Activation, transition state

It had been assnmed in the past that the main reason for development of an activated transition state with enhanced energy is a stretching of chemical bonds. Thus, in the model of Horinti and Polanyi it was assnmed that stretching of H+-H2O bonds... 

The mixture of quinone methides initially formed by combination of the coniferyl radicals in their various mcsomeric forms, i.e. (I), (III), (V), (IX) and others, can be detected by means of their characteristic spectrum with a maximum at about 312 mp (52) the haU-hfe of the mixture in 70 % aqueous dioxan is 1 hour. Those quinone methides that can rearomatize by keto-enol tautomerism, e.g. (IX), or intramolecular additions, e.g. (I) or (III) may become stabilized faster than those of type (V) which rely on addition of a foreign molecule. The quinone methides that rearomatize intramolecularly appear to react exclusively in this way, probably by a concerted mechanism that represents collapse of the activated transition state. 

The QRRK approach illustrated above also constitutes the basis to analyze the behavior of the reverse, i.e., association, reactions that proceed through chemically activated transition states. Recently Dean (1985) reformulated the unimolecular quantum-RRK method of Kassel and devised a practical method for the proper description of the fall-off behavior of bimolecular reactions, including reactions when multiple product channels are present. The method developed was shown to describe the behavior of a large variety of bimolecular reactions with considerable success (Dean and Westmoreland, 1987 Westmoreland et ai, 1986). 

Transition-state stabilization The active site often acts as a flexible molecular template that binds the substrate in a geometric structure resembling the activated transition state of the molecule (see T at the top of the curve in Figure 5.4). By stabilizing the substrate in its transition state, the enzyme greatly increases the concentration of the reactive intermediate that can be converted to product and, thus, accelerates the reaction. 

Enthalpies of activation, transition-state geometries, and primary semi-classical (without tunneling) kinetic isotope effects (KIEs) have been calculated for 11 bimolecu-lar identity proton-transfer reactions, four intramolecular proton transfers, four nonidentity proton-transfer reactions, 11 identity hydride transfers, and two 1,2-intramole-cular hydride shifts at the HF/6-311+G, MP2/6-311+G, and B3LYP/6-311+-1-G levels.134 It has been found that the KIEs are systematically smaller for hydride transfers than for proton transfers. The differences between proton and hydride transfers have been rationalized by modeling the central C H- C- unit of a proton-transfer transition state as a four-electron, three-centre (4-e 3-c) system and the same unit of a hydride-transfer transition state as a 2-e 3-c system. 

Know the meaning of equilibrium constants, reaction enthalpy, energy of activation, transition state. 

Ye S, Neese F. Nonheme oxo-iron(IY) intermediates form an oxyl radical upon approaching the C-H bond activation transition state. Proc Natl Acad Sci USA. 2011 108 1228-33. 

One of the most in ortant properties of zeolites is their ability to cany out shqie selective reactions [5]. These can be cl sified as, firstfy, product shape selective reactions in which the only products formed are those which can diffiise out of e pores of die zeolite, second, reactant shape selective reactions which occur when some of the molecules in a reactant mbcture are too large to diffiise through the catalyst pores, and, thirdfy, restricted transition-state selective reactions in which the only reactions which occur are those in which qiace exists in the pores or cavities to allow the formation of the activated transition state con lex. In some cases where the zeolite is three dimensional the gze of the channel intersections will also be a determining ictor. This unique catalytic property is related to the pore size of the zeolite and has led to the synthesis of zeohtes with a very w e range of pore gzes. 

The idea that certain bonds of the substrate are distorted upon binding to the enzyme has been suggested by several workers. This so-called rack mechanism assumes that the substrate fits loosely into the active site, but the bonds that are formed between the enzyme and the substrate are so strong that a susceptible bond within the substrate, is distorted producing the activated transition state (Fig. 4-5). In this mechanism, a portion of the... 

Figure 1.26 Highly active transition state inhibitors bearing a trifluoroacetyl moiety.

Formulation of Eq. 9 is consistent with transition-state theory, where the rate of the reaction far from equilibrium depends solely on the activity of the activated transition-state complex (Wieland et al., 1988). Equations 8 and 9 are equivalent to Eq. 2 for proton attack, where C, is equal to the surface concentration of activated complex. 

Figure 2.22. Computed structures of the C-O bond activation transition states, and barriers, for ethene" and propene epoxidation . The putative parallel TS for ethene epoxidation is generated by rotating the computed one around the O-C bond.

G.-M. Schwab (Technical University, Munich) communicated It is to be noted that the concept of endothermic chemisorption is already implicitly contained in the classical textbook representation of catalytic activation. Accordingly, activation is due to exothermic adsorption of the (active) transition state. 

From the resultant M+(CH4) complex the reaction proceeds via the C-H bond activation transition state (TS) to give the hydrido-metal-methyl cation complex, HMCHJ. In this step the C-H o-bond is broken and M-H and M-CH3 bonds are formed. Also, the oxidation number of the M-center increases by two. In order to analyze the reactivity of TMCs toward C-H (as well as H-H and C-C) bond, one has to elucidate the factors controlling thermodynamics and kinetics of the reaction M+(CH4) HMCHJ. 

There are two different ways to explain the experimentally observed temperature dependence. The first method employs the concept of the activated transition state in the absolute reaction rate theory [33]. Here it is assumed that the diffusion particle crosses an activation energy barrier between two equivalent lattice sites. One calculates the probability of the particle being on the saddle-point (transition state) and its velocity there. This implies that an equilibrium distribution of diffusing particles between normal lattice sites and the saddle-points exists. It is further assumed that the diffusing particles in the saddle-point configuration... 

Figure 4.5 shows the energies of the initial weak hydrogen-bonded adsorbed state of propylene, the proton-activated transition state and the final alkoxy product state of the protonated propylene. The structures and energies are established from DFT cluster calculations using the model structure shown in Fig. 4.5a and periodic DFT calculations using the unit cell of chabazite and the zeolitic protons (Fig. 4.5b). The cluster used in Fig.

An investigation of the C—H activation transition states for electron-deficient aromatic substrates revealed that the transition state (T.S. ) for meta-C—H bond activation has a lower energy than the corresponding transition states for activation of the ortho- (T.S. ) andpara-C—H bonds (T.S. ). The higher energy of T.S., (vs. T.S., and T.S. ) is manifested in the catalytic... 

FIGURE 25.11 Computed reaction profile (kcal/mol) for the cyclometalation of dmba-H by Pd/OAc) via a 6-membered C—H activation transition state [20]. 

FIGURE 25.18 Computed C—H activation transition states along Pathways I-III described in Figure 25.17. Non participating H atoms are omitted for clarity and distances in A [29] Adapted with permission from Ref. [29]. Copyright (2006) American Chemical Society. 

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