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排序方式: 共有838条查询结果,搜索用时 171 毫秒
21.
Hctor Salgado Zamora Benito Rizo Elena Campos Rogelio Jimnez Alicia Reyes 《Journal of heterocyclic chemistry》2004,41(1):91-94
The imidazo[1,2‐a]pyridine system was investigated as a synthon for the building of very attractive fused triazines, a planar, angular tri‐heterocycle with potential biological activity. Thus ethyl 3‐nitroimidazo[1,2‐a]pyridine‐2‐carboxylate was treated with ammonia or with an excess of primary amines to generate the corresponding substituted nitro carboxamidoimidazopyridines. The nitro substituent in the latter products, was reduced to yield 3‐amino‐2‐carboxamidoimidazo[1,2‐a]pyridine derivatives, which in turn were treated with nitrous acid to furnish 1‐oxo‐2‐substituted pyrido(1′,2′:1,2)imidazo[5,4‐d]‐1,2,3‐triazines. 相似文献
22.
Intrinsic viscosities, [η], second virial coefficients, A2, and preferential solvation coefficients, λ, for the ternary systems n-alkane (l)-butanone (2)-poly(dimethylsiloxane) (PDMS) (3), with n-alkane = n-hexane, n-heptane, n-nonane and n-undecane, have been determined at 20°. The K and a constants of the Mark-Houwink equation have been evaluated over the whole composition range of the binary solvent mixtures. Polymer (mixed solvent) interaction parameters and unperturbed dimensions have been evaluated both from A2 and [η] data, the feasibility of A2 evaluation from [η] experimental data or vice versa being discussed. Experimental and calculated (through Dondos and Patterson theory) excess free energies, GE, follow similar trends with composition; large numerical discrepancies, however, arise between both sets of GE. Maxima in [η], in a and in A2 are accompanied by inversion points in λ. The solvent mixture composition range in which PDMS is preferentially solvated by n-alkane, as well as the extent of solvation, decrease with increasing number of carbon atoms in the n-alkane. 相似文献
23.
Excitation-energy dependence of fluorescence intensity and fluorescence lifetime has been measured for 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN), 4-diisopropylaminobenzonitrile (DIABN), and 1-naphthonitrile (NN) in a supersonic free jet. In all cases, the fluorescence yield decreases rather dramatically, whereas the fluorescence lifetime decreases only moderately for S1 (pi pi*, L(b)) excess vibrational energy exceeding about 1000 cm(-1). This is confirmed by comparison of the normalized fluorescence excitation spectrum with the absorption spectrum of the compound in the vapor phase. The result indicates that the strong decrease in the relative fluorescence yield at higher energies is due mostly to a decrease in the radiative decay rate of the emitting state. Comparison of the experimental results with the TDDFT potential energy curves for excited states strongly suggests that the decrease in the radiative decay rate of the aminobenzonitriles at higher energies is due to the crossing of the pi pi* singlet state by the lower-lying pi sigma*(C[triple bond]N) singlet state of very small radiative decay rate. The threshold energy for the fluorescence "break-off" is in good agreement with the computed energy barrier for the pi pi*/pi sigma* crossing. For NN, on the other hand, the observed decrease is in fluorescence yield at higher excitation energies can best be attributed to the crossing of the pi pi* singlet state by the pi sigma* triplet state. 相似文献
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Juliana Vaz Bevilaqua Lidia M. Lima Aline Gomes Cunha Eliezer J. Barreiro Tito L. M. Alves Lucia Moreira Campos Paiva Denise M.Guimarães Freire 《Applied biochemistry and biotechnology》2005,121(1-3):117-128
The last step of the production of four phthalimide-derived acids, designed to act as antiasthma drugs, was performed by enzymatic hydrolysis of the respective methyl or ethyl esters. The esters 4-ethyl-[2-(1,3-dioxo-1,3-dihydro-2-isoindoylyl)]-phenoxyacetic methyl ester (PHT-MET), 4-ethyl-[2-(1,3-dioxo-1,3-dihydro-2-isoindoylyl)]-phenoxyacetic ethyl ester, 4-(1,3-dioxo-1,3-dihydro-2-isoindoylyl)-phenoxyacetic ethyl ester, and 2-(1,3-dioxo-1, 3-dihydro-2-isoindoylyl)-phenoxyacetic ethyl ester were hydrolyzed by immobilized lipase. The enzymatic reaction could be used only to produce the desired 4-substituted compounds. The best result that was found to hydrolysis of PHT-MET, and, therefore, that ester was selected for optimization experiments in a three-phase system. Reactions were performed with solid biocatalyst (Lipozyme® RM IM), organic solvent phase (ethyl acetate), and aqueous phase (saturated Na2CO3 solution). To optimize the reaction conditions, an experimental design optimization procedure was used. The variables studied were the amount of enzyme, the temperature, and the volume of the aqueous solution. Time course experiments were then performed for different initial enzyme concentrations (0.5, 0.9, and 1.4 UH/mL of solvent). The optimized reaction conditions found were 20 mg of Lipozyme (0.9 UH/mLsolvent) and 5.0 mL of Na2CO3(sat) at 40°C for 6 h. 相似文献
27.
P. J. Mendes J. M. Gil N. Ayres de Campos R. Peichl A. Weidinger 《Hyperfine Interactions》1983,16(1-4):791-794
The perturbation felt by181Hf probes in a181HfTa lattice loaded with 30 at% hydrogen was observed by PAC as a function of temperature. Three different interactions were identified: 1) ΝQ1=433 (6) MHz, η=0.45 2) ΝQ2=142 (9) MHz, η=0.9, and 3) ΝQ?0, σ=4–14 Μ?1 which are attributed to the Β?, ε? and α-phase in TaH system, respectively. 相似文献
28.
Samuel N. Sanders Elango Kumarasamy Kealan J. Fallon Matthew Y. Sfeir Luis M. Campos 《Chemical science》2020,11(4):1079
Singlet fission (SF) is an exciton multiplication process with the potential to raise the efficiency limit of single junction solar cells from 33% to up to 45%. Most chromophores generally undergo SF as solid-state crystals. However, when such molecules are covalently coupled, the dimers can be used as model systems to study fundamental photophysical dynamics where a singlet exciton splits into two triplet excitons within individual molecules. Here we report the synthesis and photophysical characterization of singlet fission of a hexacene dimer. Comparing the hexacene dimer to analogous tetracene and pentacene dimers reveals that excess exoergicity slows down singlet fission, similar to what is observed in molecular crystals. Conversely, the lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following the energy gap law for radiationless transitions. These results point to design rules for singlet fission chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between triplet pair and ground state should be large.We report the synthesis and photophysical characterization of highly exoergic singlet fission in a hexacene dimer revealing exciton dynamics that follow the energy gap law. 相似文献
29.
The reaction of di-p-tolyltelluride with ferric, cupric and mercuric chloride, performed in glacial acetic acid, effects the reduction of the metallic salts giving rise to di-p-tolyltellurium dichloride. In toluene solution only the ferric chloride was reduced and the anhydride of di-p-tolyltellurium hydroxychloride formed, due to the hydrolysis of the tellurium dichloride. The reaction of the mentioned telluride and benzoyl chloride gave rise to a mixture of the dichloride and the dibenzoate. The mechanism of this reaction is discussed. 相似文献
30.