Transition state

About: Transition state is a research topic. Over the lifetime, 4978 publications have been published within this topic receiving 117965 citations. The topic is also known as: transition state of elementary reaction.

Enzymatic transition state poise and transition state analogues.

Abstract: The development of kinetic isotope effect methods for enzymatic reactions has resulted in the systematic determination of enzymatic transition state structure for several distinct chemical reaction mechanisms. Although it is early in the experimental development of the method, examples of concerted nucleophilic displacements (ANDN or SN2), aromatic nucleophilic displacements (AN*DN or SNAr), and both concerted and stepwise dissociative nucleophilic displacements (DNAN and DN*AN; SN1 reactions) have been exemplified. The transition state for each reaction exhibits a characteristic extent of bond-breaking and bond-making, defined here as transition state poise. Thus, concerted nucleophilic displacements (SN2 or DNAN) exhibit various extents of residual bond order to the leaving group and to the attacking nucleophile at the transition state. Aromatic nucleophilic displacements reach their rate-limiting transition states before or after formation of the tetrahedral intermediate. Several concerted, symmetric n...

Theoretical study of the benzyl+O2 reaction: kinetics, mechanism, and product branching ratios.

Abstract: Ab initio calculations at the level of CBS-QB3 theory have been performed to investigate the potential energy surface for the reaction of benzyl radical with molecular oxygen. The reaction is shown to proceed with an exothermic barrierless addition of O2 to the benzyl radical to form benzylperoxy radical (2). The benzylperoxy radical was found to have three dissociation channels, giving benzaldehyde (4) and OH radical through the four-centered transition states (channel B), giving benzyl hydroperoxide (5) through the six-centered transition states (channel C), and giving O2-adduct (8) through the four-centered transition states (channel D), in addition to the backward reaction forming benzyl radical and O2 (channel E). The master equation analysis suggested that the rate constant for the backward reaction (E) of C6H5CH2OO → C6H5CH2 + O2 was several orders of magnitude higher that those for the product dissociation channels (B−D) for temperatures 300−1500 K and pressures 0.1−10 atm; therefore, it was also ...

Global Control of Suprathreshold Reactivity by Quantized Transition States

Abstract: The authors present evidence that the accurate quantum mechanical probability of the reaction of H with H{sub 2} is globally controlled by quantized transition states up to very high energies. The quantized transition states produce steplike features in the cumulative reaction probability curves that are analyzed up to energies of 1.6 eV; the analysis clearly associates these steps (or thresholds) with quantized dynamical bottlenecks that control the passage of reactive flux to products. They have assigned bend and stretch quantum numbers to the modes orthogonal to the reaction coordinate for all these transition states on the basis of threshold energies of semiclassical vibrationally adiabatic potential energy curves and vibrationally specific cumulative reaction probability densities.

Directing isomerization reactions of cumulenes with electric fields.

Abstract: Electric fields have been proposed as having a distinct ability to catalyze chemical reactions through the stabilization of polar or ionic intermediate transition states. Although field-assisted catalysis is being researched, the ability to catalyze reactions in solution using electric fields remains elusive and the understanding of mechanisms of such catalysis is sparse. Here we show that an electric field can catalyze the cis-to-trans isomerization of [3]cumulene derivatives in solution, in a scanning tunneling microscope. We further show that the external electric field can alter the thermodynamics inhibiting the trans-to-cis reverse reaction, endowing the selectivity toward trans isomer. Using density functional theory-based calculations, we find that the applied electric field promotes a zwitterionic resonance form, which ensures a lower energy transition state for the isomerization reaction. The field also stabilizes the trans form, relative to the cis, dictating the cis/trans thermodynamics, driving the equilibrium product exclusively toward the trans. Electric fields have been proposed as having a distinct ability to catalyse chemical reactions through stabilizing polar intermediates. Here, the authors show that electric fields can catalyse the cis-to-trans isomerization reactions of cumulenes in solution in a scanning tunnelling microscope

Coupled proton and electron transfer reactions in cytochrome oxidase.

Abstract: Cytochrome oxidase catalyzes the four-electron reduction of O2 to water and conserves the substantial free energy of the reaction in the form of a protonmotive force. For each electron, two full charges are translocated across the membrane, resulting in a voltage. One of the mechanisms to generate the charge separation in cytochrome oxidase is via a proton pump. A single reaction cycle can be monitored over the course of about 1 msec using absorption spectroscopy, revealing distinct intermediates. Thus, the reaction cycle can be studied as a series of steps. Each of the reaction steps in the catalytic cycle involves a sequence of coupled electron and proton transfer reaction, where protons are either consumed in the chemistry of water formation or pumped across the membrane. The pumping mechanism requires consideration of both the thermodynamics of the various species but also the favored kinetic pathways that assure proton pumping is unidirectional. Hence, a knowledge of transition states and transiently, poorly populated intermediates is likely to be important to understand the mechanism of the pump.