Publications

Abstract The standard (p(0) = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for air the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.

Abstract The behaviour of several high Miller index crystal surfaces of gold is examined by cyclic voltammetry in 0.01 mol dm(-3) perchloric acid solutions. The potentials of zero charge, E(sigma=0), and their temperature coefficients, dE(sigma=0)/dT for the stepped faces (332), (771), (320) and (410) are determined from a single minimum of the differential capacity curves. The results are compared with those existing for other stepped and low-index crystal faces in order to establish the relevance of the crystallographic structure in the double layer parameters. The influence of the step density and step orientation is discussed. The entropies of formation, S*, are calculated and discussed in terms of the gold-water interactions.

Abstract Hydrogen ion buffers are required for many different types of in vitro biological and chemical studies, but they may not be inert enough, thus interfering with the system under study. N-Hydroxyethylpiperazine-N'- ethanesulfonic acid (Hepes), a zwitterionic buffer in the 7.0-8.0 pH range, has been used in different biological studies. In this work, surfactant activity of Hepes and the buffer's capability to bind copper(II) were investigated through different electrochemical techniques: ion-selective electrode potentiometry, potentiometric stripping analysis, differential pulse anodic stripping voltammetry, normal pulse anodic stripping voltammetry, and alternating current polarography. Hepes concentrations between 0.1 (0.24 g liter(-1)) and 25 mmol liter(-1) (6.0 g liter(-1)) were studied at pH 8.0 (and at pH 6.5 for comparison). At pH 8.0 Hepes displayed surfactant activity, particularly when \Hepes\ > 10 mmol liter(-1) (2.4 g liter(-1)). The ability of Hepes to bind copper(II) was detected when Hepes was in large excess (\Hepes\/\Cu\ between 10,000 and 2000). Eventual implications in the bioavailability of copper(II) are also discussed. (C) 1996 Academic Press, Inc.

Abstract Pseudomonas syringae cells were exposed to 100 mu M copper alone, or to previously equilibrated copper sulfate-ligand solutions. Ligand concentrations were determined experimentally as those that reduced the free copper concentration to 5 mu M (determined with a Cu2+-selective electrode). These values were in agreement with those calculated by computational equilibrium simulation based on published stability constants. Exposure of P. syringae cells to copper sulfate, chloride, or nitrate resulted in similar high mortality, suggesting that copper was responsible for cell death. Acetate, succinate, proline, lysine, cysteine, and EDTA significantly reduced both the amount of copper bound to the cells and cell death, indicating that not only strong chelating agents but also weak and moderate copper ligands can effectively antagonize copper toxicity. However, cysteine and EDTA were considerably more effective than acetate, succinate, proline, and lysine, indicating that copper toxicity in not simply a function of free copper concentration but depends on the nature of the ligand. The results suggested that a significant fraction of copper bound to acetate, succinate, proline, or lysine was displaced to the bacteria or, alternatively, mixed copper-ligand-cell complexes could be formed. On the contrary, none of these phenomena occurred for the copper complexes with cysteine or EDTA.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation for the liquid 2-, 4-, 6-, and 8-methylquinolines: C9H6N(CH3) were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature T = 298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization for the same compounds were determined by microcalorimetry. The calorimeter was tested by measuring the enthalpies of vaporization of undecane and hexadecane. [GRPAHICS]

Abstract The simultaneous torsion and mass-loss effusion technique was used to measure the vapour pressure of crystalline bis(benzoylacetonato)beryllium (II) - Be(BZAC)(2) - and tris(benzoylacetonato) aluminium(III) - AI(BZAC)(3) -, as a function of temperature, from 415.7 K to 437.6 K and from 461.9 K to 478.2 K respectively. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation, at the mean temperature of the experimental temperature ranges, were derived for both complexes. The mean values obtained from both techniques are: Be(BZAC)(2), Delta(cr)(g)H(m)(o)(426.6 K)=151.6+/-1.8 kJ mol(-1); AI(BZAC)(3), Delta(cr)(g)H(m)(o)(470.1 K)=186.6+/-2.1 kJ mol(-1). Using the estimated value of Delta(cr)(g)C(p)(o) -50 J K-1 mol(-1), for both beryllium(Il) and aluminium(III) complexes, the standard molar enthalpies of sublimation at 298.15 K, were calculated as 158.0+/-1.8 kJ mol(-1) and 195.2+/-2.1 kJ mol(-1) respectively.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of combustion at the temperature T = 298.15K were measured by static-bomb calorimetry for crystalline 2,2'6,6'-tetraethylazobenzene N,N-dioxide, {2,6-(C2H5)(2)C6H3N(O).}2; 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene, 2,4,6-{CH3C(CH3)(2)}3C6H2NO; and 2,4,6-tri(1,1-dimethylethyl)nitrobenzene, 2,4,6-{CH3C(CH3)(2)}3C6H2NO2. The standard molar enthalpies of sublimation at T = 298.15 K of the tri(1,1-dimethylethyl) compounds were measured by microcalorimetry. [GRAPHICS] The standard molar enthalpy of decomposition of {2,6-(C2H5)(2)C6H3N(O).}2 to form 2,6-(C2H5)(2)C6H3NO(g) at T = 298.15K was measured by microcalorimetry: Delta(dec)H(m) degrees/(kJ . mol(-1)) = (200.6 +/- 5.6). Application of group-additivity schemes applied to nitrosobenzene and nitrobenzene derivatives shows that 2,4,6-tri(1,1-dimethylethyl)nitrosobenzene is unstrained whereas the corresponding nitrocompound shows considerable strain in accord with an X-ray structure analysis demonstrating that steric hindrance prevents dimerization of the nitrosoderivative. (C) 1995 Academic Press Limited.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation for crystalline 2,6-dimethylquinoline, 2,6-(CH3)(2)-C9H5N, and 2,7-dimethylquinoline, 2,7-(CH3)(2)-C9H5N, at the temperature T = 298.15 K, were derived from the standard molar energies of combustion, in oxygen, measured by static-bomb calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, were measured by microcalorimetry. [GRAPHICS]