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Encounter Complex (encounter + complex)
Selected AbstractsCleavage of CO by Mo[N(R)Ar]3 ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 23 2007Gemma Christian Abstract The reaction of MoL3 [L = NH2 and N(tBu)Ar] with CO was explored using DFT in order to rationalize why CO cleavage is not observed experimentally for this system in contrast to the corresponding N2 reaction which results in spontaneous cleavage of the N,N bond. The binding of CO to MoL3 was found to be both kinetically and thermodynamically favored over the binding of N2, with the formation of the encounter complex, L3Mo,CO, calculated to be without barrier and exothermic. While the overall reaction to form the C,MoL3 and O,MoL3 products was calculated to be energetically favorable, both the encounter complex and intermediate dimer, L3Mo,CO,MoL3, were found to be lower in energy than the products, with the final C,O cleavage step calculated to be endothermic by 169 kJ,mol,1 and 163 kJ,mol,1 for L = NH2 and N(tBu)Ar, respectively. The unfavorable CO cleavage step can be attributed to the fact that Mo does not possess the optimum d-electron configuration to sufficiently stabilise the carbide and oxide products relative to the CO-bridged intermediate dimer.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Specific Processes and Scrambling in the Dehydrogenation of Ethane and the Degenerate Hydrogen Exchange in the Gas-Phase Ion Chemistry of the Ni(C,H3,O)+/C2H6 CoupleHELVETICA CHIMICA ACTA, Issue 5 2007Maria Schlangen Abstract A mechanistically unprecedented situation characterizes the gas-phase ion chemistry of Ni(C,H3,O)+ when reacted under thermal, single-collision conditions with ethane. A dehydrogenation channel leading to Ni(C3,H7,O)+ is to 90% preceded by a complete loss of positional identity of all nine H-atoms of the encounter complex (,scrambling'), whereas ca. 10% of the reaction exhibit a selective CH bond activation of the alkane. In addition, a degenerate H exchange between ethane and the (C,H3,O) unit occurs as a side reaction, the mechanistic details of which remain unknown for the time being. [source] Free-radical 4-nitrophenylation of thieno[2,3- b]pyridine.JOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 6 2001Part 3: Consideration of mechanistic, selectivity factors involved in the substitution process A 1:1 geometrically oriented encounter complex between thieno[2,3-b]pyridine (1) and 4-nitrophenyldia-zoacetate (2) is proposed to account for the dominant formation (ca. 64%) of the 2-isomer in the mixture of 4-nitrophenyl-l isomers obtained previously. A mechanism involving one-electron transfer from 1 to 2 plus fragmentation of 2· into 4-nitrophenyl free radical, N2, and acetate ion is invoked. Formation of other isomers is discussed. It is noted that there is a close correlation between orientational rules plus mechanisms of reaction for numerous free-radical substitutions (SR) with SN reactions of alkyllithiums on furan, thiophene, N -alkylpyrroles, pyridine, and their condensed aromatic molecules, including 1, as substrates. Also isomeric selectivities for SE, SN, and SR substitutions into 1 were shown to be qualitatively consistent with one another. While SE reactions occur largely at position 3 and then at 2, SN and SR reactions occur either at 2 or 6. Selectivity for positions 4 or 5 is small or zero. [source] Feasibility of the spontaneous gas-phase proton transfer equilibria between neutral Brønsted acids and Brønsted basesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7-8 2008Peeter Burk Abstract The computational investigation of interactions of different acid,base pairs regarding the nature and extent of spontaneous proton transfer was carried out at B3LYP/6-311,+,G** level. The selected acid,base pairs include the interactions of strong base (K2O) with acids of different strength (HClO4, HCl, and HF), and strong acid (HClO4) with bases ranging from K2O (GB,=,322.8,kcal/mol) to H2O (GB,=,157.6,kcal/mol). It was shown that spontaneus, unassisted proton transfer can take place in the gas-phase reactions of strong neutral Brønsted acids and bases. The reaction might be barrierless as in case of interactions between strong acids and bases, for example perchloric acid and alkali metal oxides or potassium oxide and halogen hydrides, or involve the encounter complex (hydrogen bonded acid,base cluster), which is separated from ion-pair by the transition state. Copyright © 2008 John Wiley & Sons, Ltd. [source] Sensitized Photooxidation of Thyroidal Hormones.PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2005Evidence for Heavy Atom Effect on Singlet Molecular Oxygen [O2(, g)]-mediated Photoreactions ABSTRACT Thyronine derivatives are essential indicators of thyroid gland diseases in clinical diagnosis and are currently used as standards for developing ordinary biochemical assays. Photooxidation of gland hormones of the thyronine (TN) family and structurally related compounds (TN, 3,5-diiodo-thyronine,3,3,,5-triiodothyronine and 3,3,,5,5,-tetraiodothyronine or thyroxine) was studied using rose bengal, eosin and perinaphthenone (PN) as dye sensitizers. Tyrosine (Tyr) and two iodinated derivatives (3-iodotyrosine and 3,5-diiodotyrosine) were also included in the study for comparative purposes. Irradiation of aqueous solutions of substrates containing xanthene dyes with visible light triggers a complex series of competitive interactions, which include the triplet excited state of the dye (3Xdye*) and singlet molecular oxygen [O2(1,g)]-mediated and superoxide ion-mediated reactions. Rate constants for interaction with the 3Xdye*, attributed to an electron transfer process, are in the order of 108 -109M,1 s,1 depending on the dye and the particular substrate. The photosensitization using PN follows a pure Type-II (O2(1,g) mediated) mechanism. The presence of the phenolic group in Tyr, TN and iodinated derivatives dominates the kinetics of photooxidation of these compounds. The reactive rate constants, kr, and the quotient between reactive and overall rate constants (krlkt values, in the range of 0.7,0.06) behave in an opposite fashion compared with the overall rate constants and oxidation potentials. This apparent inconsistency was interpreted on the basis of an internal heavy atom effect, favoring the intersystem-crossing deactivation route within the encounter complex with the concomitant reduction of effective photooxidation. [source] Structure of interleukin 4 mutant E9A suggests polar steering in receptor-complex formationACTA CRYSTALLOGRAPHICA SECTION D, Issue 9 2001Martin Hülsmeyer Interleukin 4 (IL-4) is a pleiotropic cytokine which induces T-cell differentiation and class switching of B cells. It therefore plays a central role in the development of allergies and asthma. An IL-4 variant in which Glu9 was mutated to alanine shows an 800-fold drop in binding affinity towards its high-affinity receptor chain. As shown by surface plasmon resonance measurements, this mostly arises from a decreased association rate. Here, the crystal structure of this mutant is reported. It reveals that the protein has a virtually identical structure to the wild type, showing that the unusual behaviour of the mutated protein is not a consequence of misfolding. The possibility that polar interactions in the encounter complex have a steering effect is discussed. [source] | ||||||||||