slow on the NMR time-scale and exchange effects are not observed in ¹⁴N spectra. The TMAHFN/D₂O phase diagram exhibits the generic form of those of the CsPFO/water and APFO/D₂O systems, which are the only other systems composed of stable discotic micelles for which high resolution phase diagrams are currently available, but the nematic phase is displaced to smaller TMAHFN concentrations. Specifically, a discotic nematic phase N_D⁺, intermediate between an isotropic micellar phase I and a lamellar phase L, exists for weight fractions of TMAHFN between 0.149 (φ = 0.105) and 0.420 (φa = 0.325) and temperatures between 277.3 and 327.6 K.     

Orientational ordering in the nematic phase of 4-alkyl-4'-cyanobicyclohexanes

The orientational ordering of three liquid crystals (trans,trans-4-n-alkyl-4'-cyanobicyclohexanes) has been investigated using ¹³C NMR. 2D proton-encoded local field spectroscopy was used to determine the ¹³C-¹H dipolar coupling constants, from which the order parameters were calculated. Semi-empirical linear relations between the obtained order parameters and anisotropic ¹³C chemical shifts were established. It was found that the order parameters of the major axis of the cyclohexane rings are smaller than those of liquid crystals with phenyl rings, and the order parameters of the C-H bonds in the aliphatic chains also behave differently.     

The complete assignment of the h and C nmr spectra of the alkali metal salts of salinomycin and narasin

The preparation of the alkali metal salts of salinomycin and narasin, apart from lithium narasin, is described. The complete assignment of their ¹H spectra based on two dimensional COSY and NOESY spectra is reported. These asignments are then used in two dimensional ¹³C/¹H correlations via direct and long range couplings to assign the ¹³C spectra. Assignments in the ¹³C spectrum of sodium salinomycin differ in important details from those in a previous report.     

Orientational ordering in the nematic phase of 4-alkenyl-substituted bicyclohexylnitriles

The orientational ordering of a series of 4-alkenyl-substituted bicyclohexyl liquid crystals was studied by natural abundance ¹³C NMR spectroscopy. A combination of the methods of variable angle spinning (VAS) and separated local field spectroscopy (SLF) was used. Rapid sample spinning about an axis forming an angle of about 65° with respect to the magnetic field forces the nematic director to align parallel to the spinnin axis, leading to narrow peaks in the ¹³C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled ¹³C signals in the ω₂ dimension and first-order C-H splitting patterns in the ω₁ dimension, from which the C-H dipolar coupling constants for individual pairs of nuclei can be obtained. Then, the order parameters for different molecular segments can be calculated. The method was applied to five different 4-alkenyl-substituted cyanobicyclohexanes. For the compounds studied, the dependence of the ring order parameters on the alkenyl chains shows the same trend as the melting and clearing temperatures and the elastic constants.     

An NMR study of the conformational flexibility of phenyl acetate dissolved in a nematic liquid crystalline solvent

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.     

Correlation des deplacements chimiques C/H, selective pour les carbones ne portant pas d''hydrogene

By the use of weak power, off resonance, decoupling mode during the acquisition of ¹³C spectra in two-dimensional correlation ¹³C/¹H experiments, it is possible to detect selectively correlations created by long range couplings for substituted carbon atoms.     

Spectroscopic investigation of local molecular dynamics in liquid crystals I. Small molecules

The dynamics of liquid crystal systems, consisting of either small molecules or polymer chains with main-chain or side-chain mesogenic groups, can be investigated using the same spectroscopic techniques which are applied to isotropic systems. This article briefly reviews ¹H, ¹³C and ²H N.M.R. and E.S.R. spectroscopy as well as dielectric relaxation. The particular features related to the oriented nature of the liquid crystal phases are developed. Typical examples of studies performed on small-molecule liquid crystals using these various spectroscopic techniques are described.     

Multinuclear NMR studies of lead(II) soaps II. 13C and 1H studies of solid and liquid-crystalline phases

The solid and liquid-crystalline phases of two long chain lead(II) carboxylates have been studied by ¹³C and ¹H NMR spectroscopy. High resolution ¹³C NMR spectra of the solid phase of lead(II) decanoate and octadecanoate, reveal splittings of the peaks attributed to the carboxylate and adjacent methylene groups. This may result from two different environments for the carboxylate chains coordinated to the same Pb(II) ion. On going from the solid to liquid-crystalline or liquid phases, this splitting is lost, and small changes in chemical shift of the bands due to the methyl and methylene groups are observed. These are attributed to the onset of conformational disordering, and analysis of the data gives a gauche population in good agreement with that from Raman spectra and theoretical calculations. Longitudinal ¹³C relaxation times are reported. From these, and from spectral data, the -CH₃ group is seen to have different dynamics from the rest of the chain, and to retain high mobility even in the solid phase. Further information on the structural changes comes from proton NMR spectroscopy. The transverse ¹H relaxation of lead(II) decanoate shows two components, which are suggested to arise from a crystalline, and a relatively amorphous region of the compound. Study of the evolution of these as a function of temperature shows that the amorphous fraction is relatively low up to 335 K. but then increase rapidly, until at 365 K, close to the transition to the L, phase, it is 99 per cent- In parallel to these observations, changes in band shape were also observed and suggest a progressive fusion of the chains up to the temperature of this transition. The global ¹H spin-lattice relaxation was also studied as a function of temperature. and two components were observed between room temperature and approximately the tint phase transition. These are associated with populations of hydrogen nuclei with different mobilities.     

2H-N.M.R. studies of flexibility and orientational order in nematic liquid crystals

Deuteron magnetic resonance spectroscopy (²H-N.M.R.) has been used to investigate the effect of the nematic environment on the flexibility and orientational order of two perdeuteriated cyanobiphenyl homologues: 4-methyl-4'-cyanobiphenyl (1CB-d₁₁) and 4-n-pentyl-4'-cyanobiphenyl (5CB-d₁₉). The systems studied were low concentrations of 1CB-d₁₁ and 5CB-d₁₉ dissolved in the nematic phases of 5CB, N-(-4-ethoxybenzylidene)-4'-n-butylaniline (EBBA), Merck ZLI-1132 (1132) and a 55wt% mixture of 1132: EBBA. The spectra are dramatically different in these environments. Previous studies on small solutes have suggested that in the 55wt% 1132: EBBA mixture (at 301.4 K) the dominant orienting mechanism depends on the size and shape of the molecule which suggests that it is a short range repulsive interaction. This interaction has been modelled by treating the liquid crystal as an elastic continuum and the solute as a collection of van der Waals spheres which stretch the liquid crystal in the two dimensions perpendicular to the director. The distortion of the liquid crystal depends on the dimensions of the solute, and the elastic energy is described in terms of a Hooke's law force constant, k. The model is extended to include flexible liquid crystal molecules and quadrupolar couplings are calculated for each conformation of the 5CB chain. Statistical averaging over all conformations gives an excellent fit to the experimental spectrum. The results for 1CB and 5CB show that in the other nematic phases contributions from additional mechanisms must be included. Previous studies of ²H₂ and other solutes indicate that the additional mechanism is the interaction between the solute molecular quadrupole moment and the mean electric field gradient of the liquid crystal.     

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

Abstract

The compound 4-cyano-4′-(α,α,α-trifluoromethoxy)biphenyl (1OCBF₃) has been synthesized. Unlike the fully protonated analogue, 4-cyano-4′-methoxybiphenyl (1OCB), it does not show a liquid crystalline phase on cooling from the melting point (51°C) to room temperature. The transition temperature to a monotropic nematic phase was obtained as approximately 0°C by determining the transition temperatures of mixtures with 1OCB. The structures, conformational properties and orientational ordering of both 1OCB and 1OCBF₃ as solutes in a nematic solvent ZLI 1132 have been investigated via the 17 dipolar couplings obtained by analysing the proton and fluorine NMR spectra of these solutions. It is concluded that the major difference between the two molecules lies in the potential, V(φ₂), governing rotation about the ring–oxygen bonds. In 1OCB the potential has the same form as in anisole, with a minimum when the C–O bond is in the plane of the attached ring (φ₂ = 0°), and a maximum of about 15 kJ mol^?1 when φ₂ is 90°. In 1OCBF₃ the barrier to rotation about the ring–O bond decreases substantially to being near zero.     

Mesomorphic behavior of new benzothiazole liquid crystals having Schiff base linker and terminal methyl group

正A homologous series of heterocycles,6-methyl-2-(4-alkoxybenzylidenamino)benzothiazoles,were synthesized and characterized using FT-IR,~1H and ~(13)C NMR and mass spectrometric analysis.Enantiotropic nematic phase was observed for shorter members.Smectic A phase only emerged from octyloxy derivative onwards.The terminal methyl group at the benzothiazole fragment and the Schiff base linkage influenced the mesomorphic behavior of the present series.     

Synthesis, thermal stabilities, and anisotropic properties of some new isoflavone-based esters 7-decanoyloxy-3-(4'-substitutedphenyl)-4H-1-benzopyran-4-ones

A series of new isoflavonic esters 7-decanoyloxy-3-(4'-substitutedphenyl)-4H-1-benzopyran-4-ones containing a heterocycle in their central core with one flexible alkyl chain and various substituents, R (where R = F, Cl, Br, CH₃, OCH₃, or H) at 4'-position at one end, have successfully been synthesised, isolated, and characterised. The physical properties of title compounds were studied using spectroscopic techniques (Fourier transform infrared, ¹H and ¹³C nuclear magnetic resonance). The liquid crystalline properties and the textures of these compounds along with their thermal stabilities were investigated via polarising optical microscopy and differential scanning calorimetry. All the compounds except the member without the substituent were enantiotropic mesogens exhibiting smectic A and/or nematic phases. The layer periodicities in relation to different phases were substantiated by X-ray diffraction wherein the molecules within the SmA phase were found to be intercalated. The structural changes that resulted in a variation of transition temperature as well as the electronic polarisability of the respective compounds are also discussed.     

NMR spectra of salicylohydroxamic acid in DMSO-d6 solution: a DFT study

The ¹H, ¹³C and ¹H, ¹³C COSY NMR spectra of salicylohydroxamic acid (sha) were measured in DMSO-d₆ solution. The B3LYP GIAO method with the 6-311++G(d,p) basis set was chosen to reproduce the experimental spectra. All possible zusammen and entgegen conformers of monomeric sha were computed. After geometry optimisation (B3LYP/6-311++G(d,p)) only nine independent models of the molecule were shown to be stable. Additionally, the NMR chemical shifts of the Onsager model of the most stable monomer were calculated. The computed chemical shifts for the labile protons for all aforementioned geometries meaningfully underestimated experimental results suggesting the existence of the H-bonded structure of sha in DMSO solution. The most probable two dimeric structures along with two solvent-bounded aggregates were subsequently calculated at the same level of theory. The best agreement was obtained for sha H-bonded with two DMSO molecules (confirmed by the absence of concentration effect). The relative error not exceeding 10 and 4% for chemical shifts in ¹H and ¹³C NMR spectra of sha–(DMSO)₂, respectively, showed that the applied method with the B3LYP/6-311++G(d,p) basis set was efficient to predict the NMR shifts of a compound with strong H-bonds. Thus, this allows to assign properly NMR resonances to specific structure formed in DMSO solution.     

Orientational ordering of a nematic liquid crystal and its mixture with its chain-perfluorinated analogue

The orientation of different segments of 4'-cyanophenyl 4-heptylbenzoate (7CPB) has been investigated using ¹³C NMR. The method of proton-encoded local field (PELF) spectroscopy in combination with off-magic-angle spinning (OMAS) of the sample was used. High resolution 2D spectra were obtained, from which the order parameters were calculated. Linear relations between the obtained order parameters and anisotropic chemical shifts determined by 1D ¹³C NMR were established and semi-empirical parameters were achieved. A 1:2 mixture of 7CPB and its chain-perfluorinated analogue (7PFCPB) showed interesting phase behaviour with the change of temperature. It was studied by the use of ¹³C NMR and polarizing optical microscopy. The order parameters of 7CPB in the smectic A phase of the mixture were calculated using the semi-empirical parameters obtained from the 2D NMR method.     

Conformation of N,N′-bis (2,6-dimethylmorpholino), N″-dichloroacetyl phosphoric triamide: CHCl2C(O)NHP(O)[2,6(CH3)2(NC4H6O)]2. NMR and ab initio studies

Using phosphorus pentachloride as a substrate, a new carbacyclamidophosphate, N,N″-bis (2,6-dimethylmorpholino), N″-dichloroacetyl phosphoric triamide (1) has been synthesized and characterized by ¹H, ³¹P and ¹³C NMR, IR spectroscopy and elemental analysis. Due to the presence of methyl disubstituted morpholine rings and the dichloroacetamide group, several conformers can be considered for this molecule. The ³¹P{¹H} NMR spectra for the isomeric mixture of synthesized compound showed four signals with the ratio 67.1; 19.0; 12.2; 1.7, which indicates four independent conformers. The ¹H NMR spectra confirmed these results. The conformational space and the molecular geometry of the molecule in the gaseous phase have been studied using the B3LYP method of approximation, with 6-31G and 6-311++G^** basis sets.     

Magnetic field induced dipolar couplings in the pretransitional region of a nematic liquid crystal

13C and 1H NMR spectroscopy have been used to study the orientational order which develops when a nematogenic compound, 4-pentyl-4'-cyanobiphenyl (5CB), approaches the transition from the isotropic to the nematic phase at T(NI). The experiments yield values of field induced dipolar couplings, (1), between all of the directly bonded carbon and hydrogen nuclei in the molecule, and 2D(HH)B, the geminal dipolar coupling between protons in the first methylene group in the alkyl chain. The temperature dependence of these couplings shows that in each case they follow a divergence behavior governed by (T - T*)(-1),where T* is a temperature determined from the experimental data and which is close to but less than T(NI). Experiments performed at spectrometer field strengths of 11.75 and 18.79 T confirm the prediction that the induced couplings should depend on the square of the applied field strength. It was found that, within experimental error, T* is the same for each field-induced coupling, and that T(NI) - T* is the same at 11.75 and 18.79 T. It is shown that the set of field-induced couplings 1D(CHi)B obtained at a temperature close to T(NI) can be used to derive a conformer distribution for 5CB in the isotropic phase.