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排序方式: 共有533条查询结果,搜索用时 78 毫秒
121.
Julio Casado J. Ramón Leis Manuel Mosquera L. Carlos Paz M. Elena Peña 《Monatshefte für Chemie / Chemical Monthly》1984,115(2):155-163
Acetic acid/acetate ion buffer acts catalytically upon the nitrosation of amines under conditions in which the only nitrosating agents are N2O3 and NOBr, but inhibits nitrosation by H2NO
2
+
. The kinetic characteristics of these phenomena have been analysed quantitatively and compared with similar effects caused by the solventsTHF, DMSO and dioxane. The experimental results show that this behaviour is an effect of the medium.
Kinetische Untersuchungen zur Bildung von N-Nitroso-Verbindungen, 8. Mitt.: Nachweis eines Medium-Effekts von Essigsäure/Acetat-Puffer auf die Geschwindigkeitskonstante der Nitrosierung
Zusammenfassung Essigsäure/Acetat-Puffer wirkt bei der Nitrosierung von Aminen katalytisch, unter Bedingungen, wo die alleinigen nitrosierenden Agentien N2O3 und NOBr sind; andererseits wird die Nitrosierung durch H2NO 2 + unterbunden. Die kinetischen Charakteristika dieses Phänomens wurden quantitativ analysiert und mit ähnlichen Effekten der LösungsmittelTHF, DMSO und Dioxan verglichen. Die experimentellen Ergebnisse zeigen, daß dieses Verhalten auf einen Mediumeffekt zurückzuführen ist.相似文献
122.
Kinetics of the hydrolysis of bisphenol A diglycidyl ether (BADGE) in water-based food simulants 总被引:1,自引:0,他引:1
P. Paseiro Losada J. Simal Lozano S. Paz Abuín P. López Mahía J. Simal Gándara 《Fresenius' Journal of Analytical Chemistry》1993,345(7):527-532
Summary The first-order degradation kinetics of bisphenol A diglycidyl ether (BADGE; CAS No. 1675-54-3) has been studied in three water-based food simulants (3% (W/V) acetic acid, distilled water and 15% (V/V) ethanol) at various temperatures. BADGE and its first and second hydrolysis products were determined by reversed-phase high-performance liquid chromatography with fluorescence detection. Nonlinear regression was used to fit the experimental data at 40, 50 and 60°C with the proposed kinetic equations; Arrhenius' equation was then fitted to the rate constants obtained and the kinetic models were tested by comparing experimental data obtained at 70°C with the kinetic curves calculated using the rate constants predicted for this temperature. The half-life of BADGE was longest in ethanol and shortest in acetic acid. The rings opening in acetic acid appears to happen by means of active hydrogens whereas in the other simulants it is mainly influenced by the formation of acid/base adducts. The results imply that resins which comply with existing legislation on the migration of unreacted monomer may still contaminate foodstuffs. 相似文献
123.
M. M. Mato S. M. Cebreiro P. V. Verdes J. L. Legido M. I. Paz Andrade 《Journal of Thermal Analysis and Calorimetry》2005,80(2):245-251
Summary Experimental excess molar enthalpies and densities have been measured for the ternary mixture x1MTBE+x21-propanol+(1-x1-x2)octane and the involved binary mixtures at 298.15 K and atmospheric pressure. In addition, excess molar volumes were determined from the densities of the pure liquids and mixtures. A standard Calvet microcalorimeter was employed to determine the excess molar enthalpies. Densities were measured using a DMA 4500 Anton Paar densimeter. The UNIFAC group contribution model (in the versions of Larsen et al., and Gmehling et al.) has been used to estimate excess enthalpies values. Experimental data were also used to test several empirical expressions for estimating ternary properties from experimental binary results. 相似文献
124.
M. M. Mato S. M. Cebreiro J. L. Legido M. I. Paz Andrade 《Journal of Thermal Analysis and Calorimetry》2006,84(1):279-283
Experimental
excess molar volumes for the ternary system {x1MTBE+x21-propanol+(1–x1–x2)nonane}
and the three involved binary mixtures have been determined at 298.15 K and
atmospheric pressure. Excess molar volumes were determined from the densities
of the pure liquids and mixtures, using a DMA 4500 Anton Paar densimeter.
The ternary mixture shows maximum values
around the binary mixture MTBE+nonane and minimum values for the mixture MTBE+propanol.
The ternary contribution to the excess molar volume is negative, with the
exception of a range located around the rich compositions of 1-propanol.
Several empirical equations predicting ternary mixture properties from
experimental binary mixtures have been applied. 相似文献
125.
Pina J Seixas de Melo J Pina F Lodeiro C Lima JC Parola AJ Soriano C Clares MP Albelda MT Aucejo R García-España E 《Inorganic chemistry》2005,44(21):7449-7458
A new fluorescent macrocyclic structure (L1) bearing two naphthalene units at both ends of a cyclic polyaminic chain containing two phenanthroline units was investigated with potentiometric and fluorescence (steady-state and time-resolved) techniques. The fluorescence emission spectra show the simultaneous presence of three bands: a short wavelength emission band (naphthalene monomer), a middle emission band (phenanthroline emission), and a long-wavelength band. All three bands were found to be dependent on the protonation state of the macrocyclic unit (including the polyaminic and phenanthroline structures). The existence of the long-wavelength emission band is discussed and is shown to imply that a bending movement involving the two phenanthroline units leads to excimer formation. This is determined by comparison with the excimer emission formed by intermolecular association of 1,10-phenanthroline. With ligand L1, excimer formation occurs only at pH values above 4. At very acidic pH values, the protonation of the polyamine bridges is extensive leading to a rigidity of the system that precludes the bending movement. The interaction with metal cations Zn(II) and Cu(II) was also investigated. Excimer formation is, in these situations, increased with Zn(II) and decreased with Cu(II). The long-emission band is shown to present a different wavelength maximum, depending on the metal, which can be considered as a characteristic to validate the use of ligand L1 as a sensor for a given metal. 相似文献
126.
Mary C.?SalazarEmail author José L.?Paz Antonio J.?Hernández Ramón?López-Planes Marcia L.?Llanos 《Theoretical chemistry accounts》2003,110(1):42-46
We have performed ab initio fourth-order Møller–Plesset perturbation theory calculations in the framework of the supermolecule approach on the vertical excitation spectra of the weakly bound van der Waals N2–He dimer. They indicate a ``T-shaped' stablest ground N2(X1g+)–He(1S) electronic state with a well depth, De, of 21.63 cm–1 at a minimum distance, Re, of 3.44 Å and zero-point vibration correction, Do, of 7.07 cm–1. They also indicate a ``T-shaped' stablest excited conformer with Re=3.25 Å, De=36.85 cm–1 and Do=17.06 cm–1 for the N2(B3g)–He(1S) triplet electronic level. In order to investigate the use of less-demanding correlation methods, test density functional theory calculations using the mPW1PW exchange–correlation functional are also presented for comparison. 相似文献
127.
Muñoz EM López de la Paz M Jiménez-Barbero J Ellis G Pérez M Vicent C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(8):1908-1914
Carbohydrates with suitably positioned intramolecularly hydrogen-bonded hydroxyl and amide groups have the potential to act as efficient bidentate phosphate binders by taking advantage of sigma- and/or ,sigma,pi-H-bonding cooperativity in nonpolar solvents. Donor-donor 1,2-trans-diaxial amido alcohol (1) and diol (3), in which one of the donor centres is cooperative, are very efficient carbohydrate-phosphate binding motifs. We have proven and quantified the key role of hydrogen-bonding centres indirectly involved in complexation, which serve to generate an intramolecular H-bond (six-membered cis H-bond) in 1 and 3. This motif enhances the donor nature of the H-bonding centres that are directly involved in complexation. A comparison of the thermodynamic parameters of the complexes formed between phosphate and a cooperative (1-Phos) or anti-cooperative (2-Phos) bidentate H-bonded motif of a carbohydrate has allowed us to quantify the energetic advantage of H-bonding cooperativity in CDCl3 and CDCl3/CCl4 (1:1.3) (Delta Delta G degrees=-2.2 and -2.0 kcal mol(-1), respectively). The solvent dependences of the entropy and enthalpy contributions to binding provide a valuable example of the delicate balance between entropy and enthalpy that can arise for a single process, providing effective cooperative binding in terms of Delta G degrees. 相似文献
128.
João P.C ToméAugusto C Tomé Maria G.P.M.S NevesFilipe A Almeida Paz Paul J GatesJacek Klinowski José A.S Cavaleiro 《Tetrahedron》2003,59(40):7907-7913
Novel derivatives of meso-tetraphenylporphyrin with appended quinazoline moieties were synthesized, via the Diels-Alder reaction, between a 4-(porphyrinyl)pyrimidine ortho-quinodimethane and 1,4-benzoquinone, 1,4-naphthoquinone and N-(p-nitrophenyl)maleimide. The structure of one bis adduct was established by X-ray crystallography and mass spectrometry. We have unequivocally confirmed that the 2:1 adducts obtained from the reaction of pyrimidine-fused 3-sulfolenes with N-arylmaleimides have an open-chain structure and not a cyclooctapyrimidine structure, as previously published. 相似文献
129.
The potential-energy surfaces for the proton transfer in the doubly hydrogen-bonded dimer of 7-azaindole in its lowest excited electronic states were examined. The dimer with C2h symmetry in its lowest excited electronic states, 2Ag and 1Bu, undergoes concerted double-proton transfer via transition states of the same symmetry placed at energies 4.55 and 4.70 kcal/mol higher, respectively. This suggests that the activation barriers for the double-proton transfer, if any, are lower than 1 kcal/mol. Emission from the dimers resulting from the double-proton transfer involves a Stokes shift of 5605 cm(-1), as theoretically estimated from the 0-0 components of the absortion and emission transitions of the dimer. Surprisingly, however, the calculations suggest that the green emission cannot arise from the 2Ag state generated by a double-proton transfer, because this structure possesses an imaginary frequency. In the 7-azaindole dimer of Cs symmetry, the first excited electronic state, a', lies 4.9 kcal/mol below 1Bu. This excited state a' can be the starting point for single-proton transfers giving a zwitterionic form that can dissociate into the protonated and deprotonated forms of 7-azaindole, the former being electronically excited. This situation of lower symmetry is consistent with the mutational scheme proposed by Goodman [Nature (London) 378, 237 (1995)]. 相似文献
130.
Jakub Frtczak Hctor de Paz Carmona Zdenk Tiler Jos M. Hidalgo Herrador Zahra Gholami 《Molecules (Basel, Switzerland)》2021,26(23)
The Fischer–Tropsch heavy fraction is a potential feedstock for transport-fuels production through co-processing with fossil fuel fraction. However, there is still the need of developing new and green catalytic materials able to process this feedstock into valuable outputs. The present work studies the co-hydrocracking of the Fisher–Tropsch heavy fraction (FT-res.) with vacuum gas oil (VGO) at different ratios (FT-res. 9:1 VGO, FT-res. 7:3 VGO, and FT-res. 5:5 VGO) using phonolite-based catalysts (5Ni10W/Ph, 5Ni10Mo/Ph, and 5Co10Mo/Ph), paying attention to the overall conversion, yield, and selectivity of the products and properties. The co-processing experiments were carried out in an autoclave reactor at 450 °C, under 50 bars for 1 and 2 h. The phonolite-based catalysts were active in the hydrocracking of FT-res.:VGO mixtures, presenting different yields to gasoline, diesel, and jet fuel fractions, depending on the time of reaction and type of catalyst. Our results enable us to define the most suitable metal transition composition for the phonolite-based support as a hydrocracking catalyst. 相似文献