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.

[31] The use of isotope effects to determine transition-state structure for enzymic reactions

Abstract: Publisher Summary Isotope effects have been used for a long time to determine the structures of transition states for chemical reactions. Such analysis is made easier by the fact that the chemical reaction being looked at is often the sole rate-limiting step. There are two kinds of isotope effects: primary and secondary. A primary isotope effect results from substitution of a heavy atom in a position where a total bond cleavage (or bond formation) occurs. Conversely, secondary isotope effects result from isotopic substitution in a position where no bonds are made or broken, but there is a change in the strength of the bonding during the reaction (or to be precise, the bonding in the transition state differs from that in the reactant). Primary and secondary isotope effects are often treated as separate phenomena, because with deuterium the primary ones tend to be much larger than the secondary ones. There are different isotope effects on V and on V/K for each reactant, and it is not always easy to determine the intrinsic isotope effect on the bond-breaking step, which is the value that gives information on the structure of the transition state. This chapter reviews the status of techniques for determining intrinsic isotope effects and discusses the interpretation of these values in terms of transition-state structure.

Reactions of F atoms and aromatic and heterocyclic molecules: Energy distribution in the reaction complex

Abstract: This paper reports studies of the reactions of fluorine atoms and several aromatic and heterocyclic molecules. As in earlier work we have used the method of crossed molecular beams to create long‐lived reaction intermediates and to deduce from the angular distribution of the product molecules information about the energy distribution in the reaction complex. The data reported in this paper complement those collected earlier. They show that when a light group is emitted from a reaction complex randomization of the internal energy of the complex is not more rapid than reaction. When a heavy atom is emitted from the reaction complex the limited data available indicate that almost all vibrational degrees of freedom of the complex participate in the reaction. This latter observation provides an interesting connection between vibration‐translation energy exchange and the mechanism of chemical reaction.

Meisenheimer Complexes in SNAr Reactions: Intermediates or Transition States?

Abstract: Meisenheimer's missing! Anionic σ-complexes, best known as Meisenheimer complexes, have long been assumed to be intermediates in SN Ar reactions. New evidence now suggests that these intermediates may only be formed in select cases and that a concerted pathway is more common. These claims are supported by 12 C/13 C kinetic isotopes effects, determined using a new method based on 19 F NMR, and DFT calculations.

Temperature and Pressure Dependence of the Reaction of OH and CO: Master Equation Modeling on a High-Level Potential Energy Surface

Abstract: The temperature and pressure dependence of the reaction of OH + CO has been modeled using the (energy-resolved) master equation and RRKM theory. These calculations are based on the coupled-cluster potential energy surface of Yu and co-workers (Chem Phys Lett 349, 547–554, 2001). As is well known, this reaction shows a strong non-Arrhenius behavior at moderate and low temperatures because of the stabilization of the HOCO intermediate. Kinetic simulations are in excellent agreement with experiments at temperatures above 300 K, but the agreement is only modest at temperatures below 250 K. Our calculations indicate that the contribution of tunneling to the rate constant is marginal, given the small energy difference between the transition states corresponding to formation and decomposition of the HOCO intermediate. Parametric fits to the calculated rate constants are provided for modeling purposes. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 464–474, 2003