In this work, the molecular structures of the Diels–Alder adducts: ethyl (6aR/S,10aR/S)-6a,7,10,10a-tetrahydro-8,9-dimethyl-6-oxodibenzo[b,d]pyran-6a-carboxylate C18H20O41 and (6aR/S,10aR/S)-6a-acetyl-6a,7,10,10a-tetrahydro-8,9-dimethyl-6-oxodibenzo[b,d]pyran C17H18O32; and the epoxide of this last (6aR/S,7aR/S,8aS/R,9aR/S)-6a-acetyl-6a,7,7a,8a,9,9a-hexahydro-7a,8a-dimethyl-6-oxo-6H-5,8-dioxacyclopropa[b]phenanthrene C17H18O43, are comparatively analyzed. Compound 1 is triclinic, space group P-1 with a = 7.7529(6) ?, b = 9.8625(8) ?, c = 11.3103(9) ?, α = 109.387(9)°, β = 95.484(1)°, γ = 99.753(1)°, Z = 2. Compound 2 is monoclinic, space group Cc, a = 7.7285(7) ?, b = 16.6813(15) ?, c = 11.3213(10) ?, β = 92.470(2)°, Z = 4. Compound 3 is monoclinic, space group P21/ca = 11.2036(10), b = 15.8326(14), c = 8.3182(7), β = 90.600(2)°, Z = 4. The molecular structures of compounds 1–3 show conformational differences between lactone and 6a-acyl carbonyls. CO···CO dipolar interactions stabilize the less favoured
syn conformation in 2–3, conclusions are supported on theoretical calculations. The molecular structure of compound 3 demonstrates that epoxidation of 2 leads to the stereo-selective addition of the oxygen atom. 相似文献
The densities at high pressures of two dimethoxy end-capped poly(propylene glycols), CH3–O–[CH2–CH(CH3)–O]m–CH3, with average molar masses higher than 1300 g · mol?1, were measured in the range (0.1 to 60) MPa at five different temperatures from (298.15 to 398.15) K. The measurements were performed in a high-pressure vibrating tube densimeter. A correction factor, due to the viscosity of the sample, was applied to the experimental density values. The pressure–volume–temperature behavior of these lubricants was evaluated accurately over wide temperature and pressure ranges and correlated successfully with the empirical Tammann–Tait equation. The experimental data and the correlations were used to study the behavior and the influence of temperature and pressure on the isothermal compressibility, the isobaric thermal expansivity, and the internal pressure, as well as the effect of the polyether molecular structure on these properties. 相似文献
First-principles calculations within density functional theory and many-body perturbation theory have been carried out in order to investigate the structural, electronic and optical properties of undoped and doped silicon nanostructures. We consider Si nanoclusters co-doped with B and P. We find that the electronic band gap is reduced with respect to that of the undoped crystals, suggesting the possibility of impurity based engineering of electronic and optical properties of Si nanocrystals. Finally, motivated by recent suggestions concerning the chance of exploiting Ge dots for photovoltaic nanodevices, we present calculations of the electronic and optical properties of a Ge35H36 nanocrystal, and compare the results with those for the corresponding Si35H36 nanocrystals and the co-doped Si33BPH36. 相似文献
We use a predator–prey representation of an urban system to analyse how policy interventions can prevent the adverse effects of air pollution on people’s health. The number of residents is treated as prey variable, and particulate matter that consists of particles with a diameter of up to 10 micrometres (PM10) as predator variable. This representation allows integration of population trends and the effects of environmental interventions on the average level of PM10 concentration (which establishes a baseline for the potential health burden for residents). For the case of Graz, Austria, we illustrate the insights generated regarding the interdependency of market-based and technological pollution controls, and propose an indicator that assesses the cost of delayed interventions by counting additional premature deaths caused by polluted environments.
The Ramanujan Journal - Let $${bar{p}}(n)$$ denote the number of overpartitions of n. Recently, a number of congruences modulo powers of 2, 3 and 5 have been discovered. The moduli for these... 相似文献
Glass wool was placed within an NMR tube as a solid support for the covalent attachment of a molecule to allow for a simple one-dimensional 1H FT NMR solution analysis. This novel procedure avoids the use of expensive sample tubes or platforms, as required for magic angle or fast spinning, exotic pulse sequences, isotopic labeling or the use of a large number of scans to provide the ability to analyze the structure, mobility, ligand binding, and solvent interactions of the surface bound molecule. 相似文献
Protein venoms are effective cytotoxic molecules that when conveniently targeted to tumoral markers can be exploited as promising anticancer drugs. Here, it is explored whether the structurally unrelated melittin, gomesin, and CLIP71 could be functionally active when engineered, in form of GFP fusions, as self-assembling multimeric nanoparticles. Incorporated in modular constructs including a C-terminal polyhistidine tag and an N-terminal peptidic ligand of the cytokine receptor CXCR4 (overexpressed in more than 20 human neoplasias), these venoms are well produced in recombinant bacteria as proteolytically stable regular nanoparticles ranging between 12 and 35 nm. Being highly fluorescent, these materials selectively penetrate, label, and kill CXCR4+ tumor cells in a CXCR4-dependent fashion. The obtained data support the concept of recombinant venoms as promising drugs, through the precise formulation as tumor-targeted nanomaterials for selective theragnostic applications in CXCR4+ cancers. 相似文献
What happens when a C−H bond is forced to interact with unpaired pairs of electrons at a positively charged metal? Such interactions can be considered as “contra-electrostatic” H-bonds, which combine the familiar orbital interaction pattern characteristic for the covalent contribution to the conventional H-bonding with an unusual contra-electrostatic component. While electrostatics is strongly stabilizing component in the conventional C−H⋅⋅⋅X bonds where X is an electronegative main group element, it is destabilizing in the C−H⋅⋅⋅M contacts when M is Au(I), Ag(I), or Cu(I) of NHC−M−Cl systems. Such remarkable C−H⋅⋅⋅M interaction became experimentally accessible within (α-ICyDMe)MCl, NHC-Metal complexes embedded into cyclodextrins. Computational analysis of the model systems suggests that the overall interaction energies are relatively insensitive to moderate variations in the directionality of interaction between a C−H bond and the metal center, indicating stereoelectronic promiscuity of fully filled set of d-orbitals. A combination of experimental and computational data demonstrates that metal encapsulation inside the cyclodextrin cavity forces the C−H bond to point toward the metal, and reveals a still attractive “contra-electrostatic” H-bonding interaction. 相似文献
Studies of the kinetic behavior of horseradish peroxidase (HRP) at pH 8 and at room temperature indicate that the reaction
of phenol with H2O2 catalyzed by HRP exhibits normal Michaelis-Menten saturation kinetics. An irreversible reaction mechanism for the steady-state
kinetics of HRP, which is consistent with the experimental data, is considered. The second-order rate constants for the reactions
of HRP with H2O2 and compound II with phenol are 4.14 × 105 M-1s-1 and 5.54 × 104M-1s-1, respectively. 相似文献