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.

Femtosecond real‐time probing of reactions. II. The dissociation reaction of ICN

Abstract: Experimental results obtained for the dissociation reaction ICN^*→[I⋅⋅⋅CN]^(‡*)→I+CN using femtosecond transition‐state spectroscopy (FTS) are presented. The process of the I–CN bond breaking is clocked, and the transition states of the reaction are observed in real time. From the clocking experiments, a "dissociation" time of 205±30 fs was measured and was related to the length scale of the potential. The transition states live for only ∼50 fs or less, and from the observed transients we deduce some characteristics of the relevant potential energy surfaces (PES). These FTS experiments are discussed in relation to both classical and quantum mechanical models of the dynamical motion, including features of the femtosecondcoherence and alignment of fragments during recoil. The observations are related to the radial and angular properties of the PES.

Das Distortion/Interaction-Activation-Strain-Modell zur Analyse von Reaktionsgeschwindigkeiten

Abstract: The Activation Strain or Distortion/Interaction Model is a tool to analyze activation barriers that determine reaction rates. For bimolecular reactions, the activation energies are the sum of the energies to distort the reactants into geometries they have in transition states plus the energies of interaction between the two distorted molecules. The energy to distort the molecules is called the activation strain or distortion energy. This energy is the principal contribution to the activation barrier. The transition state occurs when this activation strain is overcome by stabilizing interaction energy. Following the changes in these energies along the reaction coordinate gives insights into the factors controlling reactivity. This model has been applied to reactions of all types in both organic and inorganic chemistry, including substitutions and eliminations, cycloadditions, and several types of organometallic reactions.

The roles of entropy and enthalpy in stabilizing ion-pairs at transition states in zeolite acid catalysis.

Abstract: Acidic zeolites are indispensable catalysts in the petrochemical industry because they select reactants and their chemical pathways based on size and shape. Voids of molecular dimensions confine reactive intermediates and transition states that mediate chemical reactions, stabilizing them by van der Waals interactions. This behavior is reminiscent of the solvation effects prevalent within enzyme pockets and has analogous consequences for catalytic specificity. Voids provide the “right fit” for certain transition states, reflected in their lower free energies, thus extending the catalytic diversity of zeolites well beyond simple size discrimination. This catalytic diversity is even more remarkable because acid strength is essentially unaffected by confinement among known crystalline aluminosilicates. In this Account, we discuss factors that determine the “right fit” for a specific chemical reaction, exploring predictive criteria that extend the prevailing discourse based on size and shape. We link the stru...

One-dimensional reaction coordinates for diffusive activated rate processes in many dimensions

Abstract: For multidimensional activated rate processes controlled by diffusive crossing of a saddle point region, we show that a one-dimensional reaction coordinate can be constructed even when the diffusion anisotropy is arbitrary. The rate constant, found using the potential of mean force along this coordinate, is identical to that predicted by the multidimensional Kramers–Langer theory. This reaction coordinate minimizes the one-dimensional rate constant obtained using a trial reaction coordinate and is orthogonal to the stochastic separatrix, the transition state that separates reactants from products.

Lewis acid catalysis of a Diels-Alder reaction in water

Abstract: Here we report the first detailed study of a Diels-Alder (DA) reaction that is catalyzed by Lewis acids in water. The effect of Co2+, Ni2+, Cu2+ and Zn2+ ions as Lewis acid catalysts on the rate and endo-exo selectivity of the DA reaction between the bidentate dienophiles 3-phenyl-1-(2-pyridyl)-2-propen-1-ones (1a-e) and cyclo-pentadiene (2) in water has been studied. Relative to the uncatalyzed reaction in acetonitrile, catalysis by 0.010 M CU(NO3)(2) in water accelerates the reaction by a factor of 79 300. The kinetics of the catalyzed reaction were analyzed in terms of equilibrium constants for complexation of the Lewis acid with 1a-e and rate constants for the reaction of the resulting complexes with 2. The rate enhancement imposed upon the uncatalyzed DA reaction of substrates 1 with 2 by water is much more pronounced than that for the catalyzed reaction. The increase of the endo-exo selectivity induced by water in the uncatalyzed process is completely absent for the Lewis acid catalyzed reaction. The modest solvent and substituent effects observed for the catalyzed reaction indicate that the change in charge separation during the activation process is not larger than the corresponding change For the uncatalyzed reaction.