Vapor pressure of R227ea + ethanol at 343.13 K by molecular simulation

A new molecular model for 1,1,1,2,3,3,3-heptafluoropropane (R227ea) was developed on the basis of quantum chemical calculations and optimized using experimental vapor pressure and bubble density data. In combination with an existing model for ethanol, a molecular model for the binary mixture R227ea + ethanol was defined, using the Lorentz–Berthelot combining rule. It was validated at 283.17 K, where, considering the statistical uncertainties, it agrees to the experimental vapor pressure. The vapor–liquid equilibrium, comprising both bubble line and dew line data, was predicted at 343.13 K by molecular simulation. The Peng–Robinson equation of state fails for this system.     

On-line infrared detection in aqueous micro-volume systems

Mid infrared spectroscopy is a non-destructive technique that can provide detailed information on important, molecule-specific features such as the conformation and functional groups of a large range of compounds. Infrared spectroscopy is now an established and frequently used technique for qualitative analysis, i.e. the identification of chemical constituents in a sample. In addition, its use for quantitative purposes has grown dramatically in recent years. It is important to realise that the analytical problem defines the mode of operation and implementation of the FTIR technique. This Highlight article focuses on the advantages and scope of on-line FTIR detection strategies. However, in common with all techniques, on-line FTIR detection has a number of potential shortcomings, which are also discussed.     

THE CONTROL OF METABOLISM BY BLUE LIGHT IN Acetabularia mediterranea—II. SELECTIVE TRANSLATIONAL CONTROL AND DIFFERENTIAL DEGRADATION OF ENZYMES

Abstract— During prolonged continuous irradiation with red light the specific activity of uridine 5'-diphosphoglucose (UDPG) pyrophosphorylase (uridine 5'-triphosphate: glucose 1-phosphate uridylyl-transferase EC 2.7.7.9) decreased in Acetabularia mediterranea Lamouroux (=A. acetabulum (L.) Silva). Subsequent blue light restored the original activity within a comparatively short period of 3 to 4 days. Computer-aided quantitative evaluation of density labelling experiments showed that the synthesis of the enzyme was accelerated about four-fold during the period of activation by blue light. A similar increase in the rate of synthesis was found for hydroxypyruvate reductase (EC 1.1.1.81), a control enzyme that showed no blue light-dependent changes in the specific activity under these conditions. The increase in the rate of enzyme synthesis was caused by an overall stimulation of the cytosolic translation. Degradation of UDPG pyrophosphorylase was unaffected by blue light, while the half life of hydroxypyruvate reductase was shortened about two-fold compared to continuous red light. Thus, degradation of proteins appears to be selectively light dependent in Acetabularia.
Model calculations for enzyme amount and enzyme synthesis were carried out using the measurements of enzyme activity, rates of cytosolic protein synthesis, and degradation constants of the enzymes. Assuming that activities represented amounts of the given enzymes, these calculations indicated a selective activation of UDPG pyrophosphorylase synthesis by blue light since it did not coincide with the overall stimulation of protein synthesis in the cytosol, in contrast to hydroxypyruvate reductase.     

Thermodynamic trans-Effects of the Nucleotide Base in the B12 Coenzymes

The thermodynamic effects of the nucleotide coordination on the Co-C bond strengths in the B ₁₂ coenzymes were analyzed. Methyl group transfer reactions from methylcob( III )inamides to cob( II )inamides and cob( I )inamides in neutral aqueous solution were used in equilibration experiments to determine the effect fo the intramolecular coordination of the nucleotide function on the Co-C bond dissociation energies of methylcob( III )alamin ( 4 ). In the equilibrium between 4 , cob( I )inamide ( 11 ), cob( I )alamin ( 10 ) and methylcob( III )inamide 6 (Scheme 2), 4 and 11 were found to predominate ( 4 + 11 ? 10 + 6 , equilibrium constant K_I/III≈0.004), while the equilibrium between 4 , cob( II )inamide 9 , cob( II )alamin ( 5 ), and 6 (Scheme 1) proved to be well balanced ( 4 + 9 ? 5 + 6 , equilibrium constant K_II/III=0.60). These equilibrium values indicate the nucleotide coordination to stabilize the Co–C bond in 4 both against homolysis (slight effect) and against nucleophilic heterolysis (considerable effect). They reflect a stabilization of the complete corrins 4 and 5 by the nucleotide coordination, which is also indicated for 4 and 5 by their (nucleotide) basicity. The latter information, where available for other organocobalamins, allows the analysis of the thermodynamicnucleotide trans effect there as well: e.g. in coenzyme B ₁₂ ( 1 ), the nucleotide coordination is found this way to weaken the Co–C bond towards homolysis by ca. 0.7 kcal/mol.     

Hexaphosphapentaprismane: a new gateway to organophosphorus cage compound chemistry

Several independent synthetic routes are described leading to the formation of a novel unsaturated tetracyclic phosphorus carbon cage compound tBu4C4P6 (1), which undergoes a light-induced valence isomerization to produce the first hexaphosphapentaprismane cage tBu4C4P6 (2). A second unsaturated isomer tBu4C4P6 (9) of 1 and the bis-[W(CO)5] complex 13 of 1 are stable towards similar isomerization reactions. Another starting material for the synthesis of the hexaphosphapentaprismane cage tBu4C4P6 (2) is the trimeric mercury complex [(tBu4C4P6)Hg]3 (11), which undergoes elimination of mercury to afford the title compound 2. Single-crystal X-ray structural determinations have been carried out on compounds 1, 2, 9, 11, and 13.     

Synthesis,Characterization, and Crystal Structure of Thallium(I) Complex containing Monodeprotonated 2,6‐Pyridinedicarboxylic Acid

The reaction of an ethanolic solution 2,6‐pyridinedicarboxylic acid ( 1 , LH₂) with TlNO₃ in the presence of triethylamine led to the coordination polymer [Tl(LH)]_n ( 2 ). The complex was characterized by elemental analysis, IR spectroscopy and single‐crystal X‐ray diffraction. Crystal data for 2 at –80 °C: monoclinic, space group I2/a, a = 696.1(1), b = 1190.6(2), c = 931.0(2) pm, β = 103.28(1)°, Z = 4, R₁ = 0.0256.     

Exploring potential energy surfaces for chemical reactions: an overview of some practical methods

Potential energy surfaces form a central concept in the application of electronic structure methods to the study of molecular structures, properties, and reactivities. Recent advances in tools for exploring potential energy surfaces are surveyed. Methods for geometry optimization of equilibrium structures, searching for transition states, following reaction paths and ab initio molecular dynamics are discussed. For geometry optimization, topics include methods for large molecules, QM/MM calculations, and simultaneous optimization of the wave function and the geometry. Path optimization methods and dynamics based techniques for transition state searching and reaction path following are outlined. Developments in the calculation of ab initio classical trajectories in the Born-Oppenheimer and Car-Parrinello approaches are described.