Molecular dynamics studies of melting and solid-state transitions of ammonium nitrate

Molecular dynamics simulations are used to calculate the melting point and some aspects of high-temperature solid-state phase transitions of ammonium nitrate (AN). The force field used in the simulations is that developed by Sorescu and Thompson [J. Phys. Chem. A 105, 720 (2001)] to describe the solid-state properties of the low-temperature phase-V AN. Simulations at various temperatures were performed with this force field for a 4 x 4 x 5 supercell of phase-II AN. The melting point of AN was determined from calculations on this supercell with voids introduced in the solid structure to eliminate superheating effects. The melting temperature was determined by calculating the density and the nitrogen-nitrogen radial distribution functions as functions of temperature. The melting point was predicted to be in the range 445 +/- 10 K, in excellent agreement with the experimental value of 442 K. The computed temperature dependences of the density, diffusion, and viscosity coefficient for the liquid are in good agreement with experiment. Structural changes in the perfect crystal at various temperatures were also investigated. The ammonium ions in the phase-II structure are rotationally disordered at 400 K. At higher temperatures, beginning at 530 K, the nitrate ions are essentially rotationally unhindered. The density and radial distribution functions in this temperature range show that the AN solid is superheated. The rotational disorder is qualitatively similar to that observed in the experimental phase-II to phase-I solid-state transition.     

6- and 7-aryl-1,2,4-triazolo[4,3-b]-1,2,4-triazines. Synthesis and characterization

Synthetic methods for the preparation of 6-aryl-1,2,4-triazolo[4,3-b]-1,2,4-triazines ( 1 ) and the 7-aryl isomers ( 2 ) are described. Compounds 1 were prepared from aryl glyoxaldoximes 76 via 6-aryl-1,2,4-triazin-3(2H)ones ( 75 ). A simple procedure for the preparation of the 7-aryl isomers was effected using arylglyoxals 11 and the triazoles ( 4, 12a and 12b ). However, complete regioselectivity was not realized in all cases, especially when the triazoles were substituted at the C-5 position. A regiospecific synthesis of the 7-aryl isomers 2 was developed via the 3-methylthio-5-aryl-1,2,4-triazines ( 61 ). The structure of the parent 6-phenyl derivative 5 was confirmed by x-ray crystallography.     

A combination chemical and enzymatic approach for the preparation of azole carboxamide nucleoside triphosphate

Alternative substrates for DNA and RNA polymerases offer an important set of biochemical tools. Many of the standard methods for nucleoside triphosphate synthesis fail in the cases of nonpurine and nonpyrimidine nucleosides. An efficient preparation of the 5'-O-tosylates for both the deoxy- and ribonucleosides enabled preparation of the diphosphate esters by displacement with tris(tetra-n-butylammonium) pyrophosphate. Enzymatic synthesis of the azole carboxamide deoxyribonucleoside triphosphate was based on ATP as the phosphate donor, nucleoside diphosphate kinase as the catalyst, coupled with phosphoenol pyruvate (PEP) and pyruvate kinase as an ATP regeneration system. Ribonucleoside triphosphate synthesis required PEP as the phosphate donor and pyruvate kinase as the catalyst. An optimized purification procedure based upon boronate affinity gel was developed to yield highly purified nucleoside triphosphates. The strategy outlined here provides a new and efficient method for preparation of nucleoside 5'-triphosphate and is likely applicable to a broad variety of base and sugar modified nucleoside analogues.     

The rate of O2 and CO binding to a copper complex, determined by a "flash-and-trap" technique, exceeds that for hemes

The observation and fast time-scale kinetic determination of a primary dioxygen-copper interaction have been studied. The ability to photorelease carbon monoxide from [Cu(I)(tmpa)(CO)](+) in mixtures of CO and O(2) in tetrahydrofuran (THF) between 188 and 218 K results in the observable formation of a copper-superoxide species, [Cu(II)(tmpa)(O(2)(-))](+) lambda(max) = 425 nm. Via this "flash-and-trap" technique, temperature-dependent kinetic studies on the forward reaction between dioxygen and [Cu(I)(tmpa)(thf)](+) afford activation parameters DeltaH = 7.62 kJ/mol and DeltaS = -45.1 J/mol K. The corresponding reverse reaction proceeds with DeltaH = 58.0 kJ/mol and DeltaS = 105 J/mol K. Overall thermodynamic parameters are DeltaH degrees = -48.5 kJ/mol and DeltaS degrees = -140 J/mol K. The temperature-dependent data allowed us to determine the room-temperature second-order rate constant, k(O2) = 1.3 x 10(9) M(-1) s(-1). Comparisons to copper and heme proteins and synthetic complexes are discussed.     

The dissociation quotients of formic acid in sodium chloride solutions to 200°C

The dissociation quotients of formic acid were measured potentiometrically from 25 to 200°C in NaCl solutions at ionic strengths of 0.1, 0.3 1.0, 3.0, and 5.0 mol-kg^–1. The experiments were carried out in a concentration cell with hydrogen electrodes. The resulting molal acid dissociation quotients for formic acid, as well as a set of infinite dilution literature values and a calorimetrically-determined enthalpy of reaction, were fitted by an empirical equation involving an extended Debye Hückel term and seven adjustable parameters involving functions of temperature and ionic strength. This regressional analysis yielded the following thermodynamic quantities for 25°C: logK=–3.755±0.002, H^o=–0.09±0.15 kJ-mol^–1, S^o=–72.2±0.5 J-K^–1-mol^–1, and C _p ^o =–147±4 J-K^–1-mol^–1. The isocoulombic form of the equilibrium constant is recommended for extrapolation to higher temperatures.     

Surface films and metallurgy related to lubrication and wear

The nature of the tribological surface is identified and characterized with respect to adhesion, friction, wear, and lubricating properties. Surface analysis is used to identify the role of environmental constituents on tribological behavior. The effect of solid to solid interactions for metals in contact with metals, ceramics, semiconductors, carbons, and polymers is discussed. The data presented indicate that the tribological surface is markedly different than an ideal solid surface. The environment is shown to affect strongly the behavior of two solids in contact. In certain instances, the environment can dominate surface characteristics. With metals in contact with metals, adhesion is found to be related to the cohesive binding energy. Strong adhesive bonding occurs for metals in contact with ceramics, semiconductors, carbons, and polymers. Bond strength at the interface is, with some exceptions, stronger than the bond strength of the cohesively weaker of the two materials. Many different surface properties of metals and alloys influence tribological performance. These include (1) surface energy, (2) crystallographic orientation, (3) amorphous versus the crystalline state, (4) grain boundaries, (5) texturing of the surface, (6) crystal structure, and (7) order-disorder transformations. At sufficiently high loads or sliding velocities, metals or alloy surfaces are shown to undergo recrystallization effecting tribological properties. The chemical valency, d-valence-bond characteristics, of metal surfaces affect adhesion, friction, and wear. The greater the percent d-bond character, the lower the friction and wear. The ideal tensile and shear strengths of simple transition metals correlate with friction coefficients. Results presented also show that small amounts of alloying elements in base metals can alter markedly adhesion, friction, and wear by segregating to the solid surface.     

Ternary mutual diffusion coefficients of ZnCl2−KCl−H2O at 25°C by Rayleigh interferometry

Mutual diffusion coefficients and densities were measured for aqueous ZnCl₂–KCl mixtures at 25° by using free-diffusion Rayleigh interferometry and pycnometry, respectively. The ZnCl₂ concentrations were fixed at 1.5 mol-dm^–3, whereas those of KCl were 0.5, 1.25, 2.0, or 4.0 mol-dm^–3. This corresponds to a half charged zinc-chlorine storage battery at various suporting electrolyte concentrations. The main-term coefficient of ZnCl₂ only varies by 10% with KCl concentration, whereas that of KCl varies by about 22%. The ZnCl₂ cross-term coefficient remains small and positive; in contrast the KCl cross-term coefficient goes through a maximum and is negative at high and low KCl concentrations. At KCl concentrations of 0.5 and 4.0 mol-dm^–3, solutions with the KCl c0 are statically and dynamically (diffusively) unstable at the top and bottom of the boundary. Evaluation of the parameters of the non-linear least-squares solution to the diffusion equation is difficult for the 1.25 mol-dm^–3 KCl case, since this system has nearly equal eigenvalues in its diffusion coefficient matrix.     

Bubble-size dependence of the critical electrolyte concentration for inhibition of coalescence

The interaction of pairs of bubbles with equal diameters grown on adjacent capillaries in aqueous magnesium sulfate solutions is observed for varying electrolyte concentrations and bubble diameters. As in previous investigations, a sharp transition from coalescence to bubble detachment without coalescence is observed with increasing electrolyte concentration. The critical electrolyte concentration for this transition is found to increase with decreasing bubble diameter for bubble diameters of 1.4 to 4.2 mm.     

High-Pressure 17O-NMR. Study of Vanadium (II) in Water: A Second Example of an Associative Interchange Mechanism (Ia) for Solvent Exchange on an Octahedral Divalent Transition-Metal Ion

The water exchange of [V(H₂O)₆]Cl₂ in aqueous solution has been studied as a function of temperature and pressure (up to 250 MPa), by measuring the ¹⁷O-FT-NMR. line-widths of the free water resonance at 8.13 MHz. The kinetic parameters obtained are K = 87±4 s^?1, ΔH^* = +61.8 ± 0.7 kJ mo1^?1 and ΔS^* = ?0.4±1.9 J mol^?1 K^?1. A pressure-independent volume of activation ΔV^* = ?4.1±0.1 cm³ mol^?1 is obtained, suggesting an associative interchange (I^a) mechanism for this early divalent metal ion.