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61.
Taking pyridine as a prototypal example of biologically important nitrogen bases involved in proton-coupled electron transfers, it is shown with the example of the photochemically triggered oxidation of phenol by Ru(III)(bpy)(3) that this proton acceptor partakes in a concerted pathway whose kinetic characteristics can be extracted from the overall kinetic response. The treatment of these data, implemented by the results of a parallel study carried out in heavy water, allowed the determination of the intrinsic kinetic characteristics of this proton acceptor. Comparison of the reorganization energies and of the pre-exponential factors previously derived for hydrogen phosphate and water (in water) as proton acceptors suggests that, in the case of pyridine, the proton charge is delocalized over a primary shell of water molecules firmly bound to the pyridinium cation. 相似文献
62.
Chen P El Ojaimi M Gros CP Richard P Barbe JM Guilard R Shen J Kadish KM 《Inorganic chemistry》2011,50(8):3479-3489
A series of homobimetallic manganese cofacial porphyrin-corrole dyads were synthesized and investigated as to their electrochemistry, spectroelectrochemistry, and ligand binding properties in nonaqueous media. Four dyads were investigated, each of which contained a Mn(III) corrole linked in a face-to-face arrangement with a Mn(III) porphyrin. The main difference between compounds in the series is the type of spacer, 9,9-dimethylxanthene, anthracene, dibenzofuran, or diphenylether, which determines the distance and interaction between the metallomacrocycles. Each redox process of the porphyrin-corrole dyads was assigned on the basis of spectroscopic and electrochemical data and by comparison with reactions and properties of the monocorrole and the monoporphyrin which were examined under the same solution conditions. The Mn(III) porphyrin part of the dyad undergoes two major one-electron reductions in pyridine and benzonitrile, the first of which involves a Mn(III)/Mn(II) process and the second the addition of an electron to the conjugated π-ring system of the macrocycle. The Mn(III) corrole part of the dyads also exhibits two major redox processes, one involving Mn(III)/Mn(II) and the other Mn(III) to Mn(IV) under the same solution conditions. The potentials and reversibility of each electron transfer reaction were shown to depend upon the solvent, type of spacer separating the two macrocycles, and the presence or absence of axial ligation, the latter of which was investigated in detail for the case of acetate ion which was found to bind within the cavity of the dyad to both manganese centers, both before and after the stepwise electroreduction to the Mn(II) forms of the two macrocycles. An intramolecular chloride ion exchange between the porphyrin part of the dyads which contain Mn(III)Cl and the singly oxidized corrole in the dyad is observed after the Mn(III)/Mn(IV) reaction of the corrole, suggesting that chloride is coordinated inside the cavity in the neutral compound. 相似文献
63.
Costentin C Robert M Savéant JM Tard C 《Physical chemistry chemical physics : PCCP》2011,13(12):5353-5358
Four molecules comprising a phenol moiety and a distal pyridine base connected by an intermediary H-bonding and an H-bonded alcohol group have been synthesized and their electrochemistry has been investigated by means of cyclic voltammetry. The molecules differ by the substituent at the alcohol functional carbon and by methyl groups on the pyridine. The reaction follows a concerted proton-electron transfer pathway as confirmed by the observation of a significant H/D kinetic isotope effect in all four cases. The standard rate constants characterizing each of the four compounds are analyzed in terms of reorganization energy and pre-exponential factor. Intramolecular and solvent reorganization energies appear as practically constant in the series, in which a previously investigated aminophenol is included, whereas significantly different pre-exponential factors are observed. That the latter, which is a measure of the efficiency of proton tunneling concerted with electron transfer, be substantially smaller with the H-bond relay molecules than with the aminophenol is related to the fact that two protons are moved in the first case instead of one in the second. Within the H-bond relay molecules, the pre-exponential factor varies with the substituent present at the alcohol functional carbon in the order CF(3) > H > CH(3), presumably as the result of a fine tuning of the balance between the H-bond accepting and H-bond donating properties of the central OH group. The kinetic H/D kinetic isotope effect increases accordingly in the same order. 相似文献
64.
The structure and absolute configuration of natural 8-epi-9-deoxygoniopypyrone have been confirmed by an efficient and highly diastereoselective synthesis in 15 steps from (S)-mandelic acid with an overall yield of 43%. 相似文献
65.
Abstract We study a perturbed anisotropic equation without using the knowledge of the limiting problem. This provides a different method
from that introduced by Brézis and Nirenberg [4]. Our arguments use some tools recently developed in [5, 6].
Keywords: Anisotropic critical exponent, Critical level, Compactness, Nehari manifold
Mathematics Subject Classification (2000): 35J25, 35J60, 35J65 相似文献
66.
Claudia Lederman Jean-Michel Roquejoffre Noemi Wolanski 《Comptes Rendus Mathematique》2002,334(7):569-574
This Note is devoted to the justification of an asymptotic model for quasisteady three-dimensional spherical flames proposed by G. Joulin [7]. The paper [7] derives, by means of a three-scale matched asymptotic expansion, starting from the classical thermo-diffusive model with high activation energies, an integro-differential equation for the flame radius. In the derivation, it is essential for the Lewis number – i.e., the ratio between thermal and molecular diffusion – to be strictly less than unity. In this Note, we give the main ideas of a rigorous proof of the validity of this model, under the additional restriction that the Lewis number is close to 1. To cite this article: C. Lederman et al., C. R. Acad. Sci. Paris, Ser. I 334 (2002) 569–574. 相似文献
67.
Jean-Michel Bismut 《Comptes Rendus Mathematique》2002,335(3):243-247
In this Note, we announce the vanishing of the holomorphic torsion forms of the relative de Rham complex of an equivariant fibration. To cite this article: J.-M. Bismut, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 243–247. 相似文献
68.
Hans Fritz Jean-Michel Henlin Thophile Tschamber Jacques Streith Andreas Riesen Margareta Zehnder 《Helvetica chimica acta》1988,71(4):822-834
Azeitidinodiazepines 4b and 4c react with acylnitroso dienophiles 5a–c , specifically from their convexe α-side, but in a non-regiospecific way, leading thereby stereospecifically to the expected adducts 6a–d and 7a–d . The three-dimensional structures of 6a and 7a were determined by X-ray analyses which corroborated their NMR data. OsO4 cis-glycolization occurred in good yield with the inverse adducts 7a and 7e and led to the rearranged products 10 . These latter ones result from an intramolecular N- to O-transbenzoylation of the short-lived intermediates 9 followed by fragmentation of the animal function. X-Ray analysis of 7a showed the N(10) atom to be pyramidal, a result which demonstrates that it does not have any pronounced benzamide character; otherwise no such N- to O-transbenzoylation would have taken place. Structure and relative configuration of 10a were ascertained by X-ray analysis which confirmed its NMR data as well as the stereochemical outcome of its formation. 相似文献
69.
Dominique Faure Doris Lexa Jean-Michel Savéant 《Journal of Electroanalytical Chemistry》1982,140(2):269-284
The cyclic voltammetric study of vitamin B12r in DMSO shows the importance of the base-on/base-off reaction in the electrochemical reduction mechanism. Depending upon the flux of electrons flowing through the system. part of the base-on complex is reduced through prior opening of the nucleotide side-chain which gives rise to the more easily reduced DMSO-Co(II) complex. The quantitative analysis of the variations of the peak heights with the sweep rate allows the thermodynamic and kinetic characterization of the base-on/base-off reactions to be determined. DMSO thus appears as a stronger ligand toward Co(II) than water, leading to an increased participation of the base-off complex in the reduction process. The greater stability of the DMSO complex is also related to the observation that electron transfer is significantly slower than in the case of the water complex. The importance of the ligand exchange reactions in the reduction of B12r is confirmed by the effect of pyridine additions. Three complexes then participate in the reduction process, their reduction potentials lying in the order DMSO >Py >Bzm. The reduction mechanism involving the interconversion of the three complexes is described as a function of the electrode potential, the flux of electrons and the pyridine concentration. An estimation of the equilibrium and rate constants of the three ligand exchange reactions is made, based on the variations of the cyclic voltammograms with the sweep rate and the pyridine concentration. 相似文献
70.
The 195Pt and 13C chemical shifts (δPt and δc) are reported for platinum(II), platinum(IV) and class II mixed-valence complexes, with general formula [PtL4]X2, cis- and trans-PtL2X2, PtL2X4 and Pt2L4X6 (where L may be thiourea, 2-imidazolidine-thione, tetrahydro 2-pyrimidinethione, thiocaprolactam, pyridine-2-thione and tetramethylthiourea, and X may be Cl or Br). The 195Pt chemical shifts can be understood in view of 13C data in terms of variations of electronegativities and σ-donor abilities of ligands attached to platinum. 相似文献