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121.
Miguel F. Braa Jos M. Castellano Rosa Donoso María L. Lpez Rodríguez María J. R. Yunta Nigel P. C. Walker 《Journal of heterocyclic chemistry》1990,27(2):397-399
Reaction of N-(α-acetoxy)4-pyridylmethyl]-3,5-dimethylbenzamide 3 with methyl and ethyl isocyanates afforded 1,3-dimethyl and 1,3-diethyl-4-(3,5-dimethylbenzoylamino)-2-oxoimidazolidine-5-spiro-4′-[1′,4′-dihydro-1′-acetyl]pyridine 6a,b , respectively. However, the reaction of 3 with isopropyl, t-butyl and phenyl isocyanates gave the corresponding N,N′-diurea and the dimerization compound 8 . The structure of 6a was confirmed by crystal X-ray diffraction analysis. 相似文献
122.
Beryllium precipitation from the Cu-rich matrix in a Cu–2 mass% Be–0.2 mass% Mg alloy homogenized and quenched from 1073 K was studied by differential scanning calorimetry (DSC). The DSC traces showed two main exothermic effects, A and B, each comprising two subeffects: A1 and A2 , and B1 and B2 respectively. Effects A1 and A2 correspond to the precipitation of GP zones and subsequent overlapping and independent precipitation of the phase. Only at very low heating rates can be inherited from GP zones. Effects B1 and B2 correspond to heat evolved during transitions to the states with and phases, respectively. Heat effect A can be quantitatively described in terms of solid solubilities before and after precipitation, and of the precipitation heats of the phases involved. The heat content of the combined GP zone/ phase precipitation effect was proportional to the number of beryllium atoms precipitated, yielding an average value of 21 kJ mol–1 beryllium for beryllium precipitation. It was shown that the phase arises from the combined transition from states with GP zones and phases, whereas arises from the transition of states with and phases. The apparent activation energies associated with GP zones and , and phases are 1.16±0.08, 1.18±0.07, 1.37±0.08 and 1.74±0.09 eV, respectively. These values are discussed in terms of the mobility of dissolved atoms related to the concentrations of excess vacancies and solute-vacancy complexes, and the direction of plate-like precipitate growth (either normal or perpendicular to the plate). It is inferred that the main roles of magnesium are to decrease the amount and rate of GP formation, to enhance the volume fraction of and to suppress the discontinuous precipitation of .This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
123.
J. Frau J. Donoso B. Vilanova F. Muñoz F. García Blanco 《Theoretical chemistry accounts》1993,86(3):229-239
Summary We carried out a comprehensive theoretical study on the alkaline hydrolysis of the bicyclic system of penicillins (a four-member ring fused to a thiazolidine ring) on the basis of aB
AC2 mechanism. We assayed the MINDO/3, MNDO, and AM1 semi-empirical calculation methods in order to determine their suitability for studying -lactam rings.Both the geometric and the energetic results obtained for the different intermediate states were compared with literature values — chiefly those determined byab initio methods — with which they proved to be very consistent.The conformation of the carboxyl group at position 3 was found to be rather significant to the determination of the energy of the different reaction maxima and minima. 相似文献
124.
Maria Jose Gomez Iris Martí Loreto Donoso 《Fresenius' Journal of Analytical Chemistry》1991,339(9):664-668
Summary The acid-base properties of tropical aquatic humic substances isolated from 2 Venezuelan black water rivers were studied. A model with discrete pK values was applied to potentiometric titration data, since direct determination of pK and concentration values from titration curves is not possible. In the computational procedure simulated curves have been compared with the titration curves to improve the calculations for pK and concentration values. Both rivers (Yuruaní and Aponwao) are located in a zone where the human influence is still negligible. The geomorphological characteristics of this remote area (Gran Sabana) are singular in relation to the rest of the world. The rivers have been monthly studied at sampling site, during the rainy season (May–September). Six discrete acidic groups represented well the acidic characteristics of aquatic humic substances under consideration. The average pK values were 2.74, 4.37, 5.67, 7.09, 8.73 and 10.30, and the average concentration values for each group were 2.50, 1.80, 0.94, 0.62, 0.66, and 1.31 mmol/g humic substance, respectively. Considering the factors month and river, significant quantitative, but not qualitative, differences were in general found. A comparison with data from temperate ecosystems showed that the three most acidic groups presented similar acid-base characteristics, wheras the three other groups are quite different for the tropical aquatic humic substances under study. 相似文献
125.
Abstract Measurements of the 19F nuclear relaxation times are reported for the fluorite-structured Pb0.84Bi0.16F2.16 single crystal. The formation of clusters by thermal treatment, as revealed by changes in the fluorine T 2 during temperature cycling, is discussed. 相似文献
126.
Jaime Gonzlez‐Villa Enrique Saldívar‐Guerra Ramn Enrique Díaz de Len‐Gmez Hctor Ricardo Lpez Gonzlez Jos Ramiro Infante‐Martínez 《Journal of polymer science. Part A, Polymer chemistry》2019,57(21):2157-2165
The kinetics of isothermal anionic homopolymerization of ß–myrcene (MYR) and 4–methylstyrene (4MS) in cyclohexane, initiated by n–butyllithium was studied at different temperatures (55, 63, and 71 ° C). The kinetic information obtained from the homopolymerizations was used to estimate the parameters of the Eyring equation, ΔH?= 84080 J/mol and ΔS?= ?21.9 J/mol·K for the MYR, and ΔH?= 51250 J/mol and ΔS?= ?116.8 J/mol·K for the 4MS, to calculate the apparent propagation coefficients as a function of temperature. Finally, the parameters obtained for the Eyring equation were validated by applying them in a mathematical model representing the kinetics of nonisothermal (quasi‐adiabatic) polymerization experiments of MYR and 4MS. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2157–2165 相似文献
127.
128.
Ramiro de la Vega 《Topology and its Applications》2008,155(4):225-232
We show that under the continuum hypothesis there is a compact zero-dimensional space which admits a base of pairwise homeomorphic clopen subsets but it is not an h-homogeneous space (i.e. not all of its nonempty clopen subsets are homeomorphic), partially answering a question of M.V. Matveev. Under Jensen's ? principle, we can even make the space hereditarily separable and hence, by a result of Matveev, an S-space. 相似文献
129.
Marcelo Nalin Sidney Jos Lima Ribeiro Youns Messaddeq Jos Schneider Pedro Donoso 《Comptes Rendus Chimie》2002,5(12)
A systematic investigation on glass formation in the ternary system InF3–BaF2–Sc(PO3)3n has been carried out. Scandium polyphosphate Sc(PO3)3n has been used as a third component in order to investigate the possibilities of obtaining new stable glasses. The above long-chain polyphosphate has been prepared using a specially elaborated cryo-technique, which allowed the preparation of high-purity product. Stable ternary compositions have been obtained within the compositions range (in mol%): 5–75 InF3, 0–80% BaF2, 0–50% Sc(PO3)3n. Glasses were characterized by Differential Scanning Calorimetry, vibrational spectroscopy (Raman) and 31P NMR. Structural features for the glass were put forward. Isolated P(O,F)4 groups or fluoroindated metaphosphate units could be identified depending on glass compositions. 相似文献
130.
Ramiro Pino Olivier Bokanowski Eduardo V. Ludeña Roberto López Boada 《Theoretical chemistry accounts》2009,122(3-4):189-196
We have studied photodissociation of the A state of the H2S+ ion using the quantum-chemical CAS methods, and the 12 A″ (X 2 B 1) and 14 A″ states are involved in photodissociation of the 12 A′ (A 2 A 1) state (the electronic states in dissociation were studied in the C s symmetry). The CASPT2 S-loss dissociation potential energy curve (PEC) calculations indicate that the 12 A″ and 12 A′ states correlate with the second limit [H2 + S+(2 D)] while the 14 A″ state correlates with the first limit [H2 + S+(4S)] and that there are a transition state and a local minimum along the 12 A′ PEC and the repulsive 14 A″ PEC crosses the 12 A″ and 12 A′ PECs. The CASPT2 H-loss dissociation PEC calculations indicate that the 12 A″ and 14 A″ states correlate with the first limit [HS+(X 3Σ?) + H] while the 12 A′ state correlates with the second limit [HS+(a 1Δ) + H] and that the repulsive 14 A″ PEC crosses the 12 A′ PEC. For the crossing doublet and quartet states in pairs, we performed CASSCF minimum energy crossing point (MECP) calculations, and the CASSCF spin-orbit couplings and CASPT2 energies at the MECP geometries were calculated. We examined the two previously proposed mechanisms (mechanisms I and II) for dissociation of the A state to the S+ ion, based on our calculation results. We suggest processes for dissociation of the A state to the S+ ion (processes I and II, based on mechanisms I and II, respectively) and to the SH+ ion (process III) and conclude that photodissociation of the A state mainly leads to the S+ ion via the most energetically favorable process II: A 2 A 1 (12 A′) (2.38 eV) → barrier at the linearity (2.96 eV) → X 2 B 1 (12 A″) (0.0 eV) → the 12 A″/14 A″ MECP (3.50 eV, large spin-orbit coupling) → H2 $ (X^{ 1} \Upsigma_{\text{g}}^{ + } ) $ + S+(4S) (2.92 eV) (the CASPT2 relative energy values to X 2 B 1 are given in parentheses and the largest value is 3.50 eV at the MECP). 相似文献