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921.
Klaus Rakus Sergej P. Verevkin Hans-Dieter Beckhaus Christoph Rüchardt 《欧洲无机化学杂志》1994,127(11):2225-2234
The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile ( 10a ) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile ( 10b ) decomposed heterolytically into carbenium ions and (CN)3C− anions, while 9-methyl-9-(tricyanomethyl)fluorene ( 10c ) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH ( 10c ) = 155· 2 kJ mol−1, ΔS ( 10c ) = 58· 5 J mol−1 K−1]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile ( 2 ) (422.45 kJ mol−1), 2,2-dicyanohexanenitrile ( 3 ) (349.74 kJ mol−1), 2,2-dicyano-3-phenylpropionitrile ( 4 ) (540.75 kJ mol−1), 2-butyl-2-methylhexanentrile ( 5 ) (-133.20 kJ mol−1), 2,2-dimethylpentanenitrile ( 6 ) (-45.78 kJ mol−1), and 2-methylbutyronitrile ( 7 ) (2.44 kJ mol−1) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE ( 1 ) = -18 kJ mol−1 was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol−1) and α,α-dicyanoalkyl (27 kJ mol−1) radicals, which were recalculated from bond homolysis measurements[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2). 相似文献
922.
The electron density distribution in potassiumbis-(carbonato)cuprate(II) has been analyzed using x-ray diffraction data from an earlier structure determination. While the
copper-ligand geometry is close to square planar the deformation density near the metal is strongly asymmetric. There are
local maxima near the copper atom along the line of the Cu-K vectors. These resemble features found in corresponding regions
in normal length metal-metal bonds. The observation is consistent with the long range nature of the Coulomb potential associated
with the potassium ion. 相似文献
923.
Selective detection and identification of Se containing compounds--review and recent developments 总被引:5,自引:0,他引:5
The complexity of selenium (Se) chemistry in the environment and in living organisms presents broad analytical challenges. The selective qualitative and quantitative determination of particular species of this element is vital in order to understand selenium's metabolism and significance in biology, toxicology, clinical chemistry and nutrition. This calls for state-of-the-art analytical techniques such as hyphenated methods that are reviewed with particular emphasis on interfaced separation with element-selective detection and identification of the detected selenium compounds. Atomic spectral element specific detection for monitoring chromatographic eluent enabled quantitative determination of selenium species in selenized yeast and qualitative measurement for breath samples. Gas chromatography with atomic emission detection (AED) of ethylated species and fluoroacid ion pair HPLC applied to the analysis of currently produced or archived selenized yeast and Brassica juncea have revealed the presence of a previously unrecognised Se-S amino acid, S-(methylseleno)cysteine. 相似文献
924.
José A. Rodriguez 《Theoretical chemistry accounts》2002,107(3):117-129
The surface chemistry of oxides is relevant for many technological applications: catalysis, photoelectrolysis, electronic-device
fabrication, prevention of corrosion, sensor development, etc. This article reviews recent theoretical works that deal with
the surface chemistry of oxides. The account begins with a discussion of results for the adsorption of CO and NO on oxides,
systems which have been extensively studied in the literature and constitute an ideal benchmark for testing the quality of
different levels of theory. Then, systematic studies concerned with the behavior of adsorbied alkali metals and sulfur-containing
molecules are presented. Finally, a correlation between the electronic and chemical properties of mixed-metal oxides is analyzed
and basic principles for designing chemically active oxides are introduced. Advances in theoretical methods and computer capabilities
have made possible a fundamental understanding of many phenomena associated with the chemistry of molecules on oxide surfaces.
Still many problems in this area remain as a challenge, and the approximate nature of most theoretical methods makes necessary
a close coupling between theory and experiment. Following this multidisciplinary approach, the importance of band-orbital
interactions for the reactivity of oxide surfaces has become clear. Simple models based on band-orbital mixing can explain
trends found for the interaction of many adsorbates with oxide surfaces. These simple models provide a conceptual framework
for modifying or controlling the chemical activity of pure oxides and for engineering mixed-metal oxides. In this respect,
theoretical calculations can be very useful for predicting the best ways for enhancing the reactivity of oxide systems and
reducing the waste of time, energy and materials characteristic of an empirical design.
Received: 21 June 2001 / Accepted: 8 October 2001 / Published online: 1 February 2002 相似文献
925.
Fedushkin I. L. Kurskii Yu. A. Nevodchikov V. I. Bochkarev M. N. Müchle S. Schumann H. 《Russian Chemical Bulletin》2002,51(1):160-169
ansa-Metallocenes (5:5-C24H16)M(THF)2 (M = Sm (1), Yb (2), Ca (3)) and (5:5-C24H16)MI(THF) (M = Dy (8), Er (9), Tm (10), Lu (11)) were prepared in 50—90% yields by the in situ reactions of two equivalents of potassium acenaphthylenide K+C12H8
– with MI2 or MI3, respectively. Complexes 2 and 3 were also obtained by direct reduction of acenaphthylene with ytterbium and calcium naphthalenides, respectively. An ESR signal of the acenaphthylene radical anion, which was observed upon dissolution of compound 2 in THF, indicates that the [C24H16]2–
ansa-ligand dissociated into two [C12H8]·– radical anions. Hydrolysis of complex 2 in benzene afforded 1,1",3,3"-tetrahydro-3,3"-biacenaphthylene (4) and 3,3",4,4"-tetrahydro-3,3"-biacenaphthylene (5). The reaction of complex 2 with ZrCl4 and the reaction of compound 3 with Me3SiCl proceeded with the cleavage of the C—C bond between two acenaphthylene fragments of the [C24H16]2–
ansa-ligand to produce (2-C12H8)ZrCl2(THF)3 (6) and bis(trimethylsilyl)acenaphthene (Me3Si)2C12H8 (7), respectively. Compounds 1—3, 6, 7, and 11 were characterized by 1H and 13C NMR spectroscopy. The temperature dependence of the 1H NMR spectrum of compound 11 in tetrahydrofuran is indicative of the dynamic exchange of the solvent molecules in the coordination sphere of the Lu atom. After cooling of the solution to 210 K, the dynamic process was terminated as evidenced by the nonequivalence of the 1H signals of two acenaphthylene fragments. According to the X-ray diffraction data for complex 11, dimerization of two acenaphthylene radical anions at the Lu atom gave rise to the rac-ansa-metallocene structure. In compound 11, the Lu atom is 5-coordinated by two five-membered rings of the acenaphthylene ligands and also by the I atom and the THF molecule. The coordination environment about the Lu atom is a distorted tetrahedron. The average distance between the lutetium atom and the carbon atoms of the five-membered rings is 2.623 . 相似文献
926.
The Molecule S?GeCl2. Matrix IR Investigation and Ab initio SCF Calculation Molecular S?GeCl2 is found in a matrix reaction between the high-temperature molecule Ge?S and Cl2. A structure analog to that of phosgene can be derived from the isotopical shifts (70Ge/72Ge/73Ge/74Ge/76Ge and 35Cl/37Cl) within the IR spectra. The normal coordinate analysis results for the Ge?S force constant a value of 4.21 mdyn/Å. The spectroscopic results are confirmed by ab initio SCF calculations. 相似文献
927.
The comparison of a pure electrostatic approximation and complete supermolecule SCF ab initio computations on the hydration scheme of uracil and cytosine shows that the electrostatic procedure is capable to reproduce the general aspects of the results of the supermolecule treatment provided that different distances of shortest approach be adopted for the distances between the oxygen of water and the nitrogen of NH2 or NH groups or the oxygen of C-O groups on the one hand and the oxygen of water and pyridine-type nitrogens on the other hand. 相似文献
928.
The thermodynamics of the conversion of aqueous glucose to fructose has been investigated using both heat conduction microcalorimetry
and high pressure liquid chromatography (HPLC). The reaction was carried out in both aqueous Tris/HCl buffer and in aqueous
phosphate buffer in the pH range 7–8 using the enzyme glucose isomerase and the cofactors CoCl2 and MgSO4. The temperature range over which this reaction was investigated was 298.15–358.15 K. We have found that the enthalpy of
reaction is independent of pH over the range investigated. A combined analysis of both the HPLC and microcalorimetric data
leads to the following results at 298 15 K:ΔG° = 349 ± 53 J mol-1, ΔH° = 2.78 ± 0.20 kJ mol-1, and ΔC
p
°
= 76 ± 30 J mol-1 K-1. The stated uncertainties are based upon an analysis of both the random and systematic errors inherent in the measurements.
Comparisons are made with literature data. The percent conversion of glucose to fructose has been calculated for the temperature
range 300–373.15 K. 相似文献
929.
Karen L. Kindle 《Applied biochemistry and biotechnology》1983,8(2):153-170
Thermostable α-amylases have application in a variety of industrial processes and enzymes from a substantial number of thermophilic
bacteria and fungi have been screened and characterized to varying degrees. The characteristics of these enzymes are summarized
in this review. The genetics of α-amylase production inBacillus subtilis is reviewed and classical and recombinant DNA approaches to increasing α-amylase production are discussed. 相似文献
930.
Immobilized sulfhydryl groups were prepared by partial thiolation of NH2-glass beads. The microenvironment of the immobilized SH groups was varied by different chemical modifications of neighboring
NH2 groups. Introduction of a strong charge in the surroundings of immobilized sulfhydryls results in their dramatic stabilization
against autooxidation. This effect is due to the salting of O2 from the surface microlayer of the thiolated beads. 相似文献