Thermotropic mesomorphic behavior of surfactant-encapsulated polyoxometalate hybrids

We investigate in detail novel organic-inorganic hybrid liquid crystalline materials, the complexes of surfactant-encapsulated polyoxometalate clusters (SECs), using thermal, X-ray diffraction, and FT-IR spectroscopic analyses. The differential scanning calorimetry measurements reveal four phase transitions under heating processes. We employ FT-IR spectroscopy to understand these phase behaviors. On the basis of vibration spectral assignments, the evidence suggests that the first two phase transitions are associated with the increase of gauche conformers and the disruption of alkyl chains packing in the heating run; the third phase transition is due to the full conformational disorder of alkyl chains covered on the polyoxometalates (PMs); no significant C-H stretching or wagging vibrations are observed with the fourth transition. We find that the fourth endothermic peak is sensitive to the charges of the PMs, and the transition temperature decreases from 185, 177, to 164 degrees C with decreasing PM charges from 13, 11, to 9, respectively. Interestingly, the temperatures of the first three phase transitions of SECs are essentially independent of the PM charges.     

A new electronic transition found for the aniline derivatives

From comparisons of the ultraviolet absorption spectra of aniline derivatives in the gas phase with those in solutions, it is here proposed that there is a transition from the ground state to a molecular Rydberg state between the first and second π-type transitions.     

Furnace atomization plasma emission spectrometry with He/Ar mixed gas plasmas

The influence of plasma gas composition on the operating and analytical characteristics of a furnace atomization plasma emission source (FAPES) is presented. He I and Ar I excitation temperatures increase 30% in the mixed gas plasmas whereas argon ion excitation temperatures decrease from 33 000 K to 26 000 K in the presence of He. Collisional exchange of internal energy between excited states of Ar and He accounts for these changes. Average analyte ionization temperatures (for Cr, Mn, Mg, Co, Fe, Cd and Zn), derived from the relative emission intensities of their ionic and atomic lines in a 40-MHz 50-W plasma, increase from 5270 K to 6740 K with the addition of Ar to He. Ionic line intensities increase from 10-fold (Mn) to 40-fold (Cd, Zn) with addition of Ar to the plasma while atomic line intensities increase only twofold. Limits of detection remain substantially unaltered for atomic transitions due to increased noise but are improved twofold (Cd) to 24-fold (Mn) for ionic transitions. The analytical advantages and disadvantages of mixed gas plasmas are discussed. The Ne I excitation temperature at 40 MHz and 50 W was determined to be 4330±80 K.     

Paraelectric and ferroelectric order in two-state dipolar fluids

Monte Carlo simulations are used to examine the cooperative creation of a polar state in fluids of two-state particles with nonzero dipole in the excited state. With lowering temperature such systems undergo a second-order transition from nonpolar to polar, paraelectric phase. The transition is accompanied by a dielectric anomaly of polarization susceptibility increasing by three orders of magnitude. The paraelectric phase is then followed by a formation of a nematic ferroelectric which further freezes into a fcc ferroelectric crystal by a first-order transition. A mean-field model of phase transitions is discussed.     

Investigation of poly[oligo(ethylene glycol)diacrylates] as potential stationary phases in GC

Poly[oligo(ethylene glycol)diacrylate]s were investigated as potential candidates for application as stationary phases in capillary gas chromatography. Depending on the molar mass of the monomer, up to three different phase transitions were observed in calorimetric scans of the polymers. Two transitions have been identified as the glass transition and melting of crystallites while the nature of the third effect remains unknown. The new stationary phases demonstrate a good separating ability and selectivity in the viscous‐elastic state, i.e. at temperatures above the glass transition and melting points. The new polymeric stationary phases possess selectivity similar to conventional PEG columns but provide better thermal stability than conventional PEG stationary phases.     

Interaction of an organic cation with Gibbs monolayers of n-hexadecyl phosphate

Surface phase behavior of n-hexadecyl phosphate (n-HDP) and its mixture with L-arginine (L-arg), which behaves as L-argininium cation (L-arg(+)) in aqueous solution, at a molar ratio 2:3 in Gibbs adsorption layers has been studied by film balance, Brewster angle microscopy (BAM) and surface tensiometry at 20 degrees C. The monolayers of n-HDP show three phases that are gas (G), intermediate (I) and liquid condensed (LC), and two phase transitions. A first-order G-I phase transition that is followed by a second-order I-LC phase transition is found in these monolayers. Although the monolayers of the mixtures containing n-HDP and L-arg show three phases, the nature of the middle phase is different from that of the n-HDP monolayers. The three phases observed for the mixed systems are G, liquid expanded (LE) and LC phases. A first-order G-LE phase transition is found at a low surface pressure at > or =10 degrees C. This transition is followed by another first-order LE-LC phase transition at a certain higher surface pressure. The first-order nature of the phase transitions for both the systems is confirmed by the presence of plateaus in the pi-t curves, which are accompanied by two surface phases. A second-order phase transition in the monolayers of n-HDP is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in the surface pressure. The domains formed during the first-order phase transition in the adsorption layers of n-HDP are circular and remain unaffected by changing the temperature. Although the domains of an LE phase are circular, those of an LC phase at the latter transition are fractal in the mixed system. A further branching of the arms of the fractal domains is found to occur by an increase in the temperature. All the results are explained by considering salt formation between anion from n-HDP and L-arg(+).     

A theoretical study of the ion pair SN2 reaction between lithium isocyanates with methyl fluoride with inversion and retention mechanism

This paper is devoted to a detailed theoretical study of an ion pair S_N2 reaction LiNCO+CH₃F in the gas phase and in solution at the level of MP2(full)/6-31+G**//HF/6-31+G**. Two possible reaction mechanisms, inversion and retention, are discussed. There are eight possible reaction pathways. The inversion mechanism is more favorable no matter in the gas phase or in solution based on analyses of the transition structures. Methyl isocyanate should form preferentially in the gas phase and more stable methyl cyanate is the main product in solution. The retardation of the reaction in solvents was attributed to the difference in solvation in the separated reactants and in the transition state.     

Transition probabilities for some infrared O I spectral lines—Application for determining excitation temperatures in low temperature plasmas

Intensities of selected spectral transitions of neutral oxygen emitted from a wall-stabilized arc have been measured. Applying reliable transition probability data taken from literature, Boltzmann plots have been constructed and the temperatures of the plasma have been obtained. At arc plasma conditions ensuring partial local thermal equilibrium, transition probabilities for 4 O I spectral lines from the near infrared spectral range have been obtained. These new results, together with data for two other studied O I spectral lines taken from the National Institute of Standards and Technology data base, are proposed as a suitable set of transition probabilities applicable for diagnostics of low temperature plasmas.     

四氯合铜酸二烷基铵相变的热分析和红外光谱

用DSC和TG研究了(n-C_nH_(2n+1)NH_3)_2CuCl_4(n=7-12)(记为C_nM)配合物的热稳定性和固-固相变。由红外光谱讨论了C_9Cu三个相的性质。发现C_nM的热稳定性呈奇偶效应; 主相变峰温随链长增长而升高; 相变总ΔH和ΔS也随链增长而加大; 当n≤9时, 高温相为部分无序相; 而n≤10时, 高温相为构象无序相。C_9Cu的主相变主要源自链间堆积结构变化。而在307.7 K的相变主要与烃链有序-无序变化有关。     

High resolution spectroscopic study of the first overtone of the N---H stretch and of the fundamentals of the C---H stretches in pyrrole

We have examined the first overtone N---H stretching region and the fundamental C---H stretching region of gas phase pyrrole (C₄H₅N), using high resolution Fourier transform spectra. The first overtone N---H stretch has been rotationally analysed using an asymmetric top model and was found to exhibit two separate perturbations. These perturbations produce line splittings and anomalous intensity patterns in the spectrum which are briefly discussed. Hot band transitions, one of them red-shifted and others likely to overlap the main cold transition are also discussed. Three bands observed around 3100 cm⁻¹ were also rotationally analysed, using a symmetric top Hamiltonian, and assigbed to three of the four closely overlapping fundamentals of the C---H stretch vibrations. Evidence was obtained for the fourth expected C---H fundamental.     

Kinetic appearance of first-order gas-liquid expanded and liquid expanded-liquid condensed phase transitions below the triple point

Phase diagram of Gibbs monolayers of mixtures containing n-hexadecyl phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2 has been constructed by measuring surface-pressure-time (pi-t) isotherms with film balance and by observing monolayer morphology with Brewster angle microscopy (BAM). This phase diagram shows a triple point for gas (G), liquid expanded (LE), and liquid condensed (LC) phases at around 6.7 degrees C. Above this triple point, a first-order G-LE phase transition occurring at 0 surface pressure is followed by another first-order LE-LC phase transition taking place at a certain higher surface pressure that depends upon temperature. The BAM observation supports these results. Below the triple point, the pi-t measurements show only one first-order phase transition that should be G-LC. All of these findings are in agreement with the general phase diagram of the spread monolayers. However, the BAM observation at a temperature below the triple point shows that the thermodynamically allowed G-LC phase transition is, in fact, a combination of the G-LE and LE-LC phase transitions. The latter two-phase transitions are separated by time and not by the surface pressure, indicating that the G-LC phase transition is kinetically separated into these two-phase transitions. The position of the LE phase below the triple point in the phase diagram is along the phase boundary between the G and LC phases.     

Polymérisation de monomères organiques dans des plasmas froids—I. Polymérisation de monomères acryliques déposés en couches minces

One of the applications of cold plasmas in macromolecular chemistry is the polymerization of high molecular weight acrylates in the solid or liquid phase. The reaction is controlled by several parameters (monomer functionality, power of irradiation, nature of the gas used to generate the plasma). The polymerization speed decreases with increasing monomer functionality. The strength of the electric discharge controls the creation of active species and consequently the striking speed. Polymerization speed increases with power but with a maximum depending on the type of the monomer and the nature of the ambient gas. The kinetics are affected by the gas nature (oxygen and carbon dioxide inhibit polymerization). The film surface can be degraded with incorporation of gas atoms. For instance, an argon plasma creates surface sites favourable to later oxidation. In freon 14 plasmas, the formation of C-F bonds can be observed.     

化学反应中的双稳性的临界现象及Landau相变理论

本文通过两个典型事例,即Langmuir形式的交换过程和Schlogle用以模拟一级相变的自催化反应,阐明化学反应系统的双稳性临界现象同样可以纳入Landau相变理论的模式。从而进一步揭示了双稳系统一级相变线临界点的二级(连续)相变特征,同时也揭示了作为远离平衡的化学反应双稳性临界现象的共性,以至于在更大范围内临界现象的共性。将相变的Landau理论,由原来的处于热平衡的凝聚物质推广到范围更宽广,表现更为丰富多彩的远离平衡系统的临界现象。     

About SnF2 stannous fluoride. IV. Kinetics of the α → γ and β, γ → α transitions

The kinetics of the reconstructive first order phase transitions α → γ (upon heating) and β, γ → α (upon cooling) of SnF₂ were studied by quantitative X-ray powder diffraction. The main feature of the kinetics is that these transformations are incomplete over a wide temperature range. Consequently it is not possible to fit the experimental data using the usual solid state rate expressions. Although these transitions are not reversible (large hysteresis) the empirical use of a kinetic model of a reversible transformation provides a good fit. The α → γ transformation was also investigated by D.T.A.; this transition was found to be greatly influenced by particle size: the transition temperature increases with grain size, and its enthalpy decreases. The X-ray investigation revealed that the process of the α → γ transformation involves an intermediate noncrystalline phase named the “transition phase.” It is suggested that the transition proceeds through a process of fragmentation-reconstruction.     

STATE I-STATE II TRANSITIONS IN THE THERMOPHILIC BLUE-GREEN ALGA (CYANOBACTERIUM) Synechococcus lividus*

Time courses of state I-state II transitions were measured in the thermophilic blue-green alga (Cyanobacterium), Synechococcus lividus, that was grown at 55°C. The rate of the state I–II transition using light II illumination was the same as that in the dark, and the dark state was identified to be state II. Therefore, light regulation attained by state transitions is produced by the state II–I transition induced by system I light. The redox level of plastoquinone did not affect this dark state II. Arrhenius plots of the state transitions showed a break point around 43°C that corresponded to the phase transition temperature of this alga. Since both the state I–II and II–I transitions were very much temperature-independent, we could keep the alga in either state for a long time at a “low” temperature such as room temperature. Activities of both photosystems I and II in states I and II were also measured. After a state II–I transition, the system II activity increased about 16% and at the same time, svstem I activity decreased about 30%.     

On structural phase transitions in (i-C4H9NH3)3Bi2Br9: differential scanning calorimetry, dielectric and infrared studies

Two structural phase transitions at 263 and 252 K are detected in a new isobutylammonium crystal (i-C₄H₉NH₃)₃Bi₂Br₉ by means of differential scanning calorimetry (DSC) and dielectric studies. Internal vibrations modes of (i-C₄H₉NH₃)₃Bi₂Br₉ are studied through their phase transitions using the infrared spectrscopy. The infrared studies show that the vibrational state of isobutylammonium cations changes weakly during the phase transition at 252 K. The 263 K phase transition is not reflected in the infrared spectra. The lower temperature phase transition (252 K) is believed to be governed by the reorientational motion of the isobutylammonium cations and may be classified as an ‘order–disorder' type.     

Coherent Enantiomer‐Selective Population Enrichment Using Tailored Microwave Fields

We report the experimental demonstration of coherent enantiomer‐selective enrichment of chiral molecules by employing a novel microwave five‐pulse scheme. Our results show that enantiomers can be selectively transferred to a rotational level of choice by applying sequences of resonant microwave pulses in a phase‐ and polarization‐controlled manner. This is achieved by simultaneously exciting all three kinds of electric dipole‐allowed rotational transitions and monitoring the effect on a fourth rotational transition of choice. Using molecular beams, we apply our method to two chiral terpenes and obtain a 6 % enantiomeric enrichment, which is one order of magnitude larger than that recently reported in a buffer‐gas cell experiment. This approach establishes a robust scheme for controlled manipulation of enantiomers using tailored microwave fields and opens up new avenues for chiral purification and enrichment that can be used in a broad scope of analytical or spectroscopic applications.     

Theory of phase diagrams described by thermodynamic potentials with T d symmetry

Phase diagrams of crystals induced by irreducible representations with symmetry group \(L = \bar 43m\) (T _d ) are constructed within the phenomenological theory of second-order phase transitions. A model of the Landau thermodynamic potential is studied, state equations of all symmetry-conditioned phases are obtained, and general conditions for their thermodynamic stability are formulated. Equations for the boundaries of phase areas and lines of phase transitions are obtained for the fourth order of expansion of the potential via components of the order parameter. Some types of the collapse of the multicritical point of the phase diagram for the eighth order of potential expansion are studied using computer calculations. The possible existence of phase diagrams that contain one or more triple points and areas of existence of three and four phases is shown for the first time for the potentials with the above symmetry. Examples are given of crystals that undergo phase transitions in the considered symmetry of the order parameter.     

Density functional theory study of the nematic-isotropic transition in an hybrid cell

We have employed the density functional theory formalism to investigate the nematic-isotropic capillary transitions of a nematogen confined by walls that favor antagonist orientations to the liquid crystal molecules (hybrid cell). We analyze the behavior of the capillary transition as a function of the fluid-substrate interactions and the pore width. In addition to the usual capillary transition between isotropiclike to nematiclike states, we find that this transition can be suppressed when one substrate is wet by the isotropic phase and the other by the nematic phase. Under this condition the system presents interfacelike states which allow us to continuously transform the nematiclike phase to the isotropiclike phase without undergoing a sharp phase transition. Two different mechanisms for the disappearance of the capillary transition are identified. When the director of the nematiclike state is homogeneously planar-anchored with respect to the substrates, the capillary transition ends up in a critical point. This scenario is analogous to the observed in Ising models when confined in slit pores with opposing surface fields which have critical wetting transitions. When the nematiclike state has a linearly distorted director field, the capillary transition continuously transforms in a transition between two nematiclike states.     

Effect of temperature on the surface phase behavior of n-hexadecyl dihydrogen phosphate in adsorption layers at the air-water interface

We present the adsorption kinetics and the surface phase behavior of n-hexadecyl dihydrogen phosphate (n-HDP) at the air-water interface by film balance and Brewster angle microscopy (BAM). A phase diagram, which shows a triple point at about 25.8 degrees C, is constructed by measuring the surface pressure (pi)-time (t) adsorption isotherms. Below 25.8 degrees C, each of the pi-t curves shows a plateau at about zero surface pressure indicating the existence of a first-order phase transition. The BAM observation confirms the order of this phase transition by presenting two-surface phases during this plateau. However, the BAM observation also shows clearly another second-order phase transition from an isotropic phase to a mosaic-textured liquid condensed (LC) phase. The initial phase is a gas (G) phase. Considering the peculiarity of the middle phase, we suggest this phase as an intermediate (I) phase. Above the triple point, the pi-t curves predict the existence of two-step first-order phase transitions. Similar to the results at lower temperatures, the BAM images show two-surface phases during these first-order phase transitions together with a second-order phase transition from an isotropic phase to an LC phase. These transitions are classified as a first-order G-LE (liquid expanded) phase transition, which is followed by another first-order LE-I phase transition. The second-order phase transition is an I-LC phase transition. Contrary to these results, at 36 degrees C both the pi-t measurements and the BAM observation present only two first-order phase transitions, which are G-LE at zero surface pressure and LE-LC transition at higher surface pressure. The shape of the domains during the main transitions shows a peculiar change from a circular at 20 degrees C to an elongated at 24 degrees C and finally to a circular shape at 36 degrees C. Such a change in the domain shapes has been explained considering the dehydration effect at higher temperatures as well as the nature of phases.