Tilt Of Molecules Research Articles

Molecular Mean Field Study on Even–Odd Effect in Discotic Liquid Crystals

The mean field model for rectangular lattice having herring-bone packing of tilted molecules has been extended to reproduce even–odd alternation in the transition temperatures in a discotic system by taking into account the specific change in layering interaction and molecular biaxiality for each member of a homologous series.

Structure of CdS nanoparticles containing cadmium arachidate LB films

Y-type cadmium arachidate LB films were exposed to H 2S gas for different durations. UV-VIS absorption spectra reveal an absorption edge at 455–460 nm in a 30 min H 2S exposed film which does not change appreciably on further exposure. The blue shifted absorption edge is attributed to the formation of 30 Å CdS nanoparticles. The H 2S exposed films exhibit two types of layered structures, that is, the original bilayer spacing of 55.2Å as well as an additional bilayer spacing of 44Å. The XRD peak intensities corresponding to the new bilayer spacing (44Å) increase up to 6h of H 2S exposure accompanied by an increase in bandgap absorption. These results suggest that CdS nanoparticles are formed within domains having tilted arachidate molecules distributed throughout the film without completely destroying the original layered structure of the LB film.

Supramolecular ordering of bipolar lipids from Archaea in Langmuir-Blodgett films by low-angle X-ray diffraction

The results of an investigation by X-ray diffraction on the supramolecular organization of a peculiar class of semisynthetic lipids, namely bipolar lipids from archaebacteria, when organized in layered structures in Langmuir-Blodgett films are reported. The simplicity of the experimental technique and the straightforward interpretation of the data provide a clear and unambiguous determination of the molecular conformation of bipolar lipids in LB films. The experimental data make it possible to reject unequivocally both the model of monolayers of fully extended molecules and the model of monolayers of tilted molecules, while they strongly support a structural model consisting of the stacking of bilayers made of bent U-shaped molecules. The results reported may constitute a direct clarifying contribution to the controversial matter of the molecular conformation of bipolar lipids at the air-water interface.

Tilt of N2 molecules physintercalated into C24K and C24Rb

The intercalation of molecular N2 into C24X (X = K, Rb) has been studied using the nuclear resonance photon scattering (NRPS) technique. We found that, in the physintercalated state, the nitrogen resides in the alkali planes, in a molecular form, with the molecular symmetry axis almost parallel to the graphene basal planes. In C24K, the N2 molecules are only intercalated below 200 K, while in the case of C24Rb, the physintercalation process starts at room temperature and the amount of intercalated N2 increases with decreasing T, reaching saturation at around 130K. These samples were also studied using n-diffraction and the0 N2 “small” diameter, at 120K, was found to be 0.290 nm in the (C24K + N2) system and slightly smaller, 0.279 nm, in the (C24Rb + N2) system. This difference in the measured diameters could also be attributed to a smaller tilt angle of the N2 molecule in C24Rb as compared to that in C24K.

Phase transitions in monolayers of medium-chain alcohols on water studied by sum-frequency spectroscopy and ellipsometry

Monolayers of medium-chain alcohols CH3(CH2)m–1 OH (m= 9–14) have been studied by sum-frequency vibrational spectroscopy and ellipsometry in the vicinity of the two-dimensional solid–liquid phase transition. Gauche defects are present in the solid phase and increase only slightly in the monolayer just above the phase transition. A model of the chains as freely rotating rigid rods permits a calculation of the area per molecule and chain tilt in the liquid phase. The density of the hydrocarbon chains in the liquid monolayer phase is less than that in the solid monolayer phase but significantly higher than in a bulk liquid alkane. The area per molecule, chain tilt and volume per CH2 group in the liquid phase all increase with increasing chain length.

A Molecular Meanfield Model for the Biaxial Rectangular Discotic Phase with Herring-Bone Packing of Tilted Molecules

Abstract A molecular meanfield model for a rectangular columnar to hexagonal columnar phase transition, as observed in case of the HAT series, is presented here assuming the molecules basically to be biaxial objects. The theory is developed for a rectangular lattice having herring-bone packing of the tilted molecules. Various types of phase diagrams for a homologous series are reproduced for different values of molecular biaxiality. Some of them are very similar to the experimentally observed cases but others, particularly involving biaxial nematic phase, are not yet observed. It is shown that a gradual lowering of biaxiality with the addition of flexible chain segments can lead to the full phase sequence observed in the HAT series.

Microscopic origin of spontaneous polarization in ferroelectric SC∗ liquid crystals

Abstract The origin of spontaneous polarization in the ferroelectric smectic C∗ phase is investigated within a mean-field microscopic model which describes the coupling between the tilt of molecules from the normal to the smectic layers and the rotation of a molecule around its long axis. The mean-field potential is studied which takes into account a chiral polar and a non-chiral quadrupolar biasing of the rotation of molecules around the molecular long axes. Each molecule is characterized by three transverse molecular axes: the chiral axis which turns parallel to the macroscopic C2 axis at small tilts, the polar axis in the direction of the transverse dipole moment and the quadrupolar axis which tends to be parallel to the C2 axis at very large tilts. A numerical analysis of the model shows that there are four different types of spontaneous polarization dependent on the temperature, including the sign-reversal type. The influence of three microscopic parameters, i.e. two angles between the three characte...

Correlation field, structure, and phase transition in the monolayer CO adsorbed on NaCl(100) as revealed from polarization Fourier-transform infrared spectroscopy

The polarized infrared spectra of the monolayer of natural and isotopic CO adsorbed on NaCl(100) were measured and calculated. Above 35 K the monolayer forms a p(1×1) structure with one CO molecule, aligned normal to the surface, per unit cell. Below 35 K a phase transition occurs. The CO molecules tilt, a correlation field splitting appears and a p(2×1) structure is formed with two equivalent molecules per unit cell. These are related by glide reflection.

Two‐Dimensional Phase Transitions and Correlation Field Splitting in Unimolecular Layers of CO Physisorbed on NaCI(100) as Detected by Polarization Fourier‐Transform IR Spectroscopy

AbstractA phase transition in the monolayer CO physisorbed on NaCI(100) was observed by means of polarization Fourier‐transform infrared spectroscopy. Above 35 K the spectra are in concert with molecules oriented perpendicularly to the surface, the unit cell being of p(1 × 1) symmetry. Below 35 K the molecules tilt, and a correlation field splitting appears. In this temperature range the spectra are in accordance with a p(2 × 1) structure with two equivalent molecules per unit cell, which are connected by a glide plane. Submonolayer spectra indicate random distribution of perpendicularly oriented CO molecules above 35 K, while at temperatures below 35 K ordered islands are formed.

The microscopic model of polar and quadrupolar ordering in ferroelectric smectic C*liquid crystals

Abstract Thermodynamic model, based on the Landau free energy density expansion, which includes polar as well as quadrupolar coupling between the tilt of molecules and their transverse ordering, is critically reviewed. This model can account for anomalous properties of ferroelectric liquid crystals close to the SmC* - SmA transition, but does not include thermal fluctuations of transverse molecular degrees of freedom. Those fluctuations are taken into account in the corresponding microscopic model. Comparing the results of the two models it is shown that the fluctuations decrease the anomalies in polarization behavior.

The determination of monolayer structure by infrared spectroscopy: CO 2 on NaCl(100)

Infrared spectra of a 12C 16O 2 monolayer on NaCl(100) are reported for the first time. The polarization and coverage dependence of spectroscopic signals indicate that there is only one kind of adsorption site, in which the molecular axis is tilted 68° from the surface normal. The adsorbate layer grows in the form of constant-density islands. Plane group symmetry analysis reveals a unique structure: there are two tilted molecules per layer unit cell, arranged in herringbone fashion. This structure is in quantitative agreement with the photometric observations. Splitting of the molecular v 3 band is interpreted by a vibration-vibration coupling mechanism.

Microscopic model of the spontaneous polarization in ferroelectric liquid crystals

Abstract A microscopic model is introduced which describes the coupling between the tilt of molecules from the normal to smectic layers in ferroelectric liquid crystals and the rotation of a molecule around its long axis. The single particle potential consists of a dipolar and a quadrupolar term. The microscopic model is shown to be consistent with the extended thermodynamic model, which takes into account the bilinear as well as the biquadratic coupling between the tilt of molecules and the induced transverse polarization, and which describes the anomalous thermodynamic behaviour of ferroelectric liquid crystals in the smectic C* phase. It is shown that close to the transition to the smectic A phase the thermodynamic description is exact, but overestimates the spontaneous polarization at lower temperatures. The results are compared with experimental data. It is demonstrated that the quadrupolar order is larger than the dipolar order except very close to the transition.

Thermodynamic model of ferroelectric liquid crystals and its microscopic basis

Abstract The properties of the extended thermodynamic model (Phys. Rev. A 36, 1484 (1987)) of ferroelectric liquid crystals, which can describe the anomalous thermodynamic behaviour of these systems in the chiral smectic C* phase, are reviewed. In the model the bilinear as well as biquadratic coupling between the tilt of molecules and the spontaneous polarization is included. The bilinear coupling is of a chirak nature and therefore small. This leads in the smectic C* phase close to the transition from the smectic A phase to cross-over effects between the regime, where because of small tilt only bilinear coupling is important, to the regime, where the biquadratic coupling also becomes relevant. A microscopic model is introduced, which describes the coupling between the tilt of molecules and the orientation of the molecular dipole, which is perpendicular to the long molecular axis. It is shown that the microscopic model is consistent with the extended thermodynamic model and the parameters of the thermodynamic model are expressed in terms of the microscopic parameters.

Spontaneous polarization of helicoidal ferroelectric smectic C* liquid crystals

Measurements of the spontaneous polarization of DOBAMBC in helicoidal smectic C* phase is presented. It is shown that the polarization shows an anomalous temperature dependence characterized by one square root behaviour close to Tc and by another one far from Tc. This behaviour is explained theoretically to be a consequence of a nonchiral biquadratic coupling between the polarization and the tilt of molecules in the Landau expansion of the free-energy density. Also the tilt is predicted to exhibit a (less pronounced) crossover region of the same type as the polarization within the proposed model. It is also noted that this crossover region coincides with the temperature interval where the pitch of the helix displays anomalous properties.

The influence of the boundary conditions on the character of the phase transition nematic‐smectic C

AbstractThe possibility is investigated for inducing the intermediate smectic A structure in substances with nematic and smectic C phases by the boundary forces. Different geometries are used: homeotropic, homogeneous and tilted under a different angle toward the supporting substrates of the liquid crystal cell. At homeotropic and at tilted orientation where the molecules tilt toward the supporting substrates is different from the molecule tilt in the smectic layer, an intermediate smectic A structure is observed at the transition nematicsmectic C in a narrow temperature interval. In this case the phase transition nematicsmectic C extends and transforms into phase transition of second order. At homogeneous orientation and at tilted orientation of an angle equal to the molecule tilt in the smectic layer, the transition nematic‐smetic C carried out in two stages, as partially it begins on the surface still in the nematic phase and terminates at the temperature of the phase transition N‐Sc. The influence of the boundary forces depends essentially on the energy of the phase transition N‐SC of the substances.

Resting myosin cross-bridge configuration in frog muscle thick filaments.

Clear images of myosin filaments have been seen in shadowed freeze-fracture replicas of single fibers of relaxed frog semitendinosus muscles rapidly frozen using a dual propane jet freezing device. These images have been analyzed by optical diffraction and computer averaging and have been modelled to reveal details of the myosin head configuration on the right-handed, three-stranded helix of cross-bridges. Both the characteristic 430-A and 140-150-A repeats of the myosin cross-bridge array could be seen. The measured filament backbone diameter was 140-160 A, and the outer diameter of the cross-bridge array was 300 A. Evidence is presented that suggests that the observed images are consistent with a model in which both of the heads of one myosin molecule tilt in the same direction at an angle of approximately 50-70 degrees to the normal to the filament long axis and are slewed so that they lie alongside each other and their radially projected density lies along the three right-handed helical tracks. Any perturbation of the myosin heads away from their ideal lattice sites needed to account for x-ray reflections not predicted for a perfect helix must be essentially along the three helical tracks of cross-bridges. Little trace of the presence of non-myosin proteins could be seen.

Molecular topology in crystals and fibers of hemoglobin S

Several lines of evidence indicate a close correspondence between the linear double filaments in the crystal form of hemoglobin S grown from solutions containing polyethylene glycol and the seven pairs of helical filaments that occur in the 14-filament fibers of hemoglobin S. An analysis of the adjustments to the intermolecular contacts required to convert the double filaments from crystals to fibers is presented here. In addition, postulated contacts between the helical double filaments, which are distinct from any of the contacts of the crystals, are specified for the first time. The movements from crystals to fibers are described in terms of three rotation angles: α, the inclination of the filaments with respect to the fiber axis; δ, the tilt of successive molecules along the filaments; and ω, the rotation of successive molecules along the filaments. On the basis of the fiber structure determined by three-dimensional reconstruction of electron micrographs and the assignment of filament pairs from data on incomplete fibers, the various angles have been evaluated. For the filaments at various radii in the fibers, a varies from 3 ° to 12 °, δ varies from 1 ° to 4 ° and ω is constant at 9 °. The effects of the rotations on the contacts between molecules of hemoglobin S at various positions in the fibers are characterized using surface maps based on polar coordinates. For each residue on the surface of hemoglobin the centroid position of its side-chain is located by a longitude, a latitude and an altitude. Locations on the maps are assigned for the contacts within the helical double filaments, as well as 11 classes of new contacts describing the potential interaction sites between double filaments. The resulting maps (1) deduce roles for the various α mutants of hemoglobin known to influence fiber formation that have been identified by the Benesches; (2) distinguish effects for the α chain mutants on the same ( cis) or opposite ( trans) α 1 β 1 dimer as the β6 Val in asymmetric tetramers; (3) propose new sites where effects of mutations on fiber formation may be found; and (4) suggest why some mutants may inhibit, while others enhance, fiber formation. Concerning the last point, the possibility of certain mutants “correcting” the effects of other mutants is proposed as a test of contact assignments.

THE RADIO-FREQUENCY DIELECTRIC DISPERSION IN SOLID ETHYL STEARATE

The dielectric properties of solid ethyl stearate were studied between −100° and the melting point at frequencies between 100 cycles/sec and 80 Mc/sec. The circular-arc locus lying within this frequency range in the β (tilted) form consists of a time-dependent and a time-independent part. It is more likely to arise from rotational disorder of tilted molecules than from regions of vertically stacked molecules. The width of the relaxation-time distribution function appears to decrease somewhat more rapidly with increasing temperature than would be expected for a temperature-independent distribution of heights of rotational-energy barriers.

Further observations on the structure of myelin

The changes in X-ray diffraction pattern which accompany modifications of the myelin sheath of frog sciatic nerve (details of which have been published previously) have been reconsidered in detail. It is noted that some modifications lead to increases in the diffraction spacings of the complex structure, and in the case of the dried specimens this is explained in terms of variations in the angle of tilt of lipid molecules within the layered lipoprotein structure. Approximate layer dimensions are deduced from the variation of the diffraction spacings of modified nerve, and the interpretation summarized diagrammatically. In the case of fresh nerve myelin the necessary condensation of the lipid layer thicknesses is achieved by having the longest lipid molecules stabilized individually in a curled position by association with cholesterol molecules rather than by tilting them. This enables the myelin components to retain their radial orientation, and suggests a specific function for the free cholesterol in myelin structure.

On the stability of unimolecular films. Part I.—The conditions of equilibrium

Of the factors which determine the state of equilibrium of a unimolecular film on the surface of water the two which have been subjected to the most exhaustive examination are the effects of changes in temperature and pressure. In previous communications a generalisation of the effects of kinetic agitation on the state of a film, and the conditions of equilibrium imposed by the phase rule on the various states of such films have been described. As pointed out originally by Hardy, in film formation the magnitude and direction of the adhesional forces between the surface and the polar "heads" of the spreading molecules is one of primary importance. The effects of replacement of one type of polar head by another have been examined in detail by numerous investigators; but the effect on the state of a film of one substance of changing the magnitude of this adhesion, a factor clearly as important as the conditions of temperature and pressure, has not hitherto been considered. In these communications the results of an investigation on the effects of such a change are described. Unimolecular films of long chain fatty acids on the surface of water can exist in three well-defined states, the highly dispersed or vaporous, the expanded and the condensed. The expanded state, first noted by Labrouste, was considered originally by Adam to be vaporous in character. Schofield and Rideal ( loc. cit .), however, from an examination of the isothermals of the force area curves for a series of acids concluded that expanded that the true vapour pressure of a film of myristic acid at ordinary temperatures should be about 0∙2 dynes per centimetre. A similar suggestion was made independently by Langmuir and later accepted by Adam, when with a sensitive apparatus he effected the measurement of the vapour pressure of expanded films, which proved to be in good agreement with that predicted by Schofield and Rideal. It seemed probable that expanded films consist of some arrangement of tilted molecules with the chains still enmeshed and the heads free, as suggested by Schofield and Rideal and Langmuir. Adam, however, has concluded that the heads adhere to one another with sufficient tenacity to render this view improbable.