This paper presents data on orientational ordering in a reentrant discoid nematic and on changes in the polarizability of
discogenic molecules in a homologous series. The phase transition of a nematic to the discotic column phase is accompanied
by a jump of the orientational ordering parameter and is characterized by close relationship between the orientational and
translational ordering of molecules. The lengthening of the flexible peripheral molecular chains in the reentrant nematic
phase considerably changes their conformational state, decreasing the anisotropy of their polarizability.
Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 1, pp. 89–97, 1997. 相似文献
The statistical properties of the conformational and orientational distribution of molecules with internal rotation in the isotropic and nematic phases of liquid crystal (LC) are investigated in terms of molecular statistical theory. The paper discusses the effect of mutual correlation between the molecular conformational and orientational degrees of freedom on the conformational, orientational, and mixed order parameters of molecules in LC and on the nematic–isotropic liquid transition temperature. For these order parameters, the recurrent relation method is suggested and used to derive the partial functions of the conformational and orientational distributions of molecules in LC. 相似文献
The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz
tensor components Lj and the mean molecular polarizability $
\bar \gamma
$
\bar \gamma
for five nematic liquid crystals belonging to two homological series. The dependence of Lj components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules
in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz
tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed.
The quadratic dependence $
\bar \gamma
$
\bar \gamma
(S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences $
\bar \gamma
$
\bar \gamma
(S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational
degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular
fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions. 相似文献
The heat of mixing HM has been obtained as a function of composition for nematic p-methoxybenzylidine-p-n-butylaniline (MBBA) at 25°C with two pairs of normal and branched alkane isomers (n-octane and 2,2,4-trimethylpentane (TMP), n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane (HMN)) and with cyclohexane, toluene, and carbon tetrachloride. Heats of mixing were also obtained at 55°C for isotropic MBBA with n-hexadecane, HMN, and toluene. The composition dependence of HM is strongly skewed toward high MBBA concentration for systems containing nematic MBBA, and less so for those containing MBBA in the isotropic state. Heats of solution HS at a low volume fraction (0.08) of MBBA were measured between 25°C and 75°C in the above seven solvents. The HS values change rapidly with temperature in both the nematic and isotropic temperature ranges with a small discontinuity at 43°C, the nematic-isotropic transition temperature. The data indicate: (1) a strongly temperature-dependent orientational order in pure MBBA, of long range in the nematic phase and of short range in the isotropic phase, (2) a correlation of orientations of MBBA molecules with n-alkane chains and with toluene, but not with the branched alkane molecules or cyclohexane. 相似文献
Atomistic MD simulations of water in the vicinity of oxidized amorphous atactic polystyrene are presented. The changes in the orientational and translational dynamics of water near polymer surfaces with different hydrophilicity are studied. Two main orientational relaxation processes of water molecules are distinguished: a process on a fs timescale, associated with the ballistic motion of water molecules, and a process on a ps timescale, associated with the self‐diffusion of water. The fast process is not affected by the presence of the polymeric surface. The second relaxation process differs at the interface from that in the bulk in that the dynamics of water molecules is more heterogeneous in the first. The effect of the representation of polystyrene films on the water dynamics is discussed.
Electrooptical characteristics of mesogenic chain molecules in solution and in mesophase can be described in terms of intra- and intermolecular orientational orders. The value and sign of electric birefringence δn in a solution of kinetically rigid chain molecules are determined by the combination of two factors: intramolecular orientational order which depends on the dipolar and anisotropic architecture of the molecule and intermolecular orientational order caused by the action of the external electric field E. The value and sign of the dielectric anisotropy δε of the polymer nematic phase are also determined by the combination of intra- and intermolecular orders. However, in this case the latter is not maintained by the external field but by the nematic potential of the mesophase. Therefore, comparative investigations of electrooptical properties of polymers in solutions and in nematic melts make it possible to obtain information about the intra- and intermolecular orientational orders of the molecules under investigation in these two states. These investigations were carried out using the method of electric birefringence in solutions and the method of orientational deformations of nematic textures in an electric field. The objects being investigated were nematogenic dimers and trimers. Experimental data obtained for these compounds showed the presence of intramolecular order in their molecules, which is manifested in the odd-even oscillations of the value and sign of Kerr constant K≈δn/E2 in solution and δε in the nematic phase when the number of C-C bonds in the methylene spacers of these molecules is varied. This effect is particularly dramatic in the mesophase where it is enhanced by intermolecular nematic potential. 相似文献
The influence of an intense external field on the dynamics of the nematic liquid crystal phase is investigated using a molecular dynamics simulation for the Gay-Berne nematogen under isobaric-isothermal conditions. The molecular dynamics as a function of the second-rank orientational order parameter P<2> for a system consisting of a nematic liquid crystal in the presence of an intense magnetic field is compared with that of a similar system without the field. The translational motion of molecules is determined as a function of the translational diffusion coefficient tensor and the anisotropy and compared with the values predicted theoretically. The rotational dynamics of molecules is analyzed using the first- and the second-rank orientational time correlation functions. The translational diffusion coefficient parallel with respect to the director is constrained by the intense field, although the perpendicular one is decreased as the P<2> is increased, just as it is in the system without the field. However, no essential effect of the strong magnetic field is observed in the rotational molecular dynamics. Further, the rotational diffusion coefficient parallel with respect to the director obtained from the first-rank orientational time correlation function in the simulation is qualitatively in agreement with that in the real nematic liquid crystalline molecules. The P<2> dependence of the rotational diffusion coefficient for the system with the intense magnetic field shows a tendency similar to that for the system without the field. 相似文献
The molecular structure of a polysiloxane with phenyl benzoate mesogenic side groups was investigated in an x-ray scattering study in the partially crystalline, smectic and nematic phase, and in the melt. In the crystalline phase polymer molecules have the form of straight ribbons with a double-comb-conformation. A bilayer structure is built up by regular stacking. Layers are the dominating structure element not only in the crystalline and smectic phase, but also in the nematic phase, and even in the isotropic melt. Layers are planar in the smectic phase and curved in the nematic phase, with an asymmetric distribution of the normal vectors about the director. In the isotropic melt there is evidence for the occurrence of clusters with layer-like short-range order. 相似文献
Zusammenfassung Ausgehend vom Deltafunktionspotentialmodell wurde die absolute Intensität einer Ramanlinie, die dem Quadrat der molekularen Polarisierbarkeit proportional ist, bestimmt. Auf Grund des Deltafunktionspotentialmodells ergibt sich eine Abhängigkeit der Komponente der Polarisierbarkeit in der Bindungsrichtung vonR4. Wir leiteten aus dem Deltafunktionspotentialmodell einen Ausdruck für die Ableitung der mittleren molekularen Polarisierbarkeit nach der Entfernung der Kerne aus der Gleichgewichtskonfiguration ab, der für viele zweiatomige Moleküle und für die symmetrischen Streckschwingungen vieler mehratomiger Moleküle und Ionen im elektronischen Grundzustand zutrifft. Die berechneten Werte für die Ableitung der Polarisierbarkeit stimmen meist gut mit den aus den experimentell bestimmten Ramanintensitäten erhaltenen Werten überein. Mit Hilfe des Deltafunktionspotentialmodells errechneten wir aus den experimentell bestimmten Ableitungen der Polarisierbarkeit Bindungsordnungen für verschiedene ionische Systeme und erörterten die Ergebnisse im Hinblick auf die Beiträge der - und -Elektronen zu den Ableitungen der Polarisierbarkeit von Mehrfachbindungen.
Quantum mechanical studies of absolute raman intensities (application to symmetrical stretching modes in some molecules and ions)
Absolute intensity of a Raman line which is proportional to the square of the derivative of molecular polarizability has been determined from the delta-function potential model yielding theR4-dependence of the bond parallel component of the polarizability. The delta-function potential model has been used to derive the expression for the derivative of the mean molecular polarizability with respect to a change in the internuclear distance at the equilibrium configuration for many diatomic molecules, and for the symmetrical stretching modes of many polyatomic molecules and ions in the ground electronic state. The computed polarizability derivatives are in good agreement with most of the experimental ones derived from Raman intensities. Bond orders have been computed from the experimental polarizability derivatives for various ionic systems through the use of the delta-function potential model, and the results have been discussed in relation to the contributions of the and electrons to the polarizability derivatives of multiple bonds.
Abstract The phase diagram of a mixture of the discogens triphenylene hexa-n-heptanoate (HET7) and triphenylene hexa-(2-methyl-4-n-decyloxy)benzoate is presented. This mixture provides a more accessible nematic temperature range than was previously possible. The refractive indices of one mixture composition are measured and a rather small, negative birefringence found. The two-parameter Haller extrapolation method is used to calculate the molecular polarizability ratio Δα/α. This has a rather small magnitude compared with calamitic mesogens, although the individual polarizabilities are much larger, due to the greater molecular mass of these compounds. The calculated value of Δα/α is used to give the uniaxial orientational order parameter S, whose value is comparable to those of calamitic examples. Tabulated bond polarizability data are used to predict a value for Δα/α, which is found to be similar to the experimental result. 相似文献
Underlying assumptions have been examined in scaled-particle theory for the case of a rigid-sphere solute in liquid water. As a result, it has been possible to improve upon Pierotti's corresponding analysis in a way that explicitly incorporates measured surface tensions and radial-distribution functions for pure water. It is pointed out along the way that potential energy nonadditivity should create an orientational bias for molecules in the liquid-vapor interface that is peculiar to water. Some specific conclusions have been drawn about the solvation mode for the nonpolar rigid-sphere solute.This paper is substituted for the talk given at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.Editor's note 相似文献
A perturbation expansion of the pair correlation function is used to derive the molecular field self-consistency equations for non-rigid molecules. The order parameters and the thermodynamic functions are expressed directly in terms of the segmental interaction coupling constants. The values of these constants for the 4-n-alkyl-4'-cyanobiphenyis (NCB) are determined by analysing the orientational order parameters observed by N.M.R. in the nematic phase; they are in reasonable agreement with values obtained from calculations of the nematic-isotropic transition temperatures. It is found that contributions of the isotropic intermolecular interactions to the conformational energy of the alkyl chain are comparable in magnitude to the direct intramolecular contributions. 相似文献
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. 相似文献
The preferred conformation of solidphase R4benzylidene3,4dichloroaniline molecules has been established by nuclear quadrupole resonance (NQR) combined with quantum chemical calculations. The conformational effect of substitutes reported previously is proved. The rotation angle of the plane of the aniline ring with respect to the plane of the azomethine bond is given. 相似文献
The effect of solvent ionizing ability on heterolysis rate enhances in the series 1-chloro-1-methylcyclohexane < 1-bromo-1-methylcyclohexane 1-chloro-1-methylcyclopentane < 1-bromo-1-methyl- cyclopentane. The lower sensitivity of cyclohexyl substrates compared with cyclopentyl is determined by conformational effects. Bromides are more sensitive to solvent effects than chlorides because of the stronger polarizability of the C-Br bond. 相似文献
Abstract A perturbation expansion of the pair correlation function is used to derive the molecular field self-consistency equations for non-rigid molecules. The order parameters and the thermodynamic functions are expressed directly in terms of the segmental interaction coupling constants. The values of these constants for the 4-n-alkyl-4'-cyanobiphenyis (NCB) are determined by analysing the orientational order parameters observed by N.M.R. in the nematic phase; they are in reasonable agreement with values obtained from calculations of the nematic-isotropic transition temperatures. It is found that contributions of the isotropic intermolecular interactions to the conformational energy of the alkyl chain are comparable in magnitude to the direct intramolecular contributions. 相似文献
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. 相似文献
The solvation effect on the conformational equilibrium ofL--alanine in binary water-organic solvents (H2O-acetonitrile, H2O-DMF) was studied. This manifests itself in changes in the optical activity and CD intensity. Equilibrium constants for the substitution of water molecules by organic solvent molecules were determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1054–1057, June, 1993. 相似文献
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. 相似文献