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.

Mechanism of Water Exchange for the Di- and Trivalent Metal Hexaaqua Ions of the First Transition Series

Abstract: The mechanism for the water-exchange reaction with the transition metal aqua ions from ScIII through ZnII has been investigated. The exchange mechanisms were analyzed on the previously reported model (Rotzinger, F. P. J. Am. Chem. Soc. 1996, 118, 6760) that involves the metal ion with six or seven water molecules. The structures of the reactants/products, transition states, and penta- or heptacoordinated intermediates have been computed with Hartree−Fock or CAS-SCF methods. Each type of mechanism, associative, concerted or dissociative, proceeds via a characteristic transition state. The calculated activation energies agree with the experimental ΔG⧧298 or ΔH⧧298 values, and the computed structural changes indicate whether an expansion or compression takes place during the transformation of the reactant into the transition state. These changes are in perfect agreement with the changes deduced from the experimental volumes of activation (ΔV⧧298). The motions of the ligands involved in the exchange reaction ...

Quantum theory of chemical reaction dynamics

Abstract: It is now possible to use rigorous quantum scattering theory to perform accurate calculations on the detailed state-to-state dynamics of chemical reactions in the gas phase. Calculations on simple reactions, such as H + D2 → HD + D and F + H2→ HF + H, compete with experiment in their accuracy. Recent advances in theory promise to extend such accurate predictions to more complicated reactions, such as OH + H2 → H2O + H, and even to reactions of molecules on solid surfaces. New experimental techniques for probing reaction transition states, such as negative-ion photodetachment spectroscopy and pump-probe femtosecond spectroscopy, are stimulating the development of new theories.

Determining transition-state geometries in liquids using 2D-IR.

Abstract: Many properties of chemical reactions are determined by the transition state connecting reactant and product, yet it is difficult to directly obtain any information about these short-lived structures in liquids. We show that two-dimensional infrared (2D-IR) spectroscopy can provide direct information about transition states by tracking the transformation of vibrational modes as a molecule crossed a transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement of Fe(CO)5, in which the exchange of axial and equatorial CO ligands causes an exchange of vibrational energy between the normal modes of the molecule. This energy transfer provides direct evidence regarding the time scale, transition state, and mechanism of the reaction.

Laser Probes of Conformational Isomerization in Flexible Molecules and Complexes

Abstract: Molecules with several flexible coordinates have potential energy surfaces with a large number of minima and many transition states separating them. A general experimental protocol is described that is capable of studying conformational isomerization in such circumstances, measuring the product quantum yields following conformation-specific infrared excitation, and measuring energy thresholds for isomerization of specific X --> Y reactant-product isomer pairs following excitation via stimulated emission pumping (SEP). These methods have been applied to a series of molecules of varying size and conformational complexity, including 3-indolepropionic acid (IPA), meta-ethynylstyrene, N-acetyltryptophan methyl amide (NATMA), N-acetyltryptophan amide (NATA), and melatonin. Studies of isomerization in solute-solvent complexes are also described, including a measurement of the barrier to isomerization in the IPA-H2O complex, and a unique isomerization reaction in which a single water molecule is shuttled between H-bonding sites on the trans-formanilide (TFA) molecule.

Dynamics, transition states, and timing of bond formation in Diels–Alder reactions

Abstract: The time-resolved mechanisms for eight Diels–Alder reactions have been studied by quasiclassical trajectories at 298 K, with energies and derivatives computed by UB3LYP/6-31G(d). Three of these reactions were also simulated at high temperature to compare with experimental results. The reaction trajectories require 50–150 fs on average to transverse the region near the saddle point where bonding changes occur. Even with symmetrical reactants, the trajectories invariably involve unequal bond formation in the transition state. Nevertheless, the time gap between formation of the two new bonds is shorter than a C─C vibrational period. At 298 K, most Diels–Alder reactions are concerted and stereospecific, but at high temperatures (approximately 1,000 K) a small fraction of trajectories lead to diradicals. The simulations illustrate and affirm the bottleneck property of the transition state and the close connection between dynamics and the conventional analysis based on saddle point structure.