Testing experimental analysis techniques for liquid crystals using computer simulation

Computer simulation has been used widely over the last two decades to investigate the phase behaviour and physical properties of liquid crystal systems. Here we review studies in which simulations have been used to calculate information on the physical properties of liquid crystals in a pseudo-experimental way. We concentrate on the calculation of 'experimental data' from computer simulations, from which we can extract information on the structure and dynamics of the nematic and smectic A phases. Where relevant, these are compared with both existing theoretical and experimental results. In particular, routes between the raw experimental data and physical parameters such as the orientational order parameter are highlighted and their effectiveness investigated.     

Assignment of 13C resonances of a nematic liquid crystal using off-magic angle spinning

A novel method for assigning the resonances in the 13C NMR spectrum of a static liquid crystalline sample in its nematic phase is proposed. The method is based on the fact that the carbon chemical shifts in the isotropic phase and in the oriented phase under static and off-magic angle spinning (OMAS) conditions are uniquely related by the tensorial property of the CSA tensor, requiring just one OMAS spectrum and the assignment in the isotropic phase. A computational procedure is proposed to take into account deviations arising out of non-ideal experimental conditions and the assignments are made by identifying the minimum in the differences in the frequencies between calculated and experimental line positions. Practical implementation of the method has also been demonstrated in the case of the liquid crystal N-(4-ethoxybenzylidene)-4-n-butylaniline.     

Vibrational analysis of liquid n-butylmethylether

The complex refractive index spectrum in the NIR and MIR range of liquid n-butylmethylether (NBME) was determined. Thin film recording technique was required to obtain quantitative transmission spectra in the MIR range. The experimental part was supported by a conformer population analysis and anharmonic vibrational analysis based on the B2PLYP/N07D method. The resulting population of 11 NBME conformers was analyzed and their calculated anharmonic vibrational spectra and calculated abundances (at 298 K) were used to obtain the resultant spectrum of n-butylmethylether. The band assignment procedure was based on the calculated potential energy distributions.     

Phase effect on formation of intermediates in the pulse radiolysis of 4-cyano-4'- octylbiphenyl liquid crystals

It is shown that the reaction intermediates were formed by pulse radiolysis of 4-cyano-4'- octylbiphenyl liquid crystals. The remarkable phase effect was observed in the pulse radiolysis of the liquid crystals. The optical absorption spectrum obtained in the smectic A phase of 4-cyano-4'-octylbiphenyl had no absorption maximum in the visible region, but had an absorption increasing toward the shorter wavelengths below 400 nm, while a broad absorption band with a maximum around 550 nm was observed in the nematic phase. In the isotropic liquid phase an absorption band with a maximum at 480 nm was observed, but it was observed neither in the smectic A phase nor in the nematic phase. The absorption below 400 nm was observed in the liquid phase as well as in the smectic A phase. The assignment of the absorptions observed for each phase is discussed. Then it is suggested that the structural properties could be related to the formation and behavior of reaction intermediates.     

Phase transitions in mixtures of a side-on-side chain liquid crystalline polymer and low molar mass nematic liquid crystals

Miscibility phase diagrams of mixtures of side-on side chain liquid crystalline polymers (s-SCLCP) and low molar mass liquid crystals (E48 and E44) have been established by means of polarized optical microscopy and light scattering. E48 and E44 are cyanobiphenyl-based eutectic nematic liquid crystal (LC) mixtures with nematic-isotropic transition temperatures of 93 and 105 C, respectively. The phase diagram of the s-SCLCP/E48 system reveals the coexistence of an isotropic nematic region and a single nematic phase in order of descending temperature. The single nematic phase suggests that the pair is miscible in the nematic region. On the other hand, the s-SCLCP/E44 mixture shows liquid liquid and nematic nematic coexistence phases, suggestive of the immiscibility character of the pair. These nematic phase diagrams of the s-SCLCP/E48 and s-SCLCP/E44 have been analysed in the context of the combined Flory-Huggins (FH) free energy for isotropic mixing and the Maier-Saupe (MS) free energy for nematic ordering of the mesogens. This combined FH/MS theory is capable of predicting the observed nematic phase diagrams consisting of liquid liquid, liquid nematic, nematic nematic, and the pure nematic regions. The change of colour accompanying the appearance and disappearance of the inversion walls may be attributed to the temperature dependence of birefringence.     

Differential dichroic spectroscopy in liquid crystals using unpolarized radiation

An unpolarized, differential technique is described for obtaining the dichroic spectra of uniaxial liquid crystals from a single spectral scan. Using this method, we investigate the near infrared dichoic spectrum of n-heptyl cyanobiphenyl (7CB). The temperature dependence of the orientational order parameter has been derived close to the nematic-isotropic transition and this is in good agreement with earlier Raman data. The differential method is also advantageous for dichroic studies on oriented guest molecules in liquid crystals. Illustrative results are presented for p-nitrotoluene dissolved in a nematic medium.     

Vibrational assignment and analysis for 2,3-dihydrofuran and 2,5-dihydrofuran

The vibrational spectra of 2,3-dihydrofuran and 2,5-dihydrofuran have been recorded using IR and Raman spectroscopy for the gas, liquid and solid states. A vibrational assignment consisting of a nearly complete set of vapor phase wavenumbers is proposed for both molecules based on the observed spectra and normal coordinate analyses. The normal coordinate analyses have been made by scaling the AM1 force field for each molecule with scale factors transferred from an analysis of the cyclopentene fundamental vibrations. The predicted a priori vibrational frequencies justify one reassignment of the fundamentals for 2,5-dihydrofuran from that previously reported. The vibrational assignment for 2,3-dihydrofuran is reported for the first time. Thermodynamic functions are computed for each molecule using the experimentally determined vibrational frequencies.     

Tolane oligomers: Model thermotropic liquid crystals

The liquid crystalline phases of several rigid-rod, non-polar tolane oligomers are characterized by differential scanning calorimetry and transmitted polarized light microscopy. We determine that a stable nematic phase can be formed at ambient pressure if the molecular axial ratio (length-to-width ratio) is greater than 4.5. A smectic phase forms in addition to the nematic phase if the axial ratio exceeds 6.1. Symmetrical fluorination of the terminal phenyl groups reveals that the liquid crystalline phase behaviour of these rigid rods is highly sensitive to perturbations of the charge distribution along the molecules. Nematic tolane oligomers can exhibit high stregth disclinations (s = ±3/2 and ±2) in their schlieren textures, and we discuss conditions that promote the stability of these defects.     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Vibrational analysis of 2-[4-(<Emphasis Type="Italic"> N</Emphasis> -dodecanoylamino)phenyl]-5-(4-nitrophenyl)-1,3,4-oxadiazole

The infrared absorption spectra of 2-[4-( N -dodecanoylamino)phenyl]-5-(4-nitrophenyl)-1,3,4-oxadiazole (also denoted AF51) in solution and in the solid state as well as the solid-state Raman spectrum of the powdered compound are compared with the infrared linear dichroic (LD) spectra recorded at two orthogonal polarizations. The IR-LD spectra were measured in an anisotropic solvent-the nematic liquid crystal ZLI-1695 (Merck). The solvent spectrum taken under precisely the same experimental conditions and polarized radiation azimuths was subtracted from the polarized sample spectra in order to achieve pure oriented spectra of the compound studied. These spectra were further processed using a stepwise reduction procedure that allows for assignment of vibrational modes having mutually orthogonal dipole moments.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

The vibrational spectra and rotational isomerism of 1,1,1-Trimethoxyethane

The liquid, vapour and solid-state infrared spectra of 1,1,1-trimethoxy-ethane were recorded in the region 250–4000 cm⁻¹. The laser-Raman spectrum with qualitative depolarization data was obtained for liquid only. The spectra show that there are at least two rotational isomers present in the liquid phase. The solution spectra reveal that the less polar form is the more stable in the liquid phase. The form playing an important role has GGG configuration. A vibrational assignment has been attempted for the observed infrared and Raman bands.     

Structure and vibrational frequencies of 1-naphthaldehyde based on density functional theory calculations

The mid and far FTIR and Raman spectra were measured in the liquid state. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) and standard B3LYP/6-311+G** basis set combination. The vibrational spectra were interpreted, with the aid of normal coordinate analysis based on a scaled quantum mechanical (SQM) force field. The infrared and Raman spectra were also predicted from the calculated intensities. Comparison of simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes. Unambiguous vibrational assignment of all the fundamentals was made using the total energy distribution (TED).     

Structure,hydrogen bonding and vibrational spectra of pyruvic acid

The complete vibrational spectra of liquid pyruvic acid and the infrared spectrum of crystalline pyruvic acid at about 20 K have been recorded and analyzed. A vibrational assignment is proposed based on these spectra and comparison with spectra of derivatives of pyruvic acid.The spectra of pyruvic acid can best be interpreted in terms of a cyclic hydrogen-bonded dimer structure in which the two carbonyl groups are in a trans configuration in the pure liquid phase. A similar structure has been reported for crystalline pyruvic acid by X-ray diffraction. In dilute solution the structure appears to be monomeric with an internal hydrogen bond, in essential agreement with the structures of the monomer reported from microwave spectroscopic measurements.     

Some N.M.R. experiments using lyotropic nematic liquid crystals

A series of lyotropic nematic liquid crystals based on aniso-dimensional micelles can be subdivided according to micelle shape and diamagnetic susceptibility anisotropy. The proton magnetic resonance spectra of benzene dissolved in dilute solution in several of these phases have been analysed. The signs of the partially averaged dipole-dipole coupling give information about the relative alignment of the six-fold symmetry axis of benzene, the director and the magnetic field. The system caesium perfluorooctanoate/water is unusual in two respects. The system forms a nematic phase with only a binary mixture and the disk micelles align in a perpendicular plane to the magnetic field. As a result the binary system has been studied previously in some detail (see N. Boden, S. A. Come and K. W. Jolley, 1987, J. phys. Chem.91, 4092). New measurements on the interesting phase diagram have been accomplished using caesium-133 and deuterium magnetic resonance measurements at various temperatures. Some dilatometric measurements have been made to complement the N.M.R. studies.     

The structure of acrolein in a liquid crystal phase

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.     

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 thin cell dielectric spectroscopy method for liquid crystals

A thin cell method for dielectric time domain spectroscopy is described. The cell allows one to study the dielectric dispersion and absorption spectra for both components of the dielectric permittivity tensor of uniaxial systems like nematic and smectic A and B phases. Results are presented for 4-n-pentyl-4'-cyanobiphenyl in the isotropic phase and upon orientation in the nematic phase.     

Solute induced liquid crystalline behaviour of a quaternary ammonium salt and its application to structure determination

A new liquid crystalline phase, induced by the addition of small amounts of a non-mesogenic solute (such as dimethyl sulphoxide or methyl iodide) to a quaternary ammonium salt, N -methyl- N, N, N-trioctadecylammonium iodide (MTAI), has been detected by NMR and optical microscopic studies. In some cases, there is a coexistence of nematic and smectic phases. Information on the ordering of the phases in the magnetic field of the spectrometer has been derived from NMR spectra of a dissolved molecule, 13C-enriched methyl iodide. The low order parameter of the pure thermotropic nematic phase of the salt provides firstorder spectra of the dissolved oriented molecules. Analyses of spectra of cis , cis -mucononitrile exemplifies the utility of the MTAI nematic phase in the determination of structural parameters of the solute.     

Vibrational studies of sulfamoil chloride (ClSO2NH2)

The infrared spectrum of sulfamoil chloride in the liquid phase was reinvestigated; the infrared and Raman spectra of the solid phase have also been obtained. A complete assignment of the observed bands is proposed. A subsequent normal coordinate analysis was performed. The experimental data are compared to results of ab initio and DFT (density functional theory) calculations. According to the experimental and theoretical results the main conformer of ClSO₂NH₂ possesses an anti conformation (C_s symmetry, S---Cl single bond in anti position with respect to the nitrogen lone pair).