Correlations Of Molecular Orientations Research Articles

Direct experimental assessment of the strength of orientational correlations in polar liquids

The strength of molecular orientational correlations in polar liquids is assessed by means of comparison of the diffuse scattering patterns of a liquid composed by molecules devoid of permanent electric dipole but having a weak quadrupole moment and those for a liquid composed by permanent molecular dipoles. The extent of orientational correlations within the liquid phases is in both cases assessed by comparison of the liquid radial distributions to those present in the rotator-phase (plastic) crystal phases of both compounds. For such disordered-crystal phases, information concerning orientational correlations is directly derived from the experimental scattering patterns by means of analysis of the diffuse scattering background present beneath the Bragg peaks. The results show that rather than long-ranged, orientational correlations in polar or polarizable liquids are confined within distances comprising the second coordination sphere.

Motional states of magnetic molecules and their coherent superposition

We study the coherent superposition of motional states in magnetic molecules. The distinct time evolution of each motional state in the presence of an external magnetic field and the dynamics of the molecular rotation are found to be interrelated by a molecular orientation correlation function. The correlation of two distinct molecular orientations is then investigated in terms of a dimensionless time variable and is found to reflect the inertia of the reorientation process. The relative phase in the coherent superposition gives evidence of a continuous redistribution of the interaction energy among the motional states of the superposition. Numerical simulations are provided for the prototypical magnetic molecule O2 in its ground state. Relevance is indicated for experiments on molecular interferometry, coherent control, and quantum information.

Classical Molecular Dynamics Simulation of Kappa Squared Factor in Resonance Energy Transfer for Linear Dipole Models

Molecular dynamics simulations of linear models interacting through a dipolar Kihara intermolecular potential are presented. Molecular orientation correlations are used to calculate the orientational factor kappa squared in the resonance energy transfer (RET) as a function of the intermolecular separation. The distance, R 0 (2/3), at which the simulated systems show an isotropic behavior is calculated and an analysis of the dependence of R 0 (2/3) on microscopic properties (molecular aspect ratio and dipole moment) as well on thermodynamics (temperature and density) is presented. An explanation of the use of metallic cations as probes in RET is given and some relations of our models with biological molecules are pointed out.

Correlation of molecular orientation with adhesion at polystyrene/solid interfaces

Vibrationally resonant sum-frequency generation (VR-SFG) has been used to characterize the molecular orientation of the phenyl groups at the interface between polystyrene (PS) films and surface-modified glass substrates. Both the interface structure and the film adhesion are found to vary as the substrate surface is changed from hydrophobic to hydrophilic. The improved adhesion on the hydrophilic substrate is attributed to the development of attractive hydrogen bonds between surface hydroxyl (OH) groups and the π electron cloud of the phenyl ring. This interaction is manifest by changes in the orientation distribution of the phenyl rings at the interface.

Molecular Light Scattering and Orientational Pair Correlations of Molecules in Liquid Chloro‐ and Bromobenzenes

Integral intensities and degrees of depolarization of molecular light scattering were measured for chlorobenzene, bromobenzene, o‐chlorotoluene, and o‐ and m‐dichlorobenzenes in the temperature range 20–80° at 5° intervals. After the isotropic and anisotropic components of light scattering had been isolated, the isothermal compressibility and angular correlation function, respectively, were calculated. For all compounds under study, the temperature dependences of the above characteristics have some special features in the range 40–50°, which may be explained by variation in the local structure of liquids in the stated temperature range.

Surface effects and dipolar correlations of confined and constrained liquids investigated by NMR relaxation experiments and computer simulations

Local order and molecular dynamics of liquids near surfaces strongly deviate from the behavior in the bulk. This in particular refers to liquid crystals above the bulk isotropization temperature. Transverse relaxation data of 5CB examined in porous glasses with different pore sizes are reported. A strong pore size effect was found. For the interpretation, a simple diffusion-adsorption computer simulation was carried out. Molecules can diffuse from the isotropic bulk part of the pore fluid to the ordered surface layer and vice versa. The residual dipolar correlation function is characterized by a slowly decaying tail owing to repeated returns of molecules to the surface. At each return the molecular orientation correlation is recovered as far as the surface sites visited have orientations correlated to the initial site. That is, molecular orientation is controlled by the “reorientation mediated by translational displacement” process considered in previous papers.

Collective molecular rotation in water and other simple liquids

Hyper-Rayleigh light scattering measurements of molecular motion in several liquids find that the orientations of the molecules are correlated over macroscopic distances. Such collective molecular rotation occurs in addition to the usually observed, highly localized, diffusive mode of molecular reorientation. Long range correlation of molecular orientation may have significant unrecognized effects on the structure and properties of liquids.

Structure of liquid carbon tetrachloride up to the subcritical point measured by wide Q-range neutron diffraction

Structure of liquid and gaseous CCl4 at elevated temperatures from 195 °C to the subcritical point 275 °C (Tc=285.15 °C) along the liquid–vapor coexistence curve have been studied by means of pulsed neutron diffraction. Structure factor in a wide range of momentum transfer Q from 0.03 to 20 Å−1, covering from the small angle scattering region to atomic-scale structure region, has been obtained. Long-range density fluctuation and atomic scale structure have been analyzed: (1) molecular orientational correlation still exists in liquidCCl4 at 275 °C while it disappears in its saturated vapor; (2) correlation lengths of density fluctuation are 10–13 Å for the liquid at 275 °C and 13–17 Å for its saturated vapor; and (3) molecular-density fluctuation (〈Nm2〉−〈Nm〉2)/〈Nm〉 is estimated to be 2.4 for the liquid at 275 °C and 9.3 for its saturated vapor.

Correlation of molecular orientation and shear viscosity of high molar mass polymers in dilute solution

The shear induced orientation of high molecular weight polystyrene ( M w ≌ 10 7 g mol ) in dilute solution has been determined by means of wide angle laser light scattering in the flow-plane of a Searle-type shear cell. The shear dependent molecular orientation is related to the orientation resistance parameter, m, that characterizes the influence of solution properties on the orientational dynamics of the polymer coil. For several solvents from the good to the poor, the shear dependent variation of m is compared to the viscometric behaviour of the solutions. The experiments show, that a certain critical shear rate, β crit , defined by a change in the shear dependence of m, correlates with the shear rate characterizing the transition from the non-Newtonian decrease of viscosity to a viscosity plateau. This correlation is quite obvious in the poor solvent trans-decalin and becomes less pronounced with increasing solvent quality. For the given polymer, β crit is a function of both the solvent viscosity and the intrinsic viscosity. The discussion of different theoretical approaches to the non-Newtonian flow of dilute polymer solutions demonstrates an agreement between theoretical understanding and the observed viscometric behaviour. The observed orientational behaviour below β crit , however, is not explained by present theory. This deviation is attributed to intermolecular entanglement of the very high molecular weight polymers even at rather low concentrations.

Correlation of Molecular Orientations at the Interface of Organic Double-Layered Thin Films

Correlation of molecular orientations in the respective layers of vapor-deposited double-layered thin films of N,N′-dimethylperylene-3,4,9,10-bis(dicarboximide) (DM-PBDCI) and chloroaluminum phthalocyanine (ClAlPc) has been examined by means of X-ray diffraction and (polarized) electronic absorption. After examining molecular orientations in single-layered thin films, correlated orientations in double-layered films were disclosed as ClAlPc(1 1 0)/DM-PBDCI (0 1 1)/SS and ClAlPc(1 1 0)/DM-PBDCI(0 0 2)ISS for the films prepared at r.t. and hgh temperatures, respectively, and DM-PBDCI(1 0 2)/ClAlPc(0 4 0)/SS, where SS denotes a substrate. Further, it was derived by analyzing the results of electronic absorption spectra that transition dipole moments of two different molecules facing to each other at the interface of double-layered thin films are in most cases perpendicular or parallel to one another, and this could explain the observed correlations in molecular orientations above.

Analysis of the x-ray diffuse scattering in C60 from microscopic models.

The molecular orientational correlations in the high-temperature phase of solid ${\mathrm{C}}_{60}$ have been analyzed through calculations of the diffuse scattering for different microscopic models of intermolecular interactions. A mean-field theory up to the $l=12$ terms in the multipolar expansion accounts for the main features of the complex x-ray diffuse scattering in the ${Q}_{1}\ensuremath{\simeq}3.3$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$ halo. Instabilities relative to competing phases are revealed. It is also shown that diffuse scattering calculations are good probes for testing the intermolecular interaction models.

Theory of universal fast orientational dynamics in the isotropic phase of liquid crystals

A theoretical treatment is presented that demonstrates universal dynamical behavior in the isotropic phase of liquid crystals on ultrafast time scales and short distance scales. The theoretical development generates a temperature independent power law for the short time scale decay of the molecular orientational correlation function. This provides a theoretical rationale for the postulate of universal behavior based on recent experimental observations on two liquid crystal systems. A temperature independent power law decay with the identical exponent, 0.63, was observed for the two systems. First, an alternative theoretical approach reproduces the Landau de Gennes results for the long distance scale, slow time scale orientational dynamics in the isotropic phase. This approach is also capable of examining the short distance scale and short time scale dynamics, and yields a temperature independent power law decay with exponent 0.5. Then critical correlations of fluctuations and local symmetry considerations are included. The Ising model of critical systems is employed. This detailed analysis yields the experimentally observed exponent, 0.63, without recourse to adjustable parameters. Modern theories of dynamic critical phenomena like dynamic scaling theory, the kinetic Ising model and the stochastic model of Karder–Parisi–Zhang are considered as alternative approaches. While these theories can generate some of the features found in experiment, it is not possible to reproduce the observed experimental results without internal inconsistencies or unwarranted adjustable parameters.

Heat capacities and concentration fluctuations in mixtures of 1,2-dibromoethane with alkanes

Excess volumes, excess heat capacities and light scattering have been measured for several 1,2-dibromoethane–alkane (n-heptane, n-octane, n-tetradecane, cyclooctane and 2,2,4-trimethylpentane) systems. The results show that there is good agreement between values of the concentration–concentration correlation function Scc, as calculated from light-scattering data, and the predictions of the simple rigid lattice model of Flory and Huggins, which estimates the relative effects of the differences in molecular sizes and of differences in the interaction energies of the two components. The results also confirm the utility of Scc as predictor of the existence of W-shaped excess isobaric heat capacity (CEP) curves, although other effects, such as the destruction of the (partial) correlation of molecular orientation may complicate this issue.

Monte Carlo simulation of semiflexible hard triatomic fluids

The density-density and orientational correlations in semiflexible hard triatomic fluids are investigated via Monte Carlo simulations and integral equation theory. The intramolecular correlations, intermolecular site-site and centre of mass pair correlation functions, and molecular axis and segmentsegment orientational correlation functions are monitored for several densities and molecular flexibilities. The properties of the model are found to interpolate smoothly between the limiting cases of freely jointed trimers and rigid linear trimers. A self-consistent implentation of the Chandler-Andersen (RISM) theory is found to be in qualitative and sometimes quantitative agreement with simulations for the site-site correlation functions. The importance of using the same approximations in the RISM theory for both the closure relation and the solvation potential is demonstrated.

The Onsager Approximation and Nonspecific Intermolecular Interactions in Polar Media

Onsager's model for isotropic dielectric liquids greatly overestimates the molecular orientation correlations and the dipole–dipole interaction energy and cannot be used to estimate the solvation energy.

Solution properties and characterization of polyisoprenes at a lower critical solution temperature (LCST)

This paper reports two investigations on polyisoprene. One consists of obtaining the molecular weight distribution (MWD) of cis-polyisoprene (PIP) standards by a new method using the turbidity at a lower critical solution temperature (LCST). Advantage is taken for this analysis of the fact that the MW dependence of the critical temperature is greater for systems with a narrow solubility gap, i.e. for systems whose LCST and upper critical solution temperature (UCST) are not too far apart. Typically, in n-pentane, a narrow MW standard phase-separates over 25 K. When the solution is heated by temperature increments of 2 K, 15 fractions can be identified, without physical separation. The relative amount of each phase is analysed by turbidity and the average MW of the phases by the temperature at which they phase separate. The MWs recalculated with two constants for the standards are in excellent agreement with gel permeation chromatography (g.p.c.) data. Possible effects leading to a higher polydispersity found by the LCST are discussed. Application to rubber analysis is feasible. The second investigation consists of reporting and analysing the temperatures of phase separation of medium MWD cis- and trans-PIP samples in a series of linear and branched alkanes. The occurrence of correlations of molecular orientations (CMO) in the melt, similar to those found from thermodynamic analysis of PE solutions, was considered. Trans-PIP has a LCST about 25 K lower than cis-PIP in alkanes. Linear solvents are better solvents than branched ones. The low LCSTs (100°C in n-pentane) support CMO or order in polyisoprene melt or concentrated solutions.

Molecular orientational correlation in liquid halogens

The characteristic features of structure factor Sm(Q) of liquid halogens F2, Cl2, Br2, and I2 have been discussed in terms of preferred orientational correlations introduced between the nearest neighbors. The orientational correlations estimated directly from the experimental Sm(Q)’s of these liquids are rather similar to each other and are related to a ‘‘staggered parallel’’ orientation proposed previously for liquid Br2. The characteristic features of these Sm(Q)’s are interpreted quite well by two structurally simple quantities, i.e., the ratio of intramolecular bond length to intermolecular center–center separation of correlated molecules l/Rc and that of intramolecular bond length to effective diameter of uncorrelated molecule l/σ. The characteristic features of the experimental radial distribution functions are also reproduced quite well.

Order destruction and order creation in binary mixtures of non-electrolytes

Order-destruction and order-creation during mixing are discussed for binary mixtures of various liquids (component 1) with the n-alkane series (component 2). When component 1 is a spherical molecule liquid, e.g. cyclohexane, anomalous positive contributions occur in H E and S E, but negative ones occur in C P E, Δ C V, d V E/d T and other second-order quantities. The Flory theory is used as a first approximation to the prediction of these quantities. Thermodynamic effects, not included in the theory, are attributed to a net decrease in structure during mixing because of a destruction of short-range correlations of molecular orientations (CMO) in pure n-C n . When component 1 is a plate-like molecule, e.g. 1-chloronaphthalene, these effects are decreased and reversed in sign. This has been ascribed to a net increase in structure because of a hindrance to molecular motion or a change in molecular conformation. When component 1 is a linear or branched alcohol, the n-C n are relatively inert while component 1 is associated in both the pure and solution states leading to positive or negative effects in C P E depending whether the solution or pure component 1 is more structured. The Treszczanowicz-Kehiaian model for self-associated liquids + inert solvents explains these results and also provides a general framework for the discussion of both the process of order-creation and the process of order-destruction in solution.

A neutron diffraction study of liquid formamide

Neutron diffraction data on seven different isotopes of formamide, containing 1H and 2H, are reported. Background, self-absorption of the sample and container and multiple scattering corrections were done in the previous way, whereas a new empirical procedure for the inelasticity correction is presented. After separation of the coherent scattering contribution into intramolecular and intermolecular parts the latter was analysed in terms of four expansion coefficients of the molecular pair-correlation function. These give information about the molecular centre-centre distributions and orientational correlations. It is found that in the first neighbour sphere at 4.2 Å an antiparallel orientation of the molecules is preferred, whereas a parallel orientation prevails at 5.1 Å.

Macromolecular conformational volume and thermodynamics of crystal melting

Conformational parts of the thermodynamical function change on melting, in particular the conformational volume ΔV conf, the conformational entropy ΔS conf and the pressure variation of the latter are analysed for an ideal polymer crystal. These quantities can be calculated from the data for low molecular compounds combined with the three-state rotational isomer model (with adjacent gauche states of opposite sign forbidden). The assumption of intramolecular dominance in the Gibbs energy difference between conformers in a polymethylene chain results in a negative value for ΔV conf and a positive value for the quotient ( ∂ΔS conf ∂P ) T which contradicts the available experimental data for polyethylene crystal fusion. The disagreement might be reconciled by the postulation of intermolecular contribution to the Gibbs energy difference between conformers favouring the existence of trans sequences of bonds in the chain and the correlation of molecular orientations in neighbouring chains.