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81.
82.
83.
Research supported in part by FAPESP, when the author was visiting University College Dublin, Dublin, Ireland. 相似文献
84.
The gas-phase reaction CH3SH + I2 has been studied spectrophotometrically over the temperature range of 476–604 K. It was found that the reaction undergoes H abstraction by I at ≤575 K, leading to the formation of MeSI and followed by a secondary reaction which leads to the formation of MeSSMe: Taking into consideration the effect of reaction (2), the equilibrium constant K1 (554 K) has been evaluated to be 0.025 ± 0.004. This value was combined with the estimated values S (CH3SI, g) = 73.7 ± 1.0 eu and 〈ΔC〉 = 0.87 ± 0.3 eu to obtain ΔH = 4.03 ± 0.73 kcal/mol. This yields ΔH (CH3SI, g) = 7.16 ± 0.73 kcal/mol when combined with known thermochemical values for CH3SH, HI, and I2. A kinetic study was vitiated by the concurrent heterogeneous reaction of MeSH and I2 at lower temperatures and the rather complicated chemistry occurring at elevated temperatures. However, attempts at measuring rate constants at 554 K lead to a lower limit of ΔH (CH3S·, g) ≥ 29.5 ± 2 kcal/mol when an estimated value of A = 1010.8 ± 0.2 L/mol·s for the reactionc is used. DH (CH3S–I) is estimated to be 49.3 ± 1.7 kcal/mol. The bond strengths of some divalent sulfurs and the reaction mechanisms are discussed. A crude estimate of DH0(H–CH2SH) = 96 ± 1 kcal has been obtained from the kinetic data. 相似文献
85.
Abstract Linear aldehydes trigger red emission from chloroplasts. If horseradish peroxidase is present, the aldehyde is oxidized to the next lower homologue in the triplet state, which in turn sensitizes chlorophyll fluorescence. Only certain chlorophylls are activated. 相似文献
86.
Alkene elimination, alkyl loss, styryl and alkyl migrations to oxygen are the major fragmentations of (Z)- and (E)-alkane-sulfinyl-2-phenylethenes (cis- and trans-styryl alkyl sulfoxides) under electron impact. Alkene elimination has been found to go through a McLafferty-type hydrogen rearrangement to yield the benzylic carbon and the sulfinyl oxygen, respectively. This is the most facile and interesting process for compounds with longer alkyl chains (R?C3H7), and is a unique feature for styryl sulfoxides. Product ion analyses show the migrating aptitude to carbon and to oxygen to be qualitatively similar. 相似文献
87.
Peter G. Jones Lilian Gray 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(5):o282-o283
The crystal packing of the title compound, C8H11BrN+·Br?, involves three types of secondary interaction: a classical N—H?Br? hydrogen bond, a `weak' but short C—H?Br? interaction (normalized H?Br distance of 2.66 Å) and a cation–anion Br?Br contact of 3.6331 (4) Å. The hydrogen bonds connect two cations and two anions to form rings of graph set R(14). The Br?Br contacts link these rings to form layers parallel to the bc plane. 相似文献
88.
Lilian Kao Liu 《中国化学会会志》1976,23(3):165-171
The mass spectra of saturated sulfonate esters1,2) have been reported recently. No study has been made on the vinylic methanesulfonates. In order to understand their breakdown patterns, several chloro- and fluoro-containing vinyl methanesulfonates have been made3) and their mass spectra studied. The chlorine compounds are chosen because the natural abundance of isotope 35Cl and 37Cl can serve as a internal tag. The vinyl methanesulfonates containing electronegative fluorine are used as a comparison to the chloro-compounds because of their similar electronegativities. 相似文献
89.
Cherrier MV Martin L Cavazza C Jacquamet L Lemaire D Gaillard J Fontecilla-Camps JC 《Journal of the American Chemical Society》2005,127(28):10075-10082
Because nickel is both essential and toxic to a great variety of organisms, its detection and transport is highly regulated. In Escherichia coli and other related Gram-negative bacteria, high affinity nickel transport depends on proteins expressed by the nik operon. A central actor of this process is the periplasmic NikA transport protein. A previous structural report has proposed that nickel binds to NikA as a pentahydrate species. However, both stereochemical considerations and X-ray absorption spectroscopic results are incompatible with that interpretation. Here, we report the 1.8 A resolution structure of NikA and show that it binds FeEDTA(H2O)- with very high affinity. In addition, we provide crystallographic evidence that a metal-EDTA complex was also bound to the previously reported NikA structure. Our observations strongly suggest that nickel transport in E. coli requires the binding of this metal ion to a metallophore that bears significant resemblance to EDTA. They also provide a basis for the potential use of NikA in the bioremediation of toxic transition metals and the design of artificial metalloenzymes. 相似文献
90.
Dubois L Caspar R Jacquamet L Petit PE Charlot MF Baffert C Collomb MN Deronzier A Latour JM 《Inorganic chemistry》2003,42(16):4817-4827
The dimanganese(II,II) complexes 1a [Mn(2)(L)(OAc)(2)(CH(3)OH)](ClO(4)) and 1b [Mn(2)(L)(OBz)(2)(H(2)O)](ClO(4)), where HL is the unsymmetrical phenol ligand 2-(bis-(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol, react with hydrogen peroxide in acetonitrile solution. The disproportionation reaction was monitored by electrospray ionization mass spectrometry (ESI-MS) and EPR and UV-visible spectroscopies. Extensive EPR studies have shown that a species (2) exhibiting a 16-line spectrum at g approximately 2 persists during catalysis. ESI-MS experiments conducted similarly during catalysis associate 2a with a peak at 729 (791 for 2b) corresponding to the formula [Mn(III)Mn(IV)(L)(O)(2)(OAc)](+) ([Mn(III)Mn(IV)(L)(O)(2)(OBz)](+) for 2b). At the end of the reaction, it is partly replaced by a species (3) possessing a broad unfeatured signal at g approximately 2. ESI-MS associates 3a with a peak at 713 (775 for 3b) corresponding to the formula [Mn(II)Mn(III)(L)(O)(OAc)](+) ([Mn(II)Mn(III)(L)(O)(OBz)](+) for 3b). In the presence of H(2)(18)O, these two peaks move to 733 and to 715 indicating the presence of two and one oxo ligands, respectively. When H(2)(18)O(2) is used, 2a and 3a are labeled showing that the oxo ligands come from H(2)O(2). Interestingly, when an equimolar mixture of H(2)O(2) and H(2)(18)O(2) is used, only unlabeled and doubly labeled 2a/b are formed, showing that its two oxo ligands come from the same H(2)O(2) molecule. All these experiments lead to attribute the formula [Mn(III)Mn(IV)(L)(O)(2)(OAc)](+) to 2a and to 3a the formula [Mn(II)Mn(III)(L)(O)(OAc)](+). Freeze-quench/EPR experiments revealed that 2a appears at 500 ms and that another species with a 6-line spectrum is formed transiently at ca. 100 ms. 2a was prepared by reaction of 1a with tert-butyl hydroperoxide as shown by EPR and UV-visible spectroscopies and ESI-MS experiments. Its structure was studied by X-ray absorption experiments which revealed the presence of two or three O atoms at 1.87 A and three or two N/O atoms at 2.14 A. In addition one N atom was found at a longer distance (2.3 A) and one Mn at 2.63 A. 2a can be one-electron oxidized at E(1/2) = 0.91 V(NHE) (DeltaE(1/2) = 0.08 V) leading to its Mn(IV)Mn(IV) analogue. The formation of 2a from 1a was monitored by UV-visible and X-ray absorption spectroscopies. Both concur to show that an intermediate Mn(II)Mn(III) species, resembling 4a [Mn(2)(L)(OAc)(2)(H(2)O)](ClO(4))(2), the one-electron-oxidized form of 1a, is formed initially and transforms into 2a. The structures of the active intermediates 2 and 3 are discussed in light of their spectroscopic properties, and potential mechanisms are considered and discussed in the context of the biological reaction. 相似文献