Model-independent structure and resonant X-ray spectra of intermediate smectic phases

It is shown that the orientational structure of intermediate smectic phases can be determined using the symmetry properties of the general free energy with arbitrary orientational coupling between smectic layers, without addressing a particular model. The structure of three- and four-layer intermediate phases, obtained in this way, corresponds to experimental data. The same method enables one to predict the structure of intermediate phases with periodicity of five and six layers, which have not been observed experimentally so far. The resonant X-ray spectra of the five- and six-layer intermediate phases with predicted structure have also been calculated. These spectra are characterized by a number of features which enable one to distinguish five-layer and six-layer intermediate phases from phases with smaller periods.     

Model‐independent structure and resonant X‐ray spectra of intermediate smectic phases

It is shown that the orientational structure of intermediate smectic phases can be determined using the symmetry properties of the general free energy with arbitrary orientational coupling between smectic layers, without addressing a particular model. The structure of three‐ and four‐layer intermediate phases, obtained in this way, corresponds to experimental data. The same method enables one to predict the structure of intermediate phases with periodicity of five and six layers, which have not been observed experimentally so far. The resonant X‐ray spectra of the five‐ and six‐layer intermediate phases with predicted structure have also been calculated. These spectra are characterized by a number of features which enable one to distinguish five‐layer and six‐layer intermediate phases from phases with smaller periods.     

Solid phases of cyclopentane: combined experimental and simulation study

The phase diagram of cyclopentane has been studied by powder neutron diffraction, providing diffraction patterns for phases I, II, and III, over a range of temperatures and pressures. The putative phase IV was not observed. The structure of the ordered phase III has been solved by single-crystal diffraction. Computational modeling reveals that there are many equienergetic ordered structures for cyclopentane within a small energy range. Molecular dynamics simulations reproduce the structures and diffraction patterns for phases I and III and also show an intermediate disordered phase, which is used to interpret phase II.     

The formation of ordered heterostructures during the intercalation of phosphonic acids into a layered double hydroxide

The intercalation reactions have been studied using in situ energy dispersive X-ray diffraction; ordered second stage heterostructures were observed as intermediate phases, which is almost unprecedented for rigid layer host materials.     

On the enthalpies and entropies of formation of solid nickel—zinc alloys

The enthalpies of formation of solid nickel—zinc alloys have been measured at 355 K using an isoperibol calorimeter and the technique of tin-solution calorimetry, and values have been obtained for the nickel-rich α-solid solutions and for the β₁, γ, γ₁ and δ intermediate phases. Exothermic values have been observed throughout and these have been compared with the results obtained in previous free energy studies and with those suggested by the empirical model of Miedema. The existing free energy data have been re-assessed at 900 K and the results combined with the present calorimetric enthalpies to derive entropies of formation. The possible contributions to the entropies of the phases are discussed and their Debye temperatures are estimated.     

Exploring the energy landscape of a small RNA hairpin

The energy landscape of a small RNA tetraloop hairpin is explored by temperature jump kinetics and base-substitution. The folding kinetics are single-exponential near the folding transition midpoint T(m). An additional fast phase appears below the midpoint, and an additional slow phase appears above the midpoint. Stem mutation affects the high-temperature phase, while loop mutation affects the low-temperature phase. An adjusted 2-D lattice model reproduces the temperature-dependent phases, although it oversimplifies the structural interpretation. A four-state free energy landscape model is generated based on the lattice model. This model explains the thermodynamics and multiphase kinetics over the full temperature range of the experiments. An analysis of three variants shows that one of the intermediate RNA structures is a stacking-related trap affected by stem but not loop modification, while the other is an early intermediate that forms some stem and loop structure. Even a very fast-folding 8-mer RNA with an ideal tetraloop sequence has a rugged energy landscape, ideal for testing analytical and computational models.     

High strength disclinations in the nematic and smectic states of 2,5-bis(4-alkoxybenzoyloxy)- p -benzoquinones

High strength disclinations were observed in both nematic and smectic phases exhibited by members of the 2,5-bis(4-alkoxybenzoyloxy)- p -benzoquinone series, beyond 4- n -pentoxy. The shorter members of the series show only a nematic mesophase, the highest members only a smectic mesophase, while intermediate members exhibit both types of mesophase. These high strength disclinations seen in a single component system are very stable due to energy dispersion and the high viscosity of the smectic state. All the high strength disclinations are negative in sign.     

Energetics of rotation and translation in hexagonal aggregates of extended chains

Potential energy calculations have been made for hexane aggregates in the hexagonal arrangement as models of paraffin-like solids. As a part of the general energy hypersurface the feasibility of translation and rotation of extended chains in aggregate has been examined for various degrees of packing. The pure translation is favoured over the pure rotation at all interchain separations. The energy map of coupled rotation-translation motion has been developed for the intermediate degree of packing. The energy minima on the map correspond to the parallel orientation of chain backbones and are presumed to represent the prevailing short-range structure of the rotator phase in n-alkane crystals. The calculated barriers are used for the rationalization of chain dynamics in the rotator phase, and in the disorded phases with the similar alkyl chain packing as found in crystals, monolayers and bilayers of amphiphiles. Probability of formations of the translational, rotational and conformational defects in the above systems is discussed.     

Phase diagram of Gaussian-core nematics

The authors study a simple model of a nematic liquid crystal made of parallel ellipsoidal particles interacting via a repulsive Gaussian law. After identifying the relevant solid phases of the system through a careful zero-temperature scrutiny of as many as eleven candidate crystal structures, they determine the melting temperature for various pressure values, also with the help of exact free energy calculations. Among the prominent features of this model are pressure-driven reentrant melting and the stabilization of a columnar phase for intermediate temperatures.     

Characterization of several stationary phases prepared by thermal immobilization of poly(methyltetradecylsiloxane) onto silica surfaces

Variations of a thermal immobilization procedure using poly(methyltetradecilsiloxane) and silica produced fourteen stationary phases with carbon contents of 4-18%. The stationary phases were chromatographically evaluated with the Engelhardt, SRM 870 and Tanaka tests. Classifications using USP and Euerby procedures indicate that the new immobilized phases are different from most commercial phases although there was some similarity with phases that have high ion-exchange interactions. The retention mechanism involved in the separation of basic solutes on several of the new stationary phases was studied by varying pH, type of Lewis base and the ionic strength of the eluent. The separations are strongly influenced by the chemistry of the accessible free silanols. The stationary phases present good selectivity at intermediate pH where the basic analytes were protonated, suggesting use of intermediate pH for these separations. Stability tests show that the stationary phases have poor stability at very high pH, even at 23°C, but good stability in acidic mobile phases, even at 75°C, as expected for an immobilized polymer stationary phase.     

Quantum dynamical treatment of inelastic scattering of atoms at a surface at finite temperature: the random phase thermal wave function approach

We present quantum dynamical calculations for the inelastic scattering of atoms at a nonrigid surface at finite temperature. The surface degrees of freedom are discretized and treated in a multiconfigurational wave function picture. The thermal averaging is carried out with the random phase thermal wave function approach. We show that it is sufficient to restrict the random phases to the intermediate basis of single particle functions, discuss the convergence of the method with the number of configurations and realizations, and analyze the flow of energy between different parts of the system for a range of temperatures between 4 and 500 K.     

Acrylate formation via metal-assisted C-C coupling between CO2 and C2H4: reaction mechanism as revealed from density functional calculations

The reaction path for the formation of a binuclear hydrido-acrylate complex in a CO(2)-C(2)H(4) coupling process is explored in detail by locating the key intermediates and transition states on model potential energy surfaces derived from density functional calculations on realistic models. The formation of the new C-C bond is shown to take place via oxidative coupling of coordinated CO(2) and C(2)H(4) ligands resulting in a metalla-lactone intermediate, which can rearrange to an agostic species allowing for a beta-hydrogen shift process. The overall reaction is predicted to be clearly exothermic with all intermediates lying below the reactants in energy, and the highest barrier steps correspond to C-C coupling and beta-hydrogen transfer. The phosphine ligands are found to play an important role in various phases of the reaction as their dissociation controls the coordination of CO(2), the formation of the agostic intermediate, and the dimerization process; furthermore, their presence facilitates the oxidative coupling by supplying electrons to the metal center. Our results provide a theoretical support for the reaction mechanism proposed from experimental observations. The effect of the solvent medium on the relative energy of reaction intermediates and transition states is examined and found important in order to predict reliable energetics.     

Kinetically Controlled Solid State Reactions in the System InCl3/Sn

This paper reports on studies on the reactivity of metallic Sn in a solid state reaction in the system In‐Sn‐Cl. The reaction route is investigated by thermoanalysis and XRD measurements. The reaction of InCl₃ with Sn to form InSnCl₃ does not proceed directly but in several steps giving 4 intermediate phases. It can be interpreted as a kinetically controlled reaction where intermediate phases with a smaller degree of order and higher diffusivity are preferentially formed outside thermodynamic equilibria.     

Elongated silica nanoparticles with a mesh phase mesopore structure by fluorosurfactant templating

Mesoporous silica materials with pore structures such as 2D hexagonal close packed, bicontinuous cubic, lamellar, sponge, wormhole-like, and rectangular have been made by using surfactant templating sol-gel processes. However, there are still some "intermediate" phases, in particular mesh phases, that are formed by surfactants but which have not been made into analogous silica pore structures. Here, we describe the one-step synthesis of mesoporous silica with a mesh phase pore structure. The cationic fluorinated surfactant 1,1,2,2-tetrahydroperfluorodecylpyridinium chloride (HFDePC) is used as the template. Like many fluorinated surfactants, HFDePC forms intermediate phases in water (including a mesh phase) over a wider range of compositions than do hydrocarbon surfactants. The materials produced by this technique are novel elongated particles in which the layers of the mesh phase are oriented orthogonal to the main axis of the particles.     

Intermediate nematic phases in binary systems with complex formation

The phase diagrams of some binary systems of smectogenic compounds are presented in which intermediate nematic phases occur over a large concentration range. An attempt has been made to interpret this behaviour in terms of structural differences between the smectic A phases of the components.     

A study of out-of-plane cation dynamics in a bis-thiourea pyridinium chloride inclusion compound

The out-of-plane motion of the pyridinium cation in the bis-thiourea pyridinium chloride inclusion compound has been studied in a wide temperature range using (1)H NMR, dielectric spectroscopy and quasielastic neutron scattering. The geometry of this motion is obtained from the Q-dependence of the elastic incoherent structure factor determined from the quasielastic neutron scattering measurements. We find that the pyridinium cation performs out-of-plane reorientations around the axis passing through two opposite atoms of the ring. The correlation times as a function of temperature were measured in the three known crystallographic phases, finding a good agreement between the three techniques employed. The activation energy for this motion changes from 5 ± 1 kJ mol(-1) in the low-temperature phase to 1.2 ± 0.2 kJ mol(-1) in the intermediate and high-temperature phases.     

Conversion of cellulose Iα to Iβ via a high temperature intermediate (I-HT) and other cellulose phase transformations

The observation that the hydrothermal conversion of cellulose Iα to cellulose Iβ is irreversible has been assumed to be due to the relative free energy of these polymorph phases. We propose an alternative explanation: when cooling the high temperature phase, the barrier to forming Iβ is much smaller than the barrier to forming Iα, so kinetics favor the formation of Iβ. This explanation is consistent with all available experimental data, and is consistent with the general observation of polymer solid–solid phase transformations via metastable intermediate states. While cellulose Iβ may be lower in free energy than Iα, this has not been shown experimentally. Phase transformations of other cellulose polymorphs may be subject to similar kinetic effects when converted via metastable intermediate states.     

Ab initio molecular dynamics simulation of pressure-induced phase transition in MgS

Pressure-induced phase transition in MgS is studied using a constant pressure ab initio molecular dynamics method, and a solid evidence of existence of its high-pressure phase is provided. As predicted by total energy calculations, MgS undergoes a structural phase transformation from the rocksalt structure to a CsCl-type structure under hydrostatic pressure. The transformation mechanism is characterized, and two intermediate phases having P4/nmm and P2₁/m symmetries for the rocksalt-to-CsCl-type phase transformation of MgS are proposed, which is different from the previously proposed mechanisms. We also study this phase transition using the total energy calculations. Our predicted transition parameters and bulk properties are in good agreement with the earlier first principle simulations.     

Novel intermediate phases in an ionic fluorocarbon surfactant/water system

This small angle X-ray scattering study of the lyotropic phases in the binary tetramethylammonium perfluorodecanoate/water system shows that there are no classical lyotropic mesophases present. Much of the liquid crystal region is taken up with a random mesh intermediate phase, Mh₁(0) and a phase with rhombohedral symmetry which is probably a rhombohedral mesh intermediate phase, Mh₁(R3m). This behaviour is unusual since previously these mesh phases have been associated with hydrocarbon surfactants or diblock copolymer melts. All the mesophases found have non-uniform interfacial curvature and a sufficiently strong inter-layer interaction to ensure the long range correlation of structures in some phases.     

Do one-step mechanisms always involve simultaneous evolution of electron density? QTAIM/IQA analysis of the Curtius rearrangement

The Curtius rearrangement reaction is studied by using quantum theory of atoms in molecules (QTAIM) analysis of the electron density and the interacting quantum atoms (IQA) formalism. Although the rearrangements take place in one stage, two phases are distinguished when the rearranged atom is H: the first one corresponds to the separation of N₂, and the second one to the N-H/C-H bond rearrangement. The transition state (TS) for the reaction does not represent an intermediate between reagent and product for the migration but for the isolation of the N₂ molecule. When the migration is undergone by a fluorine atom, no electronic phases can be distinguished and the process is really concerted. As the migration happens closer to the TS, the TS is more similar to the product. The IQA analysis reveals different electron density evolutions for H and F migrations, and the scarce relevance (in terms of energy) of the point where BCPs appear or disappear.