Resolving the controversy on the glass transition temperature of water?

We consider experimental data on the dynamics of water (1) in glass-forming aqueous mixtures with glass transition temperature T(g) approaching the putative T(g) = 136 K of water from above and below, (2) in confined spaces of nanometer in size, and (3) in the bulk at temperatures above the homogeneous nucleation temperature. Altogether, the considered relaxation times from the data range nearly over 15 decades from 10(-12) to 10(3) s. Assisted by the various features in the isothermal spectra and theoretical interpretation, these considerations enable us to conclude that relaxation of un-crystallized water is highly non-cooperative. The exponent β(K) of its Kohlrausch stretched exponential correlation function is not far from having the value of one, and hence the deviation from exponential time decay is slight. Albeit the temperature dependence of its α-relaxation time being non-Arrhenius, the corresponding T(g)-scaled temperature dependence has small steepness index m, likely less than 44 at T(g), and hence water is not "'fragile" as a glassformer. The separation in time scale of the α- and the β-relaxations is small at T(g), becomes smaller at higher temperatures, and they merge together shortly above T(g). From all these properties and by inference, water is highly non-cooperative as a glass-former, it has short cooperative length-scale, and possibly smaller configurational entropy and change of heat capacity at T(g) compared with other organic glass-formers. This conclusion is perhaps unsurprising because water is the smallest molecule. Our deductions from the data rule out that the T(g) of water is higher than 160 K, and suggest that it is close to the traditional value of 136 K.     

The kinetics of the polymorphic transition of the α-form of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane

The α → γ polymorphic transition in hexanitrohexaazaisowurtzitane was studied by optical microscopy, calorimetry, IR spectroscopy, thermogravimetry, and X-ray diffraction. The thermal effect of the transition was determined. The kinetics of the process is complex because of the relation between structural rearrangement and transition with the removal of water stabilizing the structure of the α polymorph. Depending on the morphological characteristics of the initial sample, the polymorphic transition can follow a frontal-heterogeneous mechanism (single crystal → polycrystalline aggregate) or a quasi-homogeneous mechanism (single crystal → single crystal).     

The rotation and the phase transition in solid C60

The interaction between C60's in solid C60 has been calculated by (exp-6-1) potential, and the cause and the controlled factor of the high rapid rotations of C60 's were discussed. In order to describe the disordered degree of C60 rotation, an equivalent M is introduced. The phase transitions at the ~260 K and at the ~90 K are studied from the viewpoint of C60 rotation. The potential barriers of the ordered rotation below the ~260 K and the disordered rotation above the ~260 K have been given, and the effect of the external pressure on the temperature of phase transition has also been given.     

Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition

The influence of the shape anisotropy of magnetic particles on the isotropic–nematic phase transition was studied in ferronematics based on the nematic liquid crystal (LC) 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT). The LC was doped with spherical or rod-like magnetic particles of different size and volume concentrations. The phase transition from isotropic to nematic phase was observed by polarising microscope as well as by capacitance measurements. The influence of the concentration and the shape anisotropy of the magnetic particles on the isotropic–nematic phase transition in LC are demonstrated here. The results are in a good agreement with recent theoretical predictions.     

Does the solid-liquid crystal phase transition provoke the spin-state change in spin-crossover metallomesogens?

Three types of interplay/synergy between spin-crossover (SCO) and liquid crystalline (LC) phase transitions can be predicted: (i) systems with coupled phase transitions, where the structural changes associated to the Cr<-->LC phase transition drives the spin-state transition, (ii) systems where both transitions coexist in the same temperature region but are not coupled, and (iii) systems with uncoupled phase transitions. Here we present a new family of Fe(II) metallomesogens based on the ligand tris[3-aza-4-((5-C(n))(6-R)(2-pyridyl))but-3-enyl]amine, with C(n) = hexyloxy, dodecyloxy, hexadecyloxy, octadecyloxy, eicosyloxy, R = hydrogen or methyl (C(n)-trenH or C(n)-trenMe), which affords examples of systems of types i, ii, and iii. Self-assembly of the ligands C(n)-trenH and C(n)-trenMe with Fe(A)2 x xH2O salts have afforded a family of complexes with general formula [Fe(C(n)-trenR)](A)2 x sH2O (s > or = 0), with A = ClO4(-), F-, Cl-, Br- and I-. Single-crystal X-ray diffraction measurements have been performed on two derivatives of this family, named as [Fe(C6-trenH)](ClO4)2 (C(6)-1) and [Fe(C6-trenMe)](ClO4)2 (C(6)-2), at 150 K for C(6)-1 and at 90 and 298 K for C(6)-2. At 150 K, C(6)-1 displays the triclinic space group P, whereas at 90 and at 298 K C(6)-2 adopts the monoclinic P2(1)/c space group. In both compounds the iron atoms adopt a pseudo-octahedral symmetry and are surrounded by six nitrogen atoms belonging to imino groups and pyridines of the ligands C(n)-trenH and C(n)-trenMe. The average Fe(II)-N bonds (1.963(2) A) at 150 K denote that C(6)-1 is in the low-spin (LS) state. For C(6)-2 the average Fe(II)-N bonds (2.007(1) A) at 90 K are characteristic of the LS state, while at 298 K they are typical for the high-spin (HS) state (2.234(3) A). Compound C(6)-1 and [Fe(C18-trenH)](ClO4)2 (C(18)-1) adopts the LS state in the temperature region between 10 and 400 K, while compound C(6)-2 and [Fe(Cn-trenMe)](ClO4)2 (n = 12 (C(12)-2), 18 (C(18)-2)) exhibit spin crossover behavior at T(1/2) centered around 140 K. The thermal spin transition is accompanied by a pronounced change of color from dark red (LS) to orange (HS). The light-induced excited spin state trapping (LIESST) effect has been investigated in compounds C(6)-2, C(12)-2 and C(18)-2. The T(1/2)LIESST is 56 K (C(6)-2), 48 K (C(16)-2), and 56 K (C(18)-2). On the basis of differential scanning calorimetry, optical polarizing microscopy, and X-ray diffraction findings for C(18)-1, C(12)-2, and C(18)-2 at high temperature a smectic mesophase SX has been identified with layered structures similar to C(6)-1 and C(6)-2. The compounds [Fe(C(n)-trenH)](Cl)2 x sH2O (n = 16 (C(16)-3, s = 3.5, C(16)-4, s = 0.5, C(16)-5, s = 0), 18 (C(18)-3, s = 3.5, C(18)-4, s = 0.5, C(18)-5, s = 0), 20 (C(20)-3, s = 3.5, C(20)-4, s = 0.5, C(20)-5, s = 0)) and [Fe(C18-tren)](F)2 x sH2O (C(18)-6, s = 3.5, C(18)-7, s = 0) show a very particular spin-state change, while [Fe(C18-tren)](Br)2 x 3H2O (C(18)-8) together with [Fe(C18-tren)](I)2 (C(18)-9) are in the LS state (10-400 K) and present mesomorphic behavior like that observed for the complexes C(18)-1, C(12)-2, and C(18)-2. In compounds C(n)-3 50% of the Fe(II) ions undergo spin-state change at T(1/2) = 375 K induced by releasing water, and in partially dehydrated compounds (s = 0.5) the Cr-->SA phase transition occurs at 287 K (C(16)-4), 301 K (C(18)-4) and 330 K (C(20)-4). For the fully dehydrated materials C(n)-5 50% of the Fe(II) ions are in the HS state and show paramagnetic behavior between 10 and 400 K. In the partially dehydrated C(n)-4 the spin transition is induced by the change of the aggregate state of matter (solid<-->liquid crystal). For compound C(18)-6 the full dehydration to C(18)-7 provokes the spin-state change of nearly 50% of the Fe(II) ions. The compounds C(n)-3 and C(18)-6 are dark purple in the LS state and become light purple-brown when 50% of the Fe(II) atoms are in the HS state.     

Self-association-driven transition of the β-peptidic H12 helix to the H18 helix

Various patterns of foldameric oligomers formed by trans-ABHC ((1S,2S,3S,5S)-2-amino-6,6-dimethylbicyclo[3.3.1]heptane-3-carboxylic acid) and β(3)-hSer residues were studied. NMR, ECD and molecular modelling demonstrated that octameric and nonameric sequences with multiple i-i+3 ABHC pair repulsions attain the β-H18 helix in CD(3)OH. As a close relative of the α-helix, this helix type is stabilized by i-i+4 backbone H-bond interactions. The formation of the β-H18 helix was found to be solvent- and concentration-dependent. Upon dilution, the β-H18 → β-H12 helix transition was revealed by concentration-dependent ECD, DOSY-NMR and TEM measurements.     

High resolution study of the E(0+g) → B3Π (0+u) transition (4400-4000 Å) of I2

The E → B transition of I₂ in the region 4400-4000 Å has been studied under high resolution using a 7.3 m vacuum Ebert spectrograph in the 14th order. The values of the constants B_e, α_e, γ_e, D_e and β_e of both E and B states determined from the analysis of the rotational structure of nine bands of the system are presented. Vibrational constants of the E state obtained from the analysis of all the bands so far reported and consistent with the precise constants of the B state given in previous work are also presented. The origin of the upper state, E(0⁺_g), of the transition is discussed.     

What is the best DFT functional for vibronic calculations? A comparison of the calculated vibronic structure of the S1-S0 transition of phenylacetylene with cavity ringdown band intensities

The sensitivity of vibronic calculations to electronic structure methods and basis sets is explored and compared to accurate relative intensities of the vibrational bands of phenylacetylene in the S(1)(A(1)B(2)) ← S(0)(X(1)A(1)) transition. To provide a better measure of vibrational band intensities, the spectrum was recorded by cavity ringdown absorption spectroscopy up to energies of 2000 cm(-1) above the band origin in a slit jet sample. The sample rotational temperature was estimated to be about 30 K, but the vibrational temperature was higher, permitting the assignment of many vibrational hot bands. The vibronic structure of the electronic transition was simulated using a combination of time-dependent density functional theory (TD-DFT) electronic structure codes, Franck-Condon integral calculations, and a second-order vibronic model developed previously [Johnson, P. M.; Xu, H. F.; Sears, T. J. J. Chem. Phys. 2006, 125, 164331]. The density functional theory (DFT) functionals B3LYP, CAM-B3LYP, and LC-BLYP were explored. The long-range-corrected functionals, CAM-B3LYP and LC-BLYP, produced better values for the equilibrium geometry transition moment, but overemphasized the vibronic coupling for some normal modes, while B3LYP provided better-balanced vibronic coupling but a poor equilibrium transition moment. Enlarging the basis set made very little difference. The cavity ringdown measurements show that earlier intensities derived from resonance-enhanced multiphoton ionization (REMPI) spectra have relative intensity errors.     

Fluorescence studies of the sol–gel transition in deoxygenated conditions

The results of steady-state and time-resolved fluorescence studies of the sol-gel transition initiated by acidic hydrolysis of silane sols of pH 1.8–5.9 are presented. The gelation process was carried out in an oxygen-free atmosphere using pyrene as a fluorescence probe at concentrations of 1×10 ^-5–1×10 ^-2 M. The silica gels were prepared from sols consisting of tetraethylorthosilicate, ethanol and water in a molar ratio of 1:6:6. The steady-state spectra recorded during gelation allowed the polarity to be determined in the microenvironment of the probe for different pH and pyrene concentration. The investigation of the kinetics of fluorescence decay permitted us to discuss the formation of pyrene aggregates due to variations occurring in the gel net.     

Extended Veytsman statistics for the solid–fluid transition in the framework of lattice fluid

Lattice fluid can describe a vapor–liquid transition but not a solid–fluid transition. In this work, we propose a simple and analytic term which yields a solid–fluid transition when coupled with a lattice based equation of state (EOS). The proposed term is derived based on the two assumptions that (1) solid can be considered as highly associated phase affected by strong attractive force and (2) this force is distinct from the conventional attractive forces yielding a vapor–liquid transition. To formulate these assumptions, we extend Veytsman statistics by modifying its density dependency. The derived term was combined with a quasi-chemical nonrandom lattice fluid theory (QLF) developed by the authors. The combined model was found to require only two parameters besides 3 QLF parameters for physical properties calculation of three phases. When tested against equilibrium properties of 8 components, the combined model was found to closely reproduce melting pressure, sublimation pressure, and vapor pressure, but underestimate solid density as well as heat of melting at the triple point temperature. It was found that the present approach can yield a solid–liquid transition at all temperatures.     

A DMA study of the suppression of the Β transition in slightly plasticized PVC blends

A new criterion for evaluating different plasticizers the ability of suppression of the transition in plasticized PVC blends. Accordingly, the suppression ability is proportional to the PVC-plasticizer compatibility, expressed either by the critical solution temperature, CST, or by the interaction parameter related to the difference between the solubility parameters of the blend components. The criterion is, however, valid for low plasticizer contents (<5%w/w) only, as long as the transitions are not overlapped by the transitions, shifted towards lower temperatures due to the effect of the plasticizer. For higher plasticizer contents the transition starts to overlap the transition and the suppression ability of the plasticizer depends increasingly on the efficiency of the plasticizer i.e. on the depression of the glass transition temperature of PVC (related to theT _g of the plasticizer). Accordingly, plasticizers with both good efficiency (lowT _g) and compatibility are more effective in the suppression than plasticizers which have only a higher compatibility but also a highT _g (i.e. reduced efficiency).One of the authors, Tiberiu Vilics, thanks the Konferenz der Deutschen Akademien der Wissenschaften; and Volkswagen Stiftung; for financial support for a research fellowship at the Institut für Makromolekulare Chemie, Freiburg. The financial support of the Deutsche Forschungsgemeinschaft (SFB 60) is greatfully acknowledged.     

The transition probabilities from the ground state to the excited J = 0 1Σu(+) levels of H2: measurements and ab initio quantum defect study

The energies, widths and absolute intensities of the P(1) v' = 0, J' = 1 absorption transitions of H(2) have been measured in the spectral range of 81-75 nm using monochromatized synchrotron radiation. This work completes and extends previous observations, in particular those of Herzberg and Jungen [J. Mol. Spectrosc. 41, 425 (1972)]. Ab initio multichannel quantum defect theory (MQDT) is used to corroborate the spectral analysis of the experimental data. Line intensities and decay widths are also calculated using MQDT.     

Model studies of the chemisorption of hydrogen and oxygen on the Au (1 0 0) surface

 The convergence of chemisorption energy for hydrogen and oxygen on gold clusters is studied. Two theoretical approaches have been employed; wavefunction methods at the self-consistent-field second–order M?ller–Plesset level and density functional theory and the two methods are compared. Relativistic effective core potentials exploited in the former approach were developed in this work. Received: 25 October 1999 / Accepted: 21 February 2001 / Published online: 11 October 2001     

Is adhesion between amorphous polymers sensitive to the bulk glass transition?

Two bulk samples of one and the same or of different amorphous polymers were brought into contact and held for a chosen period of time at a constant healing temperature (T) over the interval of T from below the bulk glass transition temperature (T _g ^bulk) by ~50 °C to above T _g ^bulk by ~10 °C. As formed adhesive joints were shear-fractured in tension at room temperature, and lap-shear strength (σ) was measured as a function of T. It has been found that σ develops with T as logσ?~?1/T both at symmetric and asymmetric interfaces of polystyrene, poly (methyl methacrylate) and poly (2,6-dimethyl-1,4-phenylene oxide). This behaviour implies that there is no discontinuity in the evolution of σ when going through T _g ^bulk, and that this process is controlled by one and the same diffusion mechanism both below and above T _g ^bulk. The results obtained indicate that the contact layer of the polymers investigated is in the viscoelastic state at T well below T _g ^bulk and support the concept of a decrease in the T _g of a near-surface layer with respect to T _g ^bulk.     

Dielectric investigation of the liquid crystal compound with the directSmA*–SmCA* phase transition

New compound showing a direct SmA*–SmC_A* phase transition was synthesised. As far as authors know there are a few pure compounds showing para- and antiferroelectric phases without SmC* between them. Direct current (DC) field applied into a planar-oriented cell induces ferroelectric SmC* phase in an investigated compound. Typical for SmC*, Goldstone mode starts to be detectable. DC field also shifts down the temperature of a SmC_A* phase creation. Moreover, modes in the appearing antiferroelectic phase are enhanced by DC field. This paper shows and discusses relations between modes detected in SmA*, SmC_A* and SmC* (SmC* phase – nucleated by DC field) phases. Parameters of observed modes are calculated using the Cole–Cole relaxation model and a calculation procedure useful especially for high frequency relaxations (higher than 200 kHz).     

High-resolution spectroscopy on the B2Σ+,ν' = 0→X2Π, ν″ =1 transition in SiCl

High-resolution laser-induced fluorescence spectra of²⁸Si³⁵Cl and ²⁸Si³⁷Cl have been observed in a molecular beam. Accurate constants describing the rotational structure in the X ²Π, ν″ = 1 as well as in the electronically excited B ²Σ⁺, ν' = 0 state are given. An inverted fine structure was found in the excited state with a spin-splitting constant γ = −31.15±0.19 MHz for Si³⁵Cl and γ = −30.62 ±0.61 MHz for Si³⁷Cl.     

Review on the transition from conventional to multi-component-based nano-high-entropy alloys—NHEAs

Journal of Thermal Analysis and Calorimetry - This paper reviews on advancement within the areas of alloy design from typical to multi-component alloys, known as ‘high-entropy...     

Kinetic hindrance of the electric field-induced blue phase I→cholesteric transition

The electric field effect on the transition of the liquid-crystalline blue phase I(BP I) to the cholesteric phase has been investigated. This field-induced transition can be overvoltaged. This effect is equivalent to the BP I supercooling and is caused by a kinetic hindrance of the molecular rearrangement within the cubic BP I structure.