Note: Equation of state and compressibility of supercooled water: simulations and experiment

The equation of state and the isothermal compressibility of supercooled water for pressures up to 3000 bar obtained from computer simulations of the TIP4P/2005 model are compared to recent experimental results. The agreement between the simulations and experimental results is quite good. This reinforces the idea that the Widom line and the liquid-liquid phase separation found in the simulations should also exist in real water.     

High-temperature behaviour of lithium borates:: Part I: Characterization and thermal stability

Nine compounds, namely Li₃BO₃, -Li₄B₂O₅, β-Li₄B₂O₅, Li₆B₄O₉, -LiBO₂, Li₂B₄O₇, Li₃B₇O₁₂, LiB₃O₅ and Li₂B₈O₁₃ in the Li₂O–B₂O₃ system have been synthesized and characterized. The unit-cell parameters, density and solubility in water at room temperature of all the compounds are reported. The densities of the compounds were found to be in the 1.90–2.50 g cm⁻³ range, while their solubility in water at room temperature was in the 0.91–8.64×10⁻² g cm⁻³ range. Determination of the thermal stability of the compounds by quenching and differential thermal analysis (DTA) showed that only -LiBO₂ and Li₂B₄O₇ retained their original symmetry up to their congruent melting at 1121 and 1188 K, respectively, in air.     

Precipitation and determination of tantalum and niobium from homogeneous solution with 3:3' :4' :5:7-pentahydroxyflavanone

3:3,:5:7-Pentahydroxyflavanone in fairly concentrated acidic solution (6-9N) does not precipitate tantalum and niobium ; however, on heating or boiling, in the presence of air, this flavanone is transformed into 3:3':4':5:7-pentahydroxyflavone, which precipitates any tantalum and niobium present in the solution. Under the precipitation conditions, racemisation of the flavanone also takes place. The racemised flavanone which is less soluble than the original d-form may accompany the tantalum and niobium precipitates without affecting the quantitative determination of these elements.

The precipitation of the tantalum and niobium complexes can be controlled by regulating the acidity and the duration of boiling, as well as the concentration of the flavanone. Experimental data and procedures are given for the precipitation and determination from homogeneous solution of tantalum and niobium complexes. Zirconium and molybdenum do not interfere with the determination. Titanium must be absent or present only in minute quantity.

Since the generation of the precipitating reagent, flavone, from the flavanone is comparatively slow, the precipitation of tantalum and niobium is uniform throughout the solution. By this technique, adsorption and co-precipitation of potassium and sulphate ions in the solution are shown to be negligible. This is in contrast to the less effective dropwise addition of the flavone reported by earlier investigators, in which adsorption and co-precipitation were pronounced.

In the present study, tantalum and niobium oxides were fused with potassium bisulphate. There is no necessity using hydrofluoric acid to dissolve these oxides and therefore no polyethylene apparatus is required.     

Encapsulation of norharmane in cyclodextrin: formation of 1:1 and 1:2 inclusion complexes

Steady state absorption and fluorometric techniques have been used to investigate the photophysics of norharmane (NHM), a bioactive fluorophore, in aqueous as well as aqueous cyclodextrin (CD) environments. The absorption and steady state fluorescence spectral studies reveal the formation of two types of inclusion complexes between the fluorophore and beta-cyclodextrin (beta-CD) depending on the relative population of the two. The stoichiometries and association constants of these complexes have been determined monitoring the fluorescence data. alpha-and gamma-cyclodextrin (alpha-CD, gamma-CD) do not have appreciable effect on the spectral pattern of the fluorophore. The differential fluorimetric behavior of NHM in different CD environments has been rationalized from the variation of the relative dimensions of the probe and the CD cavities.     

Erratum to: Electrochemical Reduction of Trichlorobiphenyls: Mechanism and Regioselectivity

Russian Journal of General Chemistry - Affilation of M. V. Sangaranarayanan should read as follows: Department of Chemistry, Indian Institute of Technology, Madras Chennai, 600036 India.     

Molecular modelling and 1H-NMR: ultimate tools for the investigation of tolbutamide: beta-cyclodextrin and tolbutamide: hydroxypropyl-beta-cyclodextrin complexes

A structural study of the inclusion compound of tolbutamide (TBM) with beta-cyclodextrin (beta-CD) and hydroxypropyl-beta-cyclodextrin (HP-beta-CD) was attempted by means of 1H-nuclear magnetic resonance (1H-NMR) experiments and computer molecular modelling. To establish the stoichiometry and stability constant of the beta-CD:TBM complex, the continuous variation method was used. The presence of true inclusion complexes between TBM and beta-CD or HP-beta-CD in solution was clearly evidenced by the 1H-NMR technique. Changes in chemical shifts of H-3 and H-5 protons, located inside the CD cavity, associated with variations in the chemical shifts of TBM aromatic protons provided clear evidence of inclusion complexation, suggesting that the phenyl moiety of the drug molecule was included in the hydrophobic cavity of CDs. This view was further supported by the observation of intermolecular NOEs between TBM and beta-CD and by the aid of a molecular modelling program, which established the most probable structure of the complex. The molecular graphic computation confirmed that the minimum energy, positioning TBM relative to beta-CD, occurs when the aromatic ring of TBM is included within the beta-CD cavity by its wider side, leaving the aliphatic chain externally, which is in good agreement with the results of 1H-NMR studies.     

High-pressure and -temperature viscosity measurements of methanol and 4:1 methanol:ethanol solution

Viscosity (eta) measurements using rolling sphere viscometry in a resistance-heated diamond-anvil pressure cell yield activation energies of 18-98 kJmol over a pressure range of 1.1-6.1 GPa for methanol and 26-78 kJmol over a pressure range of 2.9-5.4 GPa for a 4:1 methanol:ethanol solution. Nonlinear models of log eta (free-volume and power-law models) yield statistically better fits than the Arrhenius model at room temperature and extrapolate to lower glass-transition (10(12) Pa s) pressures. Glass-transition pressures for the free-volume model change little over the temperature range studied (298-338 K), whereas the power-law values converge to those of the Arrhenius model at temperatures above 320 K.     

Chemical and immunochemical characterization of the lipopolysaccharides of Yersinia kristensenii serovars 0:12,25; 0:12,26; and 0:25,35

A comparative and immunochemical characterization of the lipopolysaccharides of three serovars ofY. kristensenii has been performed and the ratios of the monosaccharides have been established. The results of the immunochemical investigations confirmed the serotyping of the microorganisms.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Branch, Academy of Sciences of the USSR, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 335–338, May–June, 1988.     

Review: Graphene-supported coordination complexes and organometallics: properties and applications

Recent advances in the functionalization of graphene (G) and graphene oxide (GO) using classical coordination complexes, as well as σ- and π-organometallic compounds as precursors, are discussed. Graphene can form hybrids via covalent or non-covalent interactions with metal complexes of carboxylates, amines, polypyridine compounds, a host of N,O-containing ligands, porphyrins, phthalocyanines, carbonyls, cyclopentadienyls, pyrene-containing moieties, and other aromatic structures. The hybrid constructs are interesting for applications in catalysis, energy storage, and corrosion inhibition and present interesting possibilities of modulating the electronic structure of graphene.     

Communication: Barium ions and helium nanodroplets: Solvation and desolvation

The solvation of Ba(+) ions created by the photoionization of barium atoms located on the surface of helium nanodroplets has been investigated. The excitation spectra corresponding to the 6p (2)P(1∕2) ← 6s (2)S(1∕2) and 6p (2)P(3∕2) ← 6s (2)S(1∕2) transitions of Ba(+) are found to be identical to those recorded in bulk He II [H. J. Reyher, H. Bauer, C. Huber, R. Mayer, A. Schafer, and A. Winnacker, Phys. Lett. A 115, 238 (1986)], indicating that the ions formed at the surface of the helium droplets become fully solvated by the helium. Time-of-flight mass spectra suggest that following the excitation of the solvated Ba(+) ions, these are being ejected from the helium droplets either as bare Ba(+) ions or as small Ba(+)He(n) (n < 20) complexes.     

Synthesis of Δ-OPC-8:0 and OPC-6:0

CuCN-catalyzed reaction of the (1R)-isomer of 4-cyclopentene-1,3-diol monoacetate with TBDPSO(CH₂)₆MgCl produced an S_N2-type product regioselectively in high yield. Mitsunobu inversion of the product and subsequent Claisen rearrangement furnished aldehyde with the two side chains, from which the title compounds were synthesized efficiently.     

FCl:PCX complexes: old and new types of halogen bonds

MP2/aug'-cc-pVTZ calculations have been performed to investigate the halogen-bonded complexes FCl:PCX, for X = NC, CN, F, H, CCH, CCF, CH(3), Li, and Na. Although stable complexes with a F-Cl···P halogen bond exist that form through the lone pair at P (configuration I), except for FCl:PCCN, the more stable complexes are those in which FCl interacts with the C≡P triple bond through a perturbed π system (configuration II). In complexes I, the nature of the halogen bond changes from traditional to chlorine-shared and the interaction energies increase, as the electron-donating ability of X increases. The anionic complex FCl:PC(-) has a chlorine-transferred halogen bond. SAPT analyses indicate that configuration I complexes with traditional halogen bonds are stabilized primarily by the dispersion interaction. The electrostatic interaction is the most important for configuration I complexes with chlorine-shared halogen bonds and for configuration II complexes except for FCl:PCNa for which the induction term is most important. The F-Cl stretching frequency is red-shifted upon complexation. EOM-CCSD/(qzp,qz2p) spin-spin coupling constants have been obtained for all FCl:PCX complexes with configuration I. (1)J(F-Cl) decreases upon complexation. (2X)J(F-P) values are quadratically dependent upon the F-P distance and are very sensitive to halogen-bond type. (1X)J(Cl-P) tends to increase as the Cl-P distance decreases but then decreases dramatically in the chlorine-transferred complex FCl:PC(-) as the Cl-P interaction approaches that of a covalent Cl-P bond. Values of (1)J(F-Cl) for configuration II are reduced relative to configuration I, reflecting the longer F-Cl distances in II compared to those of the neutral complexes of I. Although the F-P and Cl-P distances in configuration II complexes are shorter than these distances in the corresponding configuration I complexes, (2X)J(F-P) and (1X)J(Cl-P) values are significantly reduced, indicating that coupling through the perturbed C-P π bond is less efficient. The nature of F-P coupling for configuration II is also significantly different, as evidenced by the relative importance of PSO, FC, and SD components.     

Ozonolysis of alpha-pinene and beta-pinene: kinetics and mechanism

A combined quantum-chemical and RRKM/ME (ME--master equation) approach is employed to investigate the structures, energetics, and kinetics of intermediate and stable species, and the yields of stabilized carbonyl oxides and OH radicals from the alpha-pinene and beta-pinene ozonolysis reactions. The cycloaddition of O(3) is highly exothermic, with the reaction energies of 55.1 and 51.1 kcal mol(-1) for alpha- and beta-pinenes, respectively. Cleavage of primary ozonides yields carbonyl oxides with the barrier height of 12.2-17.5 kcal mol(-1). For the prompt reactions of carbonyl oxides from alpha- and beta-pinene ozonolysis, H migration to hydroperoxides represents the dominant pathway over ring closure to dioxiranes. The kinetic calculations indicate a significant portion of stabilization for alpha- and beta-carbonyl oxides. The yields of stabilized carbonyl oxides are estimated to be 0.34 for alpha-pinene and 0.22 for beta-pinene. The applicability of theoretical methods for investigation of oxidation reactions of large hydrocarbon molecules is demonstrated.     

Photochemistry of flavothione and hydroxyflavothiones: mechanisms and kinetics

In this work we present a detailed study of the mechanism of photochemistry and thermal reactions, as well as of the kinetics of flavothione (FLT) in ethanol. Furthermore, we analyzed how the hydroxysubstitution pattern of FLT influenced both the kinetics and the mechanism relative to the parent FLT. We show that the primary photochemical reaction of FLT in the absence of oxygen is hydrogen (H)-atom abstraction from the solvent by way of the excited triplet state of FLT. Several products result from thermal reactions of the resulting semireduced FLTH* radical, including more than one dimer. A full mechanism is proposed, and the relevant rate constants are evaluated. On the other hand, in the presence of oxygen and a low concentration of FLT, we found that the principal photoproduct is the parent flavone (FL). The reaction leading to photoxidation is not via 1O2 attacking a thione, but instead, it is via a reaction of the FLTH* radical with ground state oxygen. The kinetic data also demonstrate that the relative values of concentrations of reactants and the rate constants of the reactions can control the dominance of one mechanism over others. We also have examined the photochemical mechanisms and kinetics for several hydroxyflavothiones (n-OHFLT) and compared them with FLT itself. We have found that the photochemical mechanism radically changes depending on the positions of substitution. These differences are directly related to the ordering of the excited states of the n-OHFLT. Specifically, FLT with lowest 3n,pi* states (FLT, 6-hydroxyflavothione, 7-hydroxyflavothione and 7,8-dihydroxyflavothione) efficiently abstract H atoms to give the semireduced radical of the thione. The radical can (1) dimerize to form two different dimers; (2) react with oxygen to produce the parent FL; and (3) recombine with the solvent radical to yield the original FLT. In contrast, FLT with lowest 3pi,pi* states (3-hydroxyflavothione, 3,6-dihydroxyflavothione and 3,7-dihydroxyflavothione) behave as photosensitizers of oxygen to form singlet oxygen, which then reacts with the ground state of the substituted FLT. Finally, when T2(pi,pi*) is above S1(n,ppi*), as for 5-hydroxyflavothione and 5,7-dihydroxyflavothione, both the S1(n,pi*) --> T1(n,pi*) intersystem crossing and photodegradation are inefficient.     

Isolation and analysis of ginseng: advances and challenges

Ginseng occupies a prominent position in the list of best-selling natural products in the world. Because of its complex constituents, multidisciplinary techniques are needed to validate the analytical methods that support ginseng's use worldwide. In the past decade, rapid development of technology has advanced many aspects of ginseng research. The aim of this review is to illustrate the recent advances in the isolation and analysis of ginseng, and to highlight new applications and challenges. Emphasis is placed on recent trends and emerging techniques.