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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.


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TL;DR: Comparison of calculated transition states for the hydrolyses of aryl esters and the haptens intended to mimic them suggested that catalysis may be improved by hapten redesign or antibody mutagenesis.
Abstract: Is antibody catalysis of hydrolysis reactions limited by imperfect mimicry of transition states by haptenic analogs? This question was explored by comparison of calculated transition states for the hydrolyses of aryl esters and the haptens intended to mimic them The addition-elimination reaction of phenyl acetate and hydroxide ion in the gas phase was examined using ab initio calculations at the RHF/6-31+G(d)//RHF/6-31+G(d) and MP2/6-31+G(d)//RHF/6-31+G(d) levels of theory Although transition states for both addition and elimination were located, the barrier to elimination disappeared when zero point energy or electron correlation was included The reaction is therefore concerted, and the transition state for addition is the only relevant transition state in the gas phase When aqueous solvation is considered through PCM and SCI-PCM calculations, the barrier to addition is shown to increase dramatically, and a stepwise addition-elimination pathway is predicted A single transition state for the analogous reaction with p-nitrophenyl acetate was also located, and this reaction is predicted to be concerted in both the gas phase and in solution The calculated geometries, CHELPG atomic charges, and electrostatic potential surfaces of stationary points involved in hydrolysis were compared with those of model phosphonate and phosphinate haptens The haptens are better mimics of the two transition states and the tetrahedral intermediate than of the reactants or products-exactly the situation desired for catalyst generation-yet they are more analogous to the tetrahedral intermediate and the transition state for elimination than to the rate-determining addition transition state In addition, the hydrogen-bonding pattern and asymmetry of the stationary points are not faithfully reproduced by the haptens These considerations suggest that catalysis may be improved by hapten redesign or antibody mutagenesis

40 citations

Journal ArticleDOI
TL;DR: The non-Markovian theory of activated rate processes developed by Carmeli and Nitzan is applied to investigate unimolecular reactions in condensed phases with particular emphasis on the molecular size dependence of the effect of solvent friction on the reaction rate.
Abstract: The non‐Markovian theory of activated rate processes developed by Carmeli and Nitzan is applied to investigate unimolecular reactions in condensed phases with particular emphasis on the molecular size (number of internal degrees of freedom) dependence of the effect of solvent friction on the reaction rate. The model consists of one reaction coordinate coupled to n−1 nonreactive modes. The molecule solvent interaction is treated within the context of the generalized Langevin equation. The reaction dynamics may be roughly described as two consecutive processes: the well (energy diffusion) dynamics where it is assumed that fast intramolecular vibrational relaxation and slower overall molecular energy diffusion dominate the process, and the barrier dynamics where it is assumed that the motion along the reaction coordinate is only weakly coupled to the nonreactive modes. This model leads to a result for the reaction rate which, as in the one‐dimensional case, is obtained as the inverse of the sum of two times:...

40 citations

Journal ArticleDOI
TL;DR: Since the reaction is barrier-less, exoergic, and all transition states involved are located below the energy level of the separated reactants, the formation of ethynylallene is predicted to take place in low temperature atmospheres of planets and their satellites such as Titan and also in cold molecular clouds via the neutral-neutral reaction of Ethynyl radicals with allene.
Abstract: The crossed beams reaction of ground state ethynyl radicals, C2H(X2Σ+), with allene, H2CCCH2(X1A1), was conducted under single collision conditions at a collision energy of 22.0 ± 0.4 kJ mol−1. The center-of-mass functions were combined with earlier ab initio calculations and revealed that the reaction was barrier-less, proceeded via indirect reaction dynamics through an addition of the ethynyl radical to the terminal carbon atom of the allene molecule, and was terminated by atomic hydrogen emission via a tight exit transition state to form the ethynylallene product. The overall reaction was found to be exoergic by 93 ± 15 kJ mol−1. Since the reaction is barrier-less, exoergic, and all transition states involved are located below the energy level of the separated reactants, the formation of ethynylallene is predicted to take place in low temperature atmospheres of planets and their satellites such as Titan and also in cold molecular clouds via the neutral–neutral reaction of ethynyl radicals with allene. Implications to interstellar chemistry and a comparison with the chemistry of the isoelectronic cyano radical, CN(X2Σ+), are also presented.

40 citations

Journal ArticleDOI
TL;DR: In this paper, the accuracy of the variable reaction coordinate (VRC) implementation of transition state theory (TSTT) is investigated for the bimolecular reaction of H with O2 via direct comparisons with quantum scattering theory for J=0, classical trajectory simulations for a wide range of J, and experimental canonical rate constants.
Abstract: The accuracy of the variable reaction coordinate (VRC) implementation of transition state theory (TST) is investigated for the bimolecular reaction of H with O2 via direct comparisons with quantum scattering theory for J=0, classical trajectory simulations for a wide range of J, and experimental canonical rate constants. The DMBE IV potential energy surface of Varandas and co‐workers is employed in each of the theoretical calculations. The first two comparisons indicate that the VRC‐TST approach overestimates the cumulative reaction probability (CRP) for this reaction by a factor of 2.3, roughly independent of E and J for moderate energies. The trajectory simulations further indicate that this failure of TST is primarily the result of the rapid redissociation of a large fraction of the initially formed HO2. An estimate for the quantum CRP on the basis of the combined dynamical and statistical results is seen to provide a useful alternative to the more standard quasiclassical trajectory estimates. A therma...

40 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202364
2022136
2021148
2020155
2019145
2018147