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81.
Steve Scheiner Paul Redfern Eric A. Hillenbrand 《International journal of quantum chemistry》1986,29(4):817-827
Results of quantum mechanical calculations are presented that suggest a number of mechanisms whereby protons may be shifted from one group to another along an H bond. The first factor to be considered is a stretching of the bond that drastically raises the energy barrier to transfer. It is possible to predict barriers for an arbitrary system based only on results for a simple system and knowledge of the relevant bond length in the isolated subsystems. Factors that increase the intrinsic basicity of the B group in A-H-B lead not only to a lowering of the energy of the A-HB state relative to AH-B but also to a reduction in the barrier to transfer of the proton from A to B. Ions in the vicinity of the H bond exert a powerful influence and can shift the proton to the less basic group across a gradient of several pK units. Rather than shielding the proton from the external ion, the H bond acts instead to amplify the effects of the electric field. Reorientation of the A and B groups relative to one another, i.e., bends of the H bond, also produce surprisingly large changes in the relative energies of the AH-B and A-HB states. Such bends are capable of pushing the proton across to the normally less basic group, providing a mechanism of coupling conformational changes to proton ‘pumping’ activity. It is found that the high and low pH states of a given H bond can have dramatically differnt relative populations of the AH-B and A-HB configurations. These observations are explained in terms of fundamental concepts involving electrostatic interaction energies. 相似文献
82.
6-Diazopenicillanates react with allylic sulphides, selenides and bromides, to give 6,6-disubstituted penicillanates [2,3]-sigmatropic rearrangements. 相似文献
83.
Massue J Bellec N Chopin S Levillain E Roisnel T Clérac R Lorcy D 《Inorganic chemistry》2005,44(24):8740-8748
The reaction of tris(alkylthio)tetrathiafulvalene thiolates with 3-chloro-2,4-pentanedione affords tetrathiafulvalene (TTF) moieties substituted by the acetylacetone function (TTFSacacH), precursors of novel redox-active ligands: the acetylacetonate ions (TTFSacac). These TTFSacacHs have been characterized by X-ray diffraction analyses, and similar trends have been observed, such as a TTF core almost planar and the acetylacetone substituent located in a plane almost perpendicular to the plane formed by the TTF core. Their chelating ability has been demonstrated by the formation of the corresponding M(TTFSacac)2(pyridine)2 complexes in the presence of M(II)(OAc)2.H2O (M = Ni2+, Zn2+). These complexes with TTFSacac moieties, Ni(TTFSacac)2(pyridine)2, 6b, and Zn(TTFSacac)2(pyridine)2, 7b, have been characterized by X-ray diffraction analyses, showing in all structures the metal(II) center chelated by two TTFacac units in the equatorial plane and the octahedral coordination geometry around the metal completed by two axial pyridine ligands. Cyclic voltammetry and UV-visible-near infrared spectroscopic measurements have evidenced a sizable interaction between the two electroactive ligands and the stabilization of a mixed-valence state in the one-electron oxidized complexes. 相似文献
84.
85.
Shandong Yuan Sharifah Bee Derouane-Abd Hamid Yongxue Li Pinliang Ying Qin Xin Eric G Derouane Can Li 《Journal of molecular catalysis. A, Chemical》2002,180(1-2):245-258
The conversion of n-C4H10 was undertaken on MoO3/HZSM-5 catalyst at 773–973 K and the phases of molybdenum species were detected by XRD. The XRD results show that bulk MoO3 on HZSM-5 can be readily reduced by n-C4H10 to MoO2 at 773 K and MoO2 can be gradually carburized to molybdenum carbide above 813 K. The molybdenum carbide formed from the carburization of MoO2 with n-C4H10 below 893 K is -MoC1−x with fcc-structure, while hcp-molybdenum carbide formed above 933 K. During the evolution of MoO3 to MoO2 (>773 K) or the carburization of MoO2 to molybdenum carbide (>813 K), deep oxidation, cracking and coke deposition are serious, in particular at higher reaction temperatures, these lead to the poor selectivity to aromatics. Aromatization of n-C4H10 can proceed catalytically on both Mo2C/HZSM-5 and MoO2/HZSM-5, the distribution of the products for the two catalysts is similar below 813 K, but the activity for Mo2C/HZSM-5 is much higher than that for MoO2/HZSM-5. 相似文献
86.
87.
Eric R. Scerri 《Foundations of Chemistry》2004,6(3):255-257
Volume Contents
Contents of Volume 6 相似文献88.
Yüksel Tufan Nurcan Karacan J. Eric D. Davies 《Journal of inclusion phenomena and macrocyclic chemistry》1999,33(2):149-154
Four new clathrates of the formula M(Im)2Ni(CN)4·2·Dioxane (where M = Co, Ni, Cu, Cd; Im = Imidazole) have been prepared in powder form and their FT-IR and laser-Raman spectra are reported for the first time. These clathrates are analogues to the previously reported classical Hofmann type clathrates except for the copper clathrate. The Cu clathrate has different spectral features in comparison with its analogues due to the Jahn-Teller effect. 相似文献
89.
Weeber JC Finot E Legay G Cathelat A Lacroute Y Dereux A 《Langmuir : the ACS journal of surfaces and colloids》2004,20(23):10179-10185
We report on the near-field imaging of microstructured polymer layers deposited on an homogeneous metal thin film on which a surface plasmon mode is excited. The microstructures in the polymer layers are designed by electron beam lithography, and the near-field imaging is performed with a photon scanning tunneling microscope (PSTM). We show that, despite their very small height, the microstructures can be conveniently imaged with a PSTM thanks to the field enhancement at the surface of the metal thin film supporting the surface plasmon. The influence of the illumination conditions on the contrast of the PSTM images is discussed. In particular, we show that both the field enhancement and the near-field intensity distribution around the microstructures depend dramatically upon the illumination conditions, leading to the conclusion that the PSTM is well suited for spatially resolved near-field surface plasmon sensing purposes. 相似文献
90.
In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band offset in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band offset at the heterojunction, the exciton will remain the lowest-energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the offset is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases. 相似文献