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.

The chemical mechanism of nitrogenase: hydrogen tunneling and further aspects of the intramolecular mechanism for hydrogenation of η2-N2 on FeMo-co to NH3

Abstract: The preceding paper (Dalton Trans., 2008, DOI: 10.1039/b806100a) describes the logical development of a chemical mechanism for the catalysis of hydrogenation of N2 to 2NH3 that occurs at the Fe7MoS9Nc(homocitrate) cofactor (FeMo-co) of the enzyme nitrogenase. The mechanism uses a single replenishable path for serial supply of protons which become H atoms on FeMo-co, migrating to become S–H and Fe–H donors to N2 and to the intermediates that follow. This chemical catalysis at FeMo-co is distinctly intramolecular: transition states and reaction profiles for the preferred 21 step pathway were presented. This paper describes a number of alternative intermediates and pathways that were considered in developing the mechanism. These results reveal further relevant principles of the reactivity of hydrogenated FeMo-co, and the reasons why these pathways are less likely to be part of the mechanism. The intramolecular character of the mechanism, and the relatively small distances over which H atoms transfer, lead to expectations of extensive quantum mechanical hydrogen tunneling as part of the catalytic rate enhancement. This possibility is supported by comparisons of reaction profiles with those for enzyme reactions for which tunneling is established.

Computational study of the transition state for hydrogen addition to Vaska-type complexes (trans-Ir(L)2(CO)X): substituent effects on the energy barrier and the origin of the small hydrogen/deuterium kinetic isotope effect

Abstract: Ab initio molecular orbital methods have been used to study transition state properties for the concerted addition reaction of H[sub 2] to Vaska-type complexes, trans-Ir(L)[sub 2](CO)X, 1 (L = PH[sub 3] and X = F, Cl, Br, I, CN, or H; L = NH[sub 3] and X = Cl). Stationary points on the reaction path retaining the trans-L[sub 2] arrangement were located at the Hartree-Fock level using relativistic effective core potentials and valence basis sets of double-[zeta] quality. The identities of the stationary points were confirmed by normal mode analysis. Activation energy barriers were calculated with electron correlation effects included via Moller-Plesset perturbation theory carried fully through fourth order, MP4(SDTQ). The more reactive complexes feature structurally earlier transition states and larger reaction exothermicities, in accord with the Hammond postulate. The experimentally observed increase in reactivity of Ir(PPh[sub 3])[sub 2](CO)X complexes toward H[sub 2] addition upon going from X = F to X = I is reproduced well by the calculations and is interpreted to be a consequence of diminished halide-to-Ir [pi]-donation by the heavier halogens. Computed activation barriers (L = PH[sub 3]) range from 6.1 kcal/mol (X = H) to 21.4 kcal/mol (X = F). Replacing PH[sub 3] by NH[submore » 3] when X = Cl increases the barrier from 14.1 to 19.9 kcal/mol. Using conventional transition state theory, the kinetic isotope effects for H[sub 2]/D[sub 2] addition are computed to lie between 1.1 and 1.7 with larger values corresponding to earlier transition states. Judging from the computational data presented here, tunneling appears to be unimportant for H[sub 2] addition to these iridium complexes. 51 refs., 4 tabs.« less

Phenomenological description of the transition state, and the bond breaking and bond forming processes of selected elementary chemical reactions: an information-theoretic study

Abstract: Theoretic-information measures of the Shan- non type are employed to describe the course of the sim- plest hydrogen abstraction and the identity SN2 exchange chemical reactions. For these elementary chemical pro- cesses, the transition state is detected and the bond breaking/forming regions are revealed. A plausibility argument of the former is provided and verified numeri- cally. It is shown that the information entropy profiles posses much more chemically meaningful structure than the profile of the total energy for these chemical reactions. Our results support the concept of a continuum of transient of Zewail and Polanyi for the transition state rather than a single state, which is also in agreement with reaction force analyses. This is performed by following the intrinsic reaction coordinate (IRC) path calculated at the MP2 level of theory from which Shannon entropies in position and momentum spaces at the QCISD(T)/6-311??G(3df,2p) level are determined. Several selected descriptors of the density are utilized to support the observations, such as the molecular electrostatic potential, the hardness, the dipole moment along with geometrical parameters.

Transition states, avoided crossing states and valence-bond mixing: fundamental reactivity paradigms

Abstract: A chemical model has been constructed for the transition state (TS) that is otherwise defined only by mathematical terms as a saddle point on the potential-energy surface. The proposed model is the avoided crossing state (ACS) of the chemical reaction. Unlike the TS that is a priori unknown, the ACS possesses a wavefunction that is prescribed by the constraints of the avoided crossing and is explicit in terms of the participating VB configurations. These VB configurations provide simultaneously a generalized TS description along with lucid information about the chemical nature of the TS. Ab initio computations demonstrate that, for nine SN2 and nucleophilic addition reactions, the ACS is an excellent approximation for the TS. This proximity between the two structures means in turn, that the bottleneck of the reaction may be associated with the chemically well defined ACS. VB mixing ideas are used to articulate the ACS paradigm and derive electronic properties of this state and its antibonding companion state. Applications to ground-state and excited-state reactivities of electrophile–nucleophile combinations are discussed.

Calculation of multiple initial state selected reaction probabilities from Chebyshev flux-flux correlation functions: influence of reactant internal excitations on H + H2O → OH + H2.

Abstract: A Chebyshev-based flux-flux correlation function approach is introduced for calculating multiple initial state selected reaction probabilities for bimolecular reactions. Based on the quantum transition-state theory, this approach propagates, with the exact Chebyshev propagator, transition-state wave packets towards the reactant asymptote. It is accurate and efficient if many initial state selected reaction probabilities are needed. This approach is applied to the title reaction to elucidate the influence of the H2O ro-vibrational states on its reactivity. Results from several potential energy surfaces are compared.