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排序方式: 共有318条查询结果,搜索用时 140 毫秒
221.
E. Peter Kündig Chantal Grivet Eric Wenger Gerald Brnardinelli Alan F. Williams 《Helvetica chimica acta》1991,74(8):2009-2023
3-Substituted 1,2-dihydrocyclobutabenzenes (bicyclo[4. 2. 0]octa-1,3,5-triene) are readily accessible from [Cr(CO)3(1,2-dihydrocyclobutabenzene)] ( 1 ) via a two-step sequence which involves addition of a nucleophile and oxidation of the intermediate anionic cyclohexadienyl complex. Nucleophiles used include LiCMe2CN (A), LiCH2CN (B), LiC(Me)(OR)CN (C), (D), (E), LiCMe2CO2Me (F), and LiCH2CO2(t-Bu) (G). [Cr(CO)3(Indane)] ( 2 ) also reacts highly regioselectively to give α-substitution, whereas [Cr(CO)3(tetrahydronaphthalene)] ( 3 ) and [Cr(CO)3(o-xylene)] ( 4 ) give mixtures of products. In several cases, the mixtures of the intermediate anionic cyclohexadienyl complexes can be equilibrated to give, after oxidation, β-substituted derivatives of 1,2,3,4-tetrahydronaphthalene and ortho-xylene selectively. EHMO calculations were carried out, and they rationalize the observed α-regioselectivity of nucleophilic addition under kinetic control, The X-ray structures of 1 and 4 are reported and in both compounds the Cr(CO)3 group adopts in the solid state a staggered syn-conformation with respect to the substituted aromatic C-atonis. 相似文献
222.
The minimum-energy structures on the torsional potential-energy surface of 1,3-butadiene have been studied quantum mechanically using a range of models including ab initio Hartree-Fock and second-order M?ller-Plesset theories, outer valence Green's function, and density-functional theory with a hybrid functional and statistical average orbital potential model in order to understand the binding-energy (ionization energy) spectra and orbital cross sections observed by experiments. The unique full geometry optimization process locates the s-trans-1,3-butadiene as the global minimum structure and the s-gauche-1,3-butadiene as the local minimum structure. The latter possesses the dihedral angle of the central carbon bond of 32.81 degrees in agreement with the range of 30 degrees-41 degrees obtained by other theoretical models. Ionization energies in the outer valence space of the conformer pair have been obtained using Hartree-Fock, outer valence Green's function, and density-functional (statistical average orbital potentials) models, respectively. The Hartree-Fock results indicate that electron correlation (and orbital relaxation) effects become more significant towards the inner shell. The spectroscopic pole strengths calculated in the Green's function model are in the range of 0.85-0.91, suggesting that the independent particle picture is a good approximation in the present study. The binding energies from the density-functional (statisticaly averaged orbital potential) model are in good agreement with photoelectron spectroscopy, and the simulated Dyson orbitals in momentum space approximated by the density-functional orbitals using plane-wave impulse approximation agree well with those from experimental electron momentum spectroscopy. The coexistence of the conformer pair under the experimental conditions is supported by the approximated experimental binding-energy spectra due to the split conformer orbital energies, as well as the orbital momentum distributions of the mixed conformer pair observed in the orbital cross sections of electron momentum spectroscopy. 相似文献
223.
Germain Puzo Jean-Claude Prom Chantal Zedde Louis Cazaux 《Journal of mass spectrometry : JMS》1981,16(9):405-409
Electron impact mass spectrometry of an imido-sulfite 5,6-benzo-2-imino-N-(2,4,6-trichlorophenyl)-1,3-dioxa-2-thiacycloheptane shows an intense fragment ion corresponding to the expulsion of SO2H from the molecular ion. A mechanism that requires a rearrangement of the molecular ion is proposed. Structural elucidation of the [M? SO2H]+ ion was obtained by recording its metastable ion and collisional activation spectra. Comparison of these spectra with similar spectra recorded from a precursor ion of known structure demonstrated that they were identical. Thus, the results support the proposed structure which derives from the expulsion of SO2H from the molecular ion of the compound 5,6-benzo-2-imino-N-(2,4,6-trichlorophenyl)-1,3-dioxa-2-thiacycloheptane. 相似文献
224.
André-Barrès C Najjar F Bottalla AL Massou S Zedde C Baltas M Gorrichon L 《The Journal of organic chemistry》2005,70(17):6921-6924
The behavior of G3 factor and of its methylated or fluorinated analogues G3Me and G3F, was studied under Fe(II) conditions. Degradation products were isolated and characterized in each case. The use of labelled compounds allowed us to propose mechanisms in which a tertiary radical is involved. This radical rearranges by 5-exo-trig cyclization, or disproportionates in the case of G3Me. A correlation between antiplasmodial activity and stability of this radical is proposed. 相似文献
225.
Rickard GA Bergès J Houèe-Levin C Rauk A 《The journal of physical chemistry. B》2008,112(18):5774-5787
Thioredoxin controls the intracellular redox potential through a disulfide/dithiol couple. Under conditions of oxidative stress, this protein functions via one-electron exchange, in which formation of the disulfide radical anion occurs. Combined quantum mechanical (QM) and molecular mechanical (MM) calculations using two- and three-level ONIOM schemes were performed on the thioredoxin (Trx) protein of Chlamydomonas reinhardtii in its oxidized-disulfide and one-electron-reduced forms. In both cases, the active site disulfide moiety was described at the MP2(fc)/6-31+G(d) level, and larger regions of varying sizes around the active site were described at the B3LYP/6-31+G(d) level. The remainder of the 112 residues and 33 water molecules of the crystal structure (PDB entry 1EP7) were described by the AMBER force field. Adiabatic electron affinities were calculated for the disulfide bond in all systems. Separate QM or QM/QM calculations were performed on the QM regions to establish the role of the remainder of the protein on the active site properties. The radical anion species becomes more stable as the number of amide groups in the vicinity increases. One-electron reduction potentials were calculated for the small molecule models, and approximated for the protein for which the values are similar to the experimental one (approximately 0 V). This high reduction potential is due to interaction with the charged end of Lys40, as indicated by mutation in silico to norleucine. The inclusion of the protonated Asp30 side chain and a water molecule in the QM region leads to an increase in the electron affinity. Proton transfer from the Asp30 side chain to the Cys39 sulfur in the radical anion species is strongly disfavored. The radical anion is more stable than the protonated form, which is consistent with experimental results. 相似文献
226.
Dr. Chantal D. Bader Dr. Fabian Panter Dr. Ronald Garcia Dr. Egor P. Tchesnokov Dr. Sibylle Haid Christine Walt Dr. Cathrin Spröer Dr. Alexander F. Kiefer Prof. Dr. Matthias Götte Prof. Dr. Jörg Overmann Prof. Dr. Thomas Pietschmann Prof. Dr. Rolf Müller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(10):e202104484
227.
Screening of the effect of surface energy of microchannels on microfluidic emulsification 总被引:2,自引:0,他引:2
Li W Nie Z Zhang H Paquet C Seo M Garstecki P Kumacheva E 《Langmuir : the ACS journal of surfaces and colloids》2007,23(15):8010-8014
We report the results of a systematic study of the effect of the surface energy of the walls of microchannels on emulsification in parallel flow-focusing microfluidic devices. We investigated the formation of water-in-oil (W/O) and oil-in-water (O/W) emulsions and found that the stability of microfluidic emulsification depends critically on the preferential wetting of the walls of the microfluidic device by the continuous phase. The condition for stable operation of the device is, however, different than that of complete wetting of the walls by the continuous phase at equilibrium. We found that W/O emulsions form when the advancing contact angle of water on the channel wall exceeds theta approximately 92 degrees. This result is unexpected because at equilibrium even for theta < 92 degrees the microchannels would be completely wet by the organic phase. The criterion for the formation of W/O emulsions (theta > 92 degrees) is thus more stringent than the equilibrium conditions. Conversely, we observed the stable formation of O/W emulsions for theta < 92 degrees, that is, when the nonequilibrium transition to complete wetting by oil takes place. These results underlie the importance of pinning and the kinetic wetting effects in microfluidic emulsification. The results suggest that the use of parallel devices can facilitate fast screening of physicochemical conditions for emulsification. 相似文献
228.
Chatgilialoglu C Guerra M Kaloudis P Houée-Lévin C Marignier JL Swaminathan VN Carell T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(32):8979-8984
The reactions of hydrated electrons (e(aq) (-)) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t(1/2)<35 ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. gamma-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T(-)* is unable to transfer an electron to the thymine dimer 2. 相似文献
229.
The multistep synthesis of an extensive series of push-pull donor-pi-conjugated dipicolinic acid ligands is described. The charge transfer character of the ligand can be tuned by changing the donor group (CH 2R, OR, SR, or NR 2) or the nature of the conjugated backbone (phenyl, phenylethynyl, naphtylethynyl, bis(phenylethynyl), or chalcone). The photophysical properties of related D 3 symmetric europium complexes (absorption and luminescence) were measured. Experiments using two-photon sensitized luminescence of a Eu (III) complex reveal large two-photon absorption (TPA) cross-section values (775 GM at 740 nm) in dichloromethane. Furthermore, some structure-property relationships can be derived from this systematic study, allowing an optimization of TPA properties of lanthanide complexes. 相似文献
230.
D'Aléo A Picot A Beeby A Gareth Williams JA Le Guennic B Andraud C Maury O 《Inorganic chemistry》2008,47(22):10258-10268
A series of push-pull donor-pi-conjugated dipicolinic acid ligands and related tris-dipicolinate europium and lutetium complexes have been prepared. The ligands present broad absorption and emission transitions in the visible spectral range unambiguously assigned to charge-transfer transitions (CT) by means of time-dependent density functional theory calculations. The photophysical properties (absorption, emission, luminescence quantum yield, and lifetime) of the corresponding europium complexes were thoroughly investigated. Solvatochromism and temperature effects clearly confirm that Eu(III) sensitization directly occurs from the ligand CT state. In addition, modulation of the energy of the CT donating state by changing the nature of the donor fragment allows the optimal energy of the antennae for europium sensitization to be determined, and this optimal energy was found to be close to the (5)D 1 accepting state. Finally, this CT sensitization process has been successfully extended to near-infrared emitters (neodymium and ytterbium). 相似文献