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Enthalpies of protonation in aqueous solution have been determined from calorimetric measurements between 0 and 100°C for methylpentylamine, butyldimethylamine, and quinuclidine; between 0 and 125°C for di-iso-propylamine; and between 0 and 75°C for 1-azacycloheptane. Values ofpKa at 25°C have been determined from pH measurements using a glass electrode for di-iso-propyl-, methylpentyl-, and butyldimethylamine. The calorimetric results are used together with values ofpKa at 25°C to derive expressions which allow the calculation of accuratepKa values over a large temperature range. The heat capacity and entropy changes for the protonation of aqueous alkylamines found in the present and in previous studies are discussed in terms of differences in hydrophobic solvation of the alkyl groups in the neutral amines and the ions. 相似文献
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The crystal structure determination of the title compound was undertaken to determine the conformational flexibility of the steroid skeleton. 3-Methoxy-estra-1,3,5(10)-trien-17β-ol (C19H26O2) crystallizes in the space group P212121 with Z = 12, a = 12.589(2), b = 16.274(5), and c = 23.535(6) Å. The structure was solved by direct methods and refined to R = 0.056. Three symmetry independent molecules having different molecular conformations were observed in the crystal. The most flexible region of the molecules is the B ring which adopts following conformations: 7α,8β-half-chair, 8β-sofa, and an intermediate form between half-chair and sofa. 相似文献
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Gerd Weckwerth 《Journal of Radioanalytical and Nuclear Chemistry》2014,299(1):221-228
C1-carbonaceous chondrites are known to have the highest meteoritic content of volatile elements. Nevertheless, most volatile elements, such as H, He, N and O, are excluded from the accordance with the solar photosphere. In meteoritic material, which condensed closer to the sun, even moderately volatile elements could not completely condense and their C1-normalized concentration ratios with refractory elements are typically depleted. In primitive terrestrial samples these ratios of (lithophile) moderately volatile to refractory elements are even lower. This suggests an accretion of our planet from material, which condensed closer to the sun than all available meteorites surviving in significantly farer distances in the asteroid belt. Evidence to that may come from agreements between most Earth-like meteorites with the depletion pattern of moderately volatile elements in the Earth’s mantle or rather related bulk data. Therefore, two samples from Hammadah al Hamra 073 and Sahara 0182, supposed to be further members of the most Earth-like Coolidge-Loongana grouplet, have been analysed by INAA. Together with two earlier analysed samples from the grouplet the results have been used in fingerprint studies with terrestrial data. In spite of internal spreading due to sampling of inhomogeneous material, the results are in accordance with a volatility-dependent model of depletion. Abundance ratios of Fe relative to Mn, Cr, and Si measured from the grouplet-abundances are depleted relative to calculated data from the total Earth, what can be explained with a 30 %-mantle-stripping from an impact of a Mars-sized body. 相似文献
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Syvitski RT Li Y Auclair K Ortiz De Montellano PR La Mar GN 《Journal of the American Chemical Society》2002,124(48):14296-14297
Solution 1H NMR is used to probe the environments of the donor protons of eight strong hydrogen bonds on the distal side of the heme substrate in the cyanide-inhibited, substrate-bound complex of human heme oxygenase, hHO. It is demonstrated that significant magnetization transfer from the bulk water signal to the eight labile protons does not result from chemical exchange, but from direct nuclear Overhauser effect due to the dipolar interaction of these labile protons with "ordered" water molecules. The enzyme labile proton to water proton distances are estimated at approximately 3 A. It is proposed that the role of the strong hydrogen-bonding network is to immobilize numerous water molecules which both stabilize the activated hydroperoxy species and funnel protons to the active site. 相似文献