Anisotropy Of Molecular Polarizability Research Articles

Experimental determination of molecular polarizability anisotropy of nematogens by depolarized Rayleigh light scattering

The anisotropy of molecular polarizability Δα of several nematogens has been determined by depolarized Rayleigh light scattering. The experimentally determined values were found to be noticeably smaller than those obtained by MOPAC calculation. Δα values determined from order parameter S and refractive indices show reasonably good agreement with those determined in the present experiment. This result warns us that Δα calculated by MOPAC, particularly at a finite wavelength, may be larger than the true value.

Thermal, structural, and orientational relaxation of supercooled salol studied by polarization-dependent impulsive stimulated scattering

Time-resolved transient grating experiments with various polarizations are used to separate different responses and measure their dynamics in supercooled liquid salol. A contribution to signal from orientational alignment induced by flow that arises from thermal expansion is demonstrated. This contribution is distinct from that due to orientational alignment induced directly by the excitation light through the molecular polarizability anisotropy (i.e., through the optical Kerr effect). It is also distinct from signal contributions due to density modulations induced by thermal expansion. The results offer additional insight into salol dynamics and into time-dependent transient grating measurements of this class. Depending on the light polarizations used, any of the signal contributions can be eliminated or highlighted.

Electrical and Optical Properties of the Nitramine Group and Molecular Structure of Some N-Nitramines

The electrical and optical properties (dipole moment, electrooptical and optical Kerr constants, molecular polarizability anisotropy, polarizability tensor, molecular refraction) of N,N-dimethylnitramine as the simplest N NO 2 derivative were studied by a set of experimental and theoretical methods with the aim to determine the components of the polarizability ellipsoid of the N NO 2 group and develop a valence-optical scheme for calculating the optical and electrooptical parameters of N NO 2 compounds. Conjugation of the p electrons of the imide nitrogen atom with the electrons of the nitro group results in deviation of the proper- ties of N,N-dimethylnitramine from the additive values. Comparative evaluation of the parameters of the C sp 3 NO 2 ,C sp 2 NO 2 , and N NO 2 groups was made.

Physical Studies of Cyclohexane Derivatives

Mean molecular polarizabilities, polarizability anisotropies and order parameters of two homologous series of cyclohexane derivatives namely trans-4-alkyl cyclohexyl 4'-cyanο-pahenyl ester and trans-4-alkyl cyclohexyl 4'-n-pentyloxy phenyl ester are evaluated using molecular vibration approach. The variation of polarizability and polarizability anisotropy with a number of carbon atoms in the alkyl chain is studied. In addition, the dependence of order parameter on temperature is reported and discussed.

Anisotropy in molecular linear polarizability α and hyperpolarizability β: missing-orbital analysis and simplified sum-over-states calculations

We applied missing-orbital analysis to linear- and hyperpolarizabilities, α and β, respectively, obtained by simplified sum-over-states calculations and discussed the effects of π conjugation extension on α zz and α xx , as well as on diagonal β zzz and off-diagonal β zxx , respectively. The molecules studied are C 2 v molecules with donor–acceptor pair(s) linked to the π conjugation system. We found that α components are mostly constituted by the accumulation of small contributions from many configurations. Further, the dominantly contributing configuration(s) to β zzz and β zxx also play a significant role, if symmetry allowed, in α zz and α xx , respectively. Upon z-length extension in amino-nitro-polyacetylenes by increasing the number of acetylene units, the changes in α zz and α xx are almost arithmetic, thus consistent to the additivity rule of the bond polarizabilities. The incremental change in α zz is an order of magnitude larger than that in α xx , implying large anisotropy of α. Such large anisotropy is also the case for β, i.e. β zzz rapidly increases but β zxx remains mostly unchanged. When the π conjugation is extended mainly in the x-direction by replacing benzene with fused-ring(s) in 1,3-diamino-4,6-dinitrobenzene, we found a rapid increase in α xx while mostly no change in α zz , and these changes are not fully explained by the additivity rule. Anisotropic change in β is also the case and is discussed in relation to the complicated variations in two-dimensional charge-transfer structures.

Cotton–Mouton effect and optical-frequency molecular polarizability anisotropy of ethylene

New observations of the magneto-optical Cotton–Mouton effect of ethylene over a range of temperature (293.5–437.9 K) are analyzed in conjunction with the Rayleigh depolarization ratio and the mean polarizability to confirm the components of the optical-frequency ( λ=632.8 nm) molecular polarizability of this species.

Molecular Polarizabilities, Polarizability Anisotropies and Order Parameters of Nematic Liquid Crystals

AbstractMean Molecular polarizabilities, polarizability anisotropies and order parameters are evaluated in members of three homologous series, using two theoretical models. The variation of polarizabilities and their anisotropies with number of carbon atoms in the alkyl chain is studied. The temperature dependence of order parameter is also presented. The results are compared with the reported values and the merits of the different models are discussed.

CALCULATION OF THE INTERACTION POTENTIAL FOR BICHROMOPHORIC HETERO-MOLECULAR CLUSTERS

An expression for the interaction potential between two anisotropic molecules is derived. This expression is suitable for describing the van der Waals interaction between two chromophores within a bichromophoric molecular cluster. For the anthracene-naphthalene and for the benzene-biacetyl clusters the calculation predicts the existence of two isomers, in agreement with experimental observations. The model is also successfully applied to other clusters yielding better results than those obtained by alternative methods which do not take into account the anisotropy of molecular polarizability.

On the Consistency and Applicability of Internal Field Models in Nematics

Abstract Information on anisotropy of the local field of light wave in nematics is obtained mainly from refractivity measurements. Such investigations, supplemented by those concerning thermal dependence of density ρ(T), allow for determining optical polarizability anisotropy of nematogenic molecules, as well as their orientational order parameter. This paper discusses, on the basis of extensive experimental data, the following problems:•applicability of approximate relations between refractive indices of nematic liquid crystals (NLCs) and their molecular parameters for describing optical properties of nematic mesophase has been estimated;•anisotropy of the local field factor Fk has been investigated, as well as its influence on calculated anisotropy of molecular polarizability. Influence of various extrapolation procedures on calculated molecular parameters of the NLCs has been investigated.

A New Optimization of Atom Polarizabilities in Halomethanes, Aldehydes, Ketones, and Amides by Way of the Atom Dipole Interaction Model

A new optimization of atom polarizabilities is carried out by fitting observed mean molecular polarizabilities, polarizability anisotropies, and Kerr constants with values calculated by way of the atom dipole interaction (ADI) model which was used for similar purposes by this laboratory in 1972. New values are obtained for the polarizabilities of F, Cl, and Br in halomethanes, C‘ and O in aldehydes, ketones, and amides, H in the aldehyde C‘HO and the amide NH group, and N in amides. Most of the new values differ slightly from atom polarizabilities that were optimized to fit only observed mean polarizabilities in 1972, though differences up to 50% occur. The new values result in a considerably improved fit of calculated and observed anisotropies and Kerr constants, with a somewhat poorer fit to mean polarizabilities. The results are compared with those from studies by Birge, Thole, and Miller, who used modified versions of the ADI model. It is concluded that the unmodified model reproduces observed anistotropies about as well as any other model with the revised atom polarizabilities.

Determination of order parameters in realistic atom-based models of liquid crystal systems

The calculation of nematic order parameters within molecular simulations is discussed in the context of a realistic simulation of the nematogen trans-4-(trans-4-n-pentylcyclohexyl)-cyclohexylcarbonitrile (CCH5). The use of the inertia tensor, the molecular polarizability tensor and functional groups to define molecular ordering is discussed, and results are given for each of these approaches. Configurations from molecular dynamics simulations are used to determine conformationally averaged values for the molecular polarizability anisotropy of CCH5.

Calculation of intermolecular interaction in aromatic molecular clusters from direction dependent atom-pair potentials

An expression for the interaction potential between two anisotropic molecules is derived. This expression is suitable for describing the van der Waals interaction between two chromophores within a bichromophoric molecular cluster. For the anthracene–naphthalene cluster the calculation predicts the existence of two isomers, in agreement with experimental observations. The model is also successfully applied to other clusters yielding better results than those obtained by alternative methods which do not take into account the anisotropy of molecular polarizability.

Dielectric Studies for Dipole-Dipole Association of Mesogenic Molecules in Para-Xylene

Abstract Results are reported for dipole-dipole association of mesogenic molecules in para-xylene solution. Measurements of Kirkwood correlation factors for solutions of 4-n-pentyl-4-cyanophenyl cyclohexane (PCH5) and 4-n-heptyl-4-cyanophenyl cyclohexane (PCH7) are interpreted in terms of new madel by Dunmur et al. 10 for dipole-association. Results suggest that the molecular association depends on molecular shape and polarizability anisotropy of the system.

Molecular polarizabilities in gases and liquids – induced interactions in liquids – Calculations from classical and revised expressions

A comparison of the optical mean molecular polarizability ᾱ and of the molecular polarizability anisotropy γ2 between gases and liquids, based on reliable measurements is made on twelve compounds. The determinations through the spherical cavity model indicate slight decreases in ᾱ and large deviations in γ2 values from gases to liquids. A large decrease in γ2 values is observed when a corrected formula is used. It is shown by a simple electrostatic bimolecular interaction scheme that the apparent decreases observed in liquids are due to the short distance mutual interactions between the molecular dipole moments induced by the electric field of the incident optical wave. Detailed calculations are made by using the ellipsoidal cavity model, which includes the induced interactive effects in a consistent macroscopic way. They lead to ᾱ and γ2 values in liquids which are in agreement with the gas values in the case of a corrected light scattering formula. On the contrary the classical formula gives inconsistent high values. These results show the prevalence of the previously established corrected formula, and bring informations about the origin of the apparent deviations from gases to liquids.

Depolarized Rayleigh scattering of polymeric materials and their oligomers: Instrument design, calibration, and data analysis issues

The importance of obtaining dependable values of molecular optical polarizability anisotropy for polymers and their oligomers lies in their subsequent use in the rotational isomeric state theory and in calculations involving the components of the optical polarizability tensor. This work describes a depolarized Rayleigh scattering photometer able to determine optical polarizability anisotropy of oligomers, melts, and polymer compounds in solution with a combination of exceptional accuracy and high sensitivity. Two separate calibration techniques are reported that ensure the accuracy and preciseness of the results obtained from the measurements. Finally, data analysis procedures are reported that show the sensitivity of the results to the internal field correction.

Effective optical anisotropies of simple molecular liquids from the depolarized Rayleigh spectra

Depolarized Rayleigh spectra (DRS) of the following liquids C 6H 6, C 6H 5CH 3, CS 2 CCl 4 and n-alkanes: C 5H 12, C 6H 14, C 7H 16, C 8H 18, have been studied. The applied apparatus permitted exact determination of the Lorentz type central line as well as its exponential wing. The percent share of the interaction induced contribution to DRSS as well as the square of the effective anisotropy of molecular polarizability due to interaction induced mechanisms have been determined.

Nematic Ordering in nOCB Series from Molecular Polarizabilities

Abstract The molecular polarizabilities (α0, αe,) of the four members of the homologous series of cyano-alkyloxy-biphenyl (CnH2n+1.O.φ.φ.CN for n = 5 to 8) in the nematic range have been calculated using two different internal field models. The additive rule of bond polarizabilities has been adopted to get the mean polarizability (α) and the principal molecular polarizability anisotropy (α1 - α1) values for these compounds. The orientational order parameters have been calculated from these results. The ordering exhibits an odd-even alternation along the series. The results have been compared with different theories. Possible causes for deviation from the theories have been discussed. Besides the relative stabilities of the alkyloxy cyano biphenyls have also been discussed in terms of the molecular parameters.

Computer simulation study of the Rayleigh and Raman spectra of fluid N2

Computer simulations of the Rayleigh and Raman spectral time-correlation functions (TCFs) are reported for N2 at 323 K and densities ranging from 0.48 to 2.5 times the critical value. The results are compared with experiment. Particular attention is paid to the density dependence of the spectral intensity due to the correlated permanent polarizability and to the interaction-induced contributions to the polarizability. The partial cancellations that occur between various two-, three-, and four-body terms in the cross and collision induced (CI) parts of the spectral TCFs are evaluated. It is shown that these terms are significant in both the Raman and the Rayleigh spectra, but cancellation greatly reduces their net contribution at all densities studied. The weak but uncancelled TCFs that are associated with orientational correlations of the molecular polarizability anisotropies are shown to be a significant part of the high density Rayleigh TCFs. It is argued that the long-range nature of this TCF means that its simulated values are poorly known.

Refractometric studies of mesogenic molecules in isotropic solutions

Abstract The Lorenz-Lorentz equation for binary isotropic mixtures, consisting of anisotropic molecules, has been derived using the point-dipole approximation taking into account only pair molecular correlations. This permits the calculation of the effect of molecular correlations on the refractive index. Special attention has been paid to the case of infinite dilution in the solvents consisting of isotropic molecules, where an experimental check of the equations is possible. The specific refraction of some solutes with different molecular polarizability anisotropy and polarity has been studied in various solvents. It has been shown, that the main theoretically predicted features are observed in these experiments, but for a quantitative comparison information on the two-particle distribution function is needed.

Molecular Polarizability Anisotropies and Order Parameters of Di-n-Alkyl 4-[4-(4-N-Octyloxy-Benzoyloxy)-Benzoyloxy]-Benzylidenemalonates

Abstract For the homologous series of the swallow-tailed compounds the relative molecular polarizability anisotropies and the temperature dependence of the order parameter in the nematic and smectic A phases were determined from refractive indices using the isotropic internal field model of Vuks and the mean field fitting method of Tough and Bradshaw. The Maier-Saupe mean field interaction parameter is in the nematic phase nearly constant and shows in dependence on alkyl chain length a strong alternation in the smectic A phase. The relative polarizability anisotropy is decreasing alternately in both phase types with extension of the alkyl chains of the swallow tails.