Search results for "reaction rate constant"
showing 10 items of 210 documents
Reversible Dioxygen Binding and Phenol Oxygenation in a Tyrosinase Model System
2000
The complex [Cu2(L-66)]2+ (L-66 = a,a'-bis¿bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino¿-m-xylene) undergoes fully reversible oxygenation at low temperature in acetone. The optical [lambda(max) = 362 (epsilon 15000), 455 (epsilon 2000), and 550 nm (epsilon 900M(-1)cm(-1))] and resonance Raman features (760 cm(-1), shifted to 719cm(-1)(-1) with 18O2) of the dioxygen adduct [Cu2(L-66)(O2)]2+ indicate that it is a mu-eta2:eta2-peroxodicopper(II) complex. The kinetics of dioxygen binding, studied at - 78 degrees C, gave the rate constant k1 = 1.1M(-1) 5(-1) for adduct formation, and k(-1) =7.8 x 10(-5)s(-1), for dioxygen release from the Cu2O2 complex. From these values, the O2 binding consta…
Convergence of Theory and Experiment on the Role of Preorganization, Quantum Tunneling, and Enzyme Motions into Flavoenzyme-Catalyzed Hydride Transfer
2017
Hydride transfer is one of the most common reactions catalyzed by enzymatic systems, and it has become an object of study because of possible significant quantum tunneling effects. In the present work, we provide a combination of theoretical QM/MM simulations and experimental measurements of the rate constants and kinetic isotopic effects (KIEs) for the hydride transfer reaction catalyzed by morphinone reductase, MR. Quantum mechanical tunneling coefficients, computed in the framework of variational transition-state theory, play a significant role in this reaction, reaching values of 23.8 ± 5.5 for the lightest isotopologue—one of the largest values reported for enzymatic systems. This pred…
Transition state theory thermal rate constants and RRKM-based branching ratios for the N((2)D) + CH(4) reaction based on multi-state and multi-refere…
2012
International audience; Multireference single and double configuration interaction (MRCI) calculations including Davidson (+Q) or Pople (+P) corrections have been conducted in this work for the reactants, products, and extrema of the doublet ground state potential energy surface involved in the N(2D) + CH4 reaction. Such highly correlated ab initio calculations are then compared with previous PMP4, CCSD(T), W1, and DFT/B3LYP studies. Large relative differences are observed in particular for the transition state in the entrance channel resolving the disagreement between previous ab initio calculations. We confirm the existence of a small but positive potential barrier (3.86 +/- 0.84 kJ mol-1…
Mechanism of Reaction of Melatonin with Human Myeloperoxidase
2001
Recently, it was suggested that melatonin (N-acetyl-5-methoxytryptamine) is oxidized by activated neutrophils in a reaction most probably involving myeloperoxidase (Biochem. Biophys. Res. Commun. (2000) 279, 657-662). Myeloperoxidase (MPO) is the most abundant protein of neutrophils and is involved in killing invading pathogens. To clarify if melatonin is a substrate of MPO, we investigated the oxidation of melatonin by its redox intermediates compounds I and II using transient-state spectral and kinetic measurements at 25 degrees C. Spectral and kinetic analysis revealed that both compound I and compound II oxidize melatonin via one-electron processes. The second-order rate constant measur…
Quartz Crystal Microbalance And Electrical Impedance Characterization Of Nickel Dissolution Process.
2005
Abstract. The anodic nickel dissolution in acid media is analysed by means of EQCM and EIS techniques. The experimental impedance spectra have been fitted to the equivalent circuit which corresponds to two consecutive electron transfers followed by a Ni(II) desorption. That way rate constants and surface concentrations of the Ni(0) and Ni(I) species are obtained. EQCM also provides information about the mechanism of deposition and passivation of nickel as well as the hydrogen evolution.
Mössbauer effect study of the temperature and pressure dependence of the singlet-quintet intersystem crossing dynamics in an iron(II) spin crossover …
1988
The lineshapes of Mossbauer spectra of the iron(II) spin crossover complex [Fe(6-mepy)3 tren] (PF6)2 are affected by the dynamics of the HS⇌LS equilibrium. The lineshapes are reproduced with a stochastic two-state-relaxation-model yielding rate constants similar to those determined for related complexes in solution. Application of an external pressure of 150 MPa increases the relaxation rate.
Magnetic predissociation in Te 2 B 1 u
1997
We report a registration of magnetic predissociation (MPD) of electronically excited molecules via peculiarities in magnetic field B induced alignment-orientation conversion (AOC). Non-linear magnetic energy shift and heterogeneous MPD produce dispersion type fluorescence circularity signals C(B) of different sign. Measurements on B 1 u - , v(J) equals 2(96) state of 130 Te 2 yielded natural C v het equals plus or minus 6 s -1/2 and magnetic (alpha) v het equals minus or plus 9 multiplied by 10 3 s -1/2 T -1 rate constants of heterogeneous PD, supposing that the B1 u - state PD takes place through O u - state continuum.
Effect of Lithium Perchlorate on the Kinetics of the Anionic Polymerization of Methyl Methacrylate in Tetrahydrofuran
1999
The kinetics of the anionic polymerization of methyl methacrylate in the presence of lithium perchlorate (LiClO 4 ) are investigated in THF using 1,1-diphenylhexyllithium as initiator in a flow-tube reactor between -30 and 0 °C. The rate constants of propagation determined in the presence of LiClO 4 are lower than those obtained in the absence of the salt, similar to the effect observed for LiCl. For propagation, the reaction order with respect to active center concentration is found to be 0.5 in both cases, which indicates that LiClO 4 does not effectively perturb the aggregation of the enolate ion pair. The formation of various mixed aggregates is proposed. The polydispersity index of the…
Inverse solvent effects in the heterogeneous and homogeneous epoxidation of cis-2-heptene with [2-percarboxyethyl]-functionalized silica and meta-chl…
2014
The rate constants for the epoxidation of cis-2-heptene with [2-percarboxyethyl]-functionalized silica (1a) and meta-chloroperbenzoic acid (mCPBA) (1b) in different solvents have been determined at temperatures in the −10 to 40 °C range. The heterogeneous epoxidation exhibits a dependence of the reaction rate on solvent polarity opposite to its homogeneous counterpart and anomalous activation parameters in n-hexane, which are interpreted in terms of the surface-promoted solvent structure at the solid–liquid interface. The results show that highly polar solvents can strongly inhibit heterogeneous reactions performed with silica-supported reagents or catalysts.
Kinetik der bromierung von phenolen und phenolischen mehrkernverbindungen, 3. Die reaktionsfähigkeit isomerer dreikernverbindungen und einer homologe…
1976
Die Geschwindigkeit der Bromierung von drei isomeren Dreikernverbindungen und einer homologen Reihe von Mehrkernverbindungen mit bis zu sechs Methylphenolbausteinen pro Molekul wurde in Eisessig bei 22°C UV-spektroskopisch gemessen. Alle Verbindungen waren so gewahlt, das sie nur eine reaktionsfahige ortho-Stellung in bezug auf die Hydroxy-Gruppe besasen. Es zeigte sich, das die Reaktionsgeschwindigkeit im wesentlichen dadurch bestimmt wird, ob zwischen den phenolischen Hydroxy-Gruppen des reagierenden Endbausteins und des unmittelbar benachbarten Bausteins eine intramolekulare Wasserstoffbrucke moglich ist. Ist dies nicht der Fall, so sind die Geschwindigkeitskonstanten zweiter und dritter…