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.

Quantum chemical studies of zeolite proton catalyzed reactions

Abstract: Theoretical chemistry applied to zeolite acid catalysis is becoming an important tool in the understanding of the adsorption and interaction of guest molecules with the zeolitic lattice. Especially the understanding of the mechanisms by which zeolite catalyzed chemical reactions proceed becomes possible. It is shown here that the old interpretation of carbonium and carbenium ions as intermediates for zeolite catalyzed reactions has to be replaced by a new approach in terms of positively charged transition states that are strongly stabilized by the zeolitic lattice. The large deprotonation energy of the acidic zeolite is overcome by stabilization of the intermediate or transition state positive charge by the negative charge left in the lattice. The zeolitic sites responsible for the adsorption and/or reaction of guest molecules are the Bronsted-acid and Lewis-base sites. We also show that different transition states are responsible for different kinds of reactions, such as cracking, dehydrogenation, etc.

Hydrogenation of two-electron mixed-valence iridium alkyl complexes.

Abstract: Two-electron mixed-valence complexes of the general formula (tfepma)3Ir20,IIRBr [tfepma = bis(bis(trifluoroethoxy)phosphino)methylamine, MeN[P(OCH2CF3)2]2, and R = CH3 (2), CH2C(CH3)3 (3)] have been synthesized and structurally characterized and their reactivity with H2 investigated. Hydrogenation of 2 and 3 proceeds in a cascade reaction to produce alkane upon initial H2 addition, followed by the formation of the Ir2I,III binuclear trihydride−bromide complex (tfepma)3Ir2I,IIIH3Br (4) upon the incorporation of a second molecule of H2. Hydrogenation of two-electron mixed-valence di-iridium alkyl complexes is examined with nonlocal density-functional calculations. H2 attacks the IrII metal center prior to alkyl protonation to produce an η2−H2 complex. Transition states link all intermediates to a complex that has the same regiochemistry as the crystallographically determined final product. Calculated atomic charges suggest that the second H2 molecule is homolytically cleaved within the di-iridium coordinati...

The thermodynamics and kinetics of protein folding : a lattice model analysis of multiple pathways with intermediates

Abstract: The kinetics and thermodynamics of folding of a representative sequence of a 125-residue protein model subject to Monte Carlo dynamics on a simple cubic lattice were investigated. The diverse trajectories that lead to the native state can be classified into a relatively small number of average pathways: a “fast track” in which the chain forms a stable core that folds directly to the native state and several “slow tracks” in which particular contacts form before the core is complete and direct the chain to a misfolded intermediate. Rearrangement from the intermediates to the native state is slow because it requires breaking stable contacts, which involve primarily surface residues. The transition state for folding is identified by activated dynamics simulations and consists of a reduced version of the core in the absence of other (native and nonnative) contacts which slow folding. Each track involves an ensemble of structures that can be characterized by two progress coordinates for the reaction. These coordinates are based on a comparison of folding and nonfolding trajectories: one coordinate monitors the formation of the core and the other monitors whether the chain is trapped in a long-lived intermediate. From Monte Carlo simulations, we obtain an estimate for the density of states and calculate equilibrium averages, including the free energy, energy, and entropy, as functions of the two coordinates. The thermodynamics are in good agreement with the observed kinetics; the transition states correspond to plateaus or barriers in the free energy while the intermediates are energetically stabilized local free energy minima. The complexities of the folding mechanism bear a striking similarity to those observed experimentally for lysozyme, a well-studied protein of comparable size.