Stopped-flow kinetic/spectroscopic study

A detailed kinetic study has been carried out for the formation of 2 from 1 and O2. Figure 4 shows the kinetic scheme deduced from a low-temperature stopped-flow kinetic/spectroscopic study. The 1 1 intermediate [LCu(02)] species was detected at... 

In agreement with this mechanism, a stopped-flow IR spectroscopic study on the metalation of iV,iV-dialkylbenzamides with s-BuLi/TMEDA [57] suggested the presence of amide-Li complex intamediates before the formation of the lithiated product. Theoretical (MNDO) evidence was also found for the formation of weakly chelated complexes suggesting that a coordination event leads to a deprotonation process [58,59]. Inter- and intramolecular kinetic isotope effect (KIE) studies also support a CIPE mechanism [60, 61]. 

Khan, M.J., Joginadha Swamy, M., Krishna Sastry, M.V., Umadevi Sajjan, S., Patanjali, S.R., Rao, P., Swarnalatha, G.V., Banerjee, P. and Surolia, A. (1988). Saccharide binding to three Gal/GalNac specific lectins Fluorescence, spectroscopic and stopped-flow kinetic studies. Glycoconjugate Journal, 5, 75-84. 

The proposed mechanism for the O2 reduction cycle of the cytochrome c oxidase enzyme is outlined in Figure 6. 8 A two-step 2-e reduction process converts the inactive oxidized Fe3+-Cu2+ bimetallic site to an active Fe +-Cu+ state. Recent kinetic/spectroscopic studies implicate Cub as the initial binding site for dioxygen, thus affirming (for the first time) that Cu(I)/02 interactions are important in CcO. This evidence comes from flash-photolysis of CuB-CO protein derivatives as well as from observations (stopped-flow kinetics/spectroscopy) regarding the direct reaction of O2 with reduced CcO. The... 

If an intermediate is not sufficiently stable to be isolated, it might nevertheless be formed in sufficient concentration to be detected spectroscopically. Techniques used for this purpose include UV—vis spectroscopy in stopped-flow kinetics experiments for relatively stable intermediates or IR spectroscopy in matrix isolation spectroscopy for more reactive species. For photochemical reactions, we can detect transient spectra of intermediates in the millisecond to microsecond ( conventional" flash spectroscopy) or nanosecond to picosecond or femtosecond (laser flash spectroscopy) time scale. In all cases we must be certain that the spectra observed are indeed indicative of the presence of the proposed intermediate and only the proposed intermediate. Theoretical calculations have been useful in determining the spectroscopic properties of a proposed intermediate, whether it is likely to be sufficiently stable for detection, and the t)q e of experiment most likely to detect it. In addition, kinetic studies may suggest optimum conditions for spectroscopic detection of an intermediate. ... 

Catalytic water splitting has been studied extensively with several experimental techniques, including X-ray diffraction, cycHc voltammetry, stopped-flow kinetics, and several spectroscopic methods (NMR, IR, UV—VIS, etc.). The insight given by these techniques is often limited by the rather complex nature of the oxidation chemistry involved. Once the... 

The reaction of alkyl- and halo-substituted phenols with Cr02Cl2 results mainly in the formation of quinones and diphenoquinones. Phenoxyl radicals are involved as intermediates. The mechanism of oxidation of a-hydroxycar-boxylic acids by pyridinium chlorochromate involves a rate-limiting hydride transfer. In the reaction of HOCD2CO2H, a kinetic isotope effect (W d) = 5.80 has been determined. Spectroscopic evidence for Cr(IV) and Cr(V) species has been obtained in the oxidation of alkylaromatics by chromyl acetate in acetic anhydride.Stopped-flow and esr studies show two stages (equations 22, 23) in the reactions of RCH2Ph, with both rates being decreased on deuteration at... 

The recent explosion of interest in cationic research does not, however, stem from further refinements in the clean handling of sensitive materials. Wistfully, in this respect, the authors record a steady decline in standards. As a result, spectacular improvements due to modern spectroscopic stopped-flow methods, and to instrumental advances in unravelling chemical mechanisms generally, have perhaps been slower than elsewiiere to emerge in studies of cationic polymerisation mechanism. Only in 1976 do we find — in the work of Kunitake and Takarabe — the first thorough examination of the kinetics of initiation of a dimerisation ( ) based on spectroscopic stopped-flow measurements. 

Silverman s studies on mechanism based MAO inactivation have provided overwhelming support for the role of electron transfer in the MAO catalyzed dealkylation of amines. It must be mentioned however that spectroscopic attempts for detecting the radical ion intermediates have hitherto been unsuccessful. Yasanobu and coworkers could not find EPR spectral evidence for radical intermediates in MAO-catalyzed oxidation of benzylamine [205]. Miller et al. failed to observe the flavin semiquinone or an amine-flavin adduct in rapid-scan-stopped flow spectroscopy [206]. The only time-dependent absorption change observed in this study was the bleaching of the oxidized flavin. Furthermore, no influence of a magnetic field up to 6500 G was observed on the rate of MAO B reduction. The reaction rates of systems with kinetically significant radical pair intermediates are known to be altered... 

On the coordination chemistry side, ligand substitution on metal complexes in ILs has attracted quite some interest. This is mainly due to the fact that both spectroscopic and catalytic properties are strongly governed by the nature of the ligands and the stability of their bond to a metal center. Begel et al. have studied the role of different ILs on ligand substitution reactions on [Pt(terpy)Cl]+ (terpy = 2,2 6, 2"-terpyridine) with thiourea with stopped-flow techniques. The substitution kinetics show similar trends if compared to conventional solvents with similar polarities. Moreover, much like in conventional solvents, the authors find an associative character of the substitution reaction [205], These results are essentially supporting an earlier study by Weber et al., who found the same behavior [206],... 

A versatile and classical method for studying kinetic reactions and other kinetic phenomena on short time scales is the use of a stopped-flow apparatus (SFA) for fast reproducible mixing and then to apply, e.g., spectroscopic methods for detection. In this technique, the reactants are rapidly mixed in a mixing chamber, usually under full turbulent flow that ensures fast homogenization on length scales down to nanometers [99]. Provided that short, synchronized acquisitions can be made. X-ray or neutron scattering can be used to probe kinetic transitions and other processes directly by measuring the temporal evolution of the intensity of the (mixed) sample. 

The lack of simple spectroscopic handles for the CA systems has made kinetic studies somewhat difficult. In one study, the C linebroadening method has been used. The most common technique is the stopped-flow indicator method in which a solution of CO /HCOj is mixed with a solution of buffer, enzyme and indicator. The small pH change caused by formation or loss of due to reaction (8.22) can be monitored spectrophotometrically. More recently, the method of choice has been to use 0-enriched COj/HCOj" in combination with a mass... 

Reaction of traws-l,3-diphenyl-l-butene (D), the trans ethylenic dimer of styrene, with trifluoro-methanesulfonic acid in dichloromethane has been performed at temperatures lower than room temperature using a stopped-flow technique with real time UV-visible spectroscopic detection. The main product of the reaction was the dimer of D. A transient absorption at 340 nm has been assigned to 1,3-diphenylbutylium, a model for the polystyryl cation. Other absorptions at 349 nm and 505 nm have also been observed and were assigned to an allylic cation, l,3-diphenyl-l-buten-3-ylium, resulting from hydride abstraction from D. This species was very stable at temperatures lower than -30°C. A general mechanism was proposed based on a kinetic study of the reactions involved. 

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