Kinetic studies theoretical calculations

Evidence has been presented for both these mechanistic postulates. Such evidence has come from kinetic and structural data as well as isotope effect studies. Theoretical calculations aimed at explaining the observed kinetic isotope effects support the (3+2) cycloaddition mechanism. Two questions, however, remain to be answered. The mechanistic details of oxidation of an alkene by 0s04 in the absence of L have not been studied in great detail. Also the changes, if any, that are effected by the introduction of L to such a mechanism are not known. 

Rigo e.t al,6j were the first to propose that head addition does occur but is immediately followed by a 1,2-chlorine atom shift. The viability of 1,2-chlorine atom shifts is well established in model studies and theoretical calculations.64 Experimental support for this occurring during VC polymerization has been provided by NMR studies on reduced PVC /lfl Starnes et a/.61 proposed that head addition is followed by one or two 1,2-chlorine atom shifts to give chloromethyl or dichloroethyl branch structures respectively (Scheme 4.8). There also is kinetic data to support this hypothesis. 

A comprehensive experimental and theoretical study was undertaken on the reaction, which was shown to be a concerted reductive elimination process kinetic studies were consistent with reductive elimination, and DFT calculations on complex 3 (Fig. 13.1) supported an associative reductive elimination pathway with a small... 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

Jakdetchai and Nakajima/Wang and coworkers—theoretical models favor redox mechanism. Beginning in 2002, a number of theoretical models were published in Theochem studying the water-gas shift reaction over Cu(110), Cu(lll), and Cu(100) surfaces. Perhaps the first was by Jakdetchai and Nakajima,325 relying on the AMI method. The main goal of the study was (1) to determine whether or not theoretical calculations are consistent with a redox or associative (e.g., formate) mechanism and (2) whether the kinetics are described best by a Langmuir-Hinshel-wood expression or an Eley-Rideal expression. That is, in the case of a redox model, does the adsorbed O adatom react with adsorbed CO or directly with gas phase CO Their approximate A//a[Pg.205]

The Dotz reaction mechanism has received further support from kinetic and theoretical studies. An early kinetic investigation [37] and the observation that the reaction of the metal carbene with the alkyne is supressed in the presence of external carbon monoxide [38] indicated that the rate-determining step is a reversible decarbonylation of the original carbene complex. Additional evidence for the Dotz mechanistic hyphotesis has been provided by extended Hiickel molecular orbital [23, 24] and quantum chemical calculations [25],... 

Kinetic studies of the unnatural 6-a -epimer of ampicillin, fi-ept-ampicillin (154), have revealed an intramolecular process not undergone by ampicillin (or other natural /3-substituted penicillins) At pH 6-9, intramolecular attack of the jS-lactam carbonyl group by the side-chain amino group of (154) yields a stable piperazine-2,5-dione derivative (155). Theoretical calculations show that the intramolecular aminolysis of 6-epi-ampicillin nucleophilic attack occurs from the a-face of the -lactam ring with an activation energy of 14.4kcalmor In other respects, the hydrolysis of the b-a-epimer is unexceptional. 

Theoretical evidence [Hartree-Fock (RHF) calculations and density functional theory] has been obtained for a concerted mechanism of oxirane cleavage and A-ring formation in oxidosqualene cyclization. A common concerted mechanistic pathway has been demonstrated for the acid-catalysed cyclization of 5,6-unsaturated oxiranes in chemical and enzymic systems. For example, the conversion of (24) into (26) proceeds via (25) and not via a discrete carbocation (27). Kinetic studies and other evidence are presented for various systems. 

In addition to these few examples of kinetic studies related to acid-base equilibria in azoles, there are reports in the literature on activation energies calculated theoretically for proton transfer involving azoles (80CCC3482 80MI3 84JPC5882 86BSF429). Generally, the inclusion of solvent molecules is necessary to find a double minimum potential function. 

Kinetic measurements and theoretical calculations provided evidence that carbonyl oxide intermediates (75) are an important source of HO radicals in gas-phase ozonolysis.1332 1333 New studies with a wide range of variously substituted... 

From the data on the kinetics of et decay in vitrified solutions of (1) and (2), by using eqns. (15) and (17)-(20) theoretical curves of etr decay in the matrices (3) and (4) have been calculated. In calculating the tunneling distances f tl (f) and Rh(t) use was made of an equation of the type of eqn. (34) of Chap. 4, i.e. no precursors of et r were assumed to decay by reactions with B, and B2 acceptors. From Fig. 21, it can be seen that the results of the theoretical calculation of the et decay kinetics in matrices containing a mixture of the acceptors Bj and B, and the bifunctional acceptor Th-B.. by means of eqns. (20) and (15) (the solid and broken lines, respectively) are in excellent agreement with experiment. Good agreement of experiment and calculation via eqn. (20) has also been obtained for the other acceptors in vitreous MTHF and in ethyl alcohol that were studied in ref. 91. Note that... 

Kinetic studies. Excellent agreement was obtained for various Km and Ki values determined for compounds common to a number of the reactions catalyzed (37, 40, 41, 46, 113). Activities observed when several substrates were combined in assay mixtures agreed well with theoretical values calculated on the basis of the involvement of a single enzyme (37, 46)-... 

A low-temperature study in superacid media of mono-, di-, and tri-protonated thiourea has been earned out.302 The experimental results were confirmed by theoretical calculations. Monoprotonation occurs at sulfur and, whereas the mono- and di-protonated forms are thermodynamically stable, the triprotonated ion is only kinetically stable. The pyrolysis of A-acctylthiourea and A.A -diacetyIthiourea (335) are ultimolecular first-order eliminations.83 Acid-catalysed ethanolysis of N,N -di- and tri-substituted aryl- and alkylaryl-thioureas gives 0-ethyl AAaryl thiocarbamates and amines.303 The acid-catalysed hydrolysis of thiourea was first order in thiourea and acid.304... 

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