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91.
Selective bond formations are one of the most important reactions in organic synthesis. In the Lewis acid mediated electrophile reactions of carbonyls, the selective formation of a carbonyl–acid complex plays a critical role in determining selectivity, which is based on the difference in the coordinative interaction between the carbonyl and Lewis acid center. Although this strategy has attained progress in selective bond formations, the discrimination between similarly sized aromatic and aliphatic carbonyls that have no functional anchors to strongly interact with the metal center still remains a challenging issue. Herein, this work focuses on molecular recognition driven by dispersion interactions within some aromatic moieties. A Lewis acid catalyst with a π-space cavity, which is referred to as a π-pocket, as the recognition site for aromatic carbonyls is designed. Cage-shaped borates 1 B with various π-pockets demonstrated significant chemoselectivity for aromatic aldehydes 3 b – f over that of aliphatic 3 a in competitive hetero-Diels–Alder reactions. The effectiveness of our catalysts was also evidenced by intramolecular recognition of the aromatic carbonyl within a dicarbonyl substrate. Mechanistic and theoretical studies demonstrated that the selective activation of aromatic substrates was driven by the preorganization step with a larger dispersion interaction, rather than the rate-determining step of the C−C bond formation, and this was likely to contribute to the preferred activation of aromatic substrates over that of aliphatic ones.  相似文献   
92.
93.
We introduce various spaces of functions of bounded mean oscillation (BMO) defined in a domain by taking into account the behavior of functions near the boundary. Then we establish several equivalences of these spaces. Moreover, we compare our space with a BMO space introduced by Miyachi. As an application we prove that the heat and the Stokes semigroup are analytic in such a type of spaces.  相似文献   
94.
95.
Reported here is the N2 cleavage of a one‐electron oxidation reaction using trans‐[Mo(depe)2(N2)2] ( 1 ) (depe=Et2PCH2CH2PEt2), which is a classical molybdenum(0)‐dinitrogen complex supported by two bidentate phosphine ligands. The molybdenum(IV) terminal nitride complex [Mo(depe)2N][BArf4] ( 2 ) (BArf4=B(3,5‐(CF3)2C6H3)4) is synthesized by the one‐electron oxidation of 1 upon addition of a mild oxidant, [Cp2Fe][BArf4] (Cp=C5H5), and proceeds by N2 cleavage from a MoII‐N=N‐MoII structure. In addition, the electrochemical oxidation reaction for 1 also cleaved the N2 ligand to give 2 . The dimeric Mo complex with a bridging N2 is detected by in situ resonance Raman and in situ UV‐vis spectroscopies during the electrochemical oxidation reaction for 1 . Density‐functional theory (DFT) calculations reveal that the unstable monomeric oxidized MoI species is converted into 2 via the dimeric structure involving a zigzag transition state.  相似文献   
96.
We have studied the electronic excitations of fluorinated copper phthalocyanine, CuPcF16, thin films using electron energy-loss spectroscopy in transmission. This allowed for the derivation of the dielectric function in a wide energy range. Furthermore, we have analyzed the lowest excitation feature using a Lorentz model, which enabled the determination of the dielectric background in the energy range of the gap excitation, and an analysis of the momentum dependence of the spectral intensities at low energies.  相似文献   
97.
Inelastic X‐ray scattering (IXS) measurements have been performed on an MgO single crystal in order to evaluate IXS as a methodology for accurate and precise determination of elastic constants and sound velocities. By performing the IXS experiment using a 12‐analyzer array, the complete set of single‐crystal elastic constants of MgO were determined to a precision better than 0.8% (sound velocities to better than 0.2%). The results are consistent with values in the literature. The precision and accuracy of this work, which is significantly better than other published work to date, demonstrates the potential of IXS in determining elastic properties.  相似文献   
98.
A novel tensile-testing machine has been manufactured by which the true stress-strain relation of a transparent plastic film at different rates of strain can be obtained by an optical means. The stress can be measured by the deformation of a spring setup, parallel with the specimen. Both the strain of the specimen and the deformation of the spring are, at the same time, photographed by a cylindrical lens placed on a rotating drum which is synchronized with the rate of tension. Therefore, various thin plastic films can be actually applied to this tensile-testing machine by which the states of rheological flow and phenomena of rupture can be determined accurately.  相似文献   
99.
100.
We investigate the resonant two-magnon Raman scattering in two-dimensional (2D) Mott insulators by using a half-filled 2D Hubbard model in the strong coupling limit. By performing numerical diagonalization calculations for small clusters, we find that the Raman intensity is enhanced when the incoming photon energy is not near the optical absorption edge but well above it, being consistent with experimental data. The absence of resonance near the gap edge is associated with the presence of background spins, while photoexcited states for resonance are found to be characterized by the charge degree of freedom. The resonance mechanism is different from those proposed previously.  相似文献   
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