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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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Journal ArticleDOI
TL;DR: The ability of uranium monoxide cations, UO+ and UO2+, to activate the O-H bond of H2O was studied by using two different approaches of the density functional theory, using relativistic small-core pseudopotentials and B3LYP hybrid functional.
Abstract: The ability of uranium monoxide cations, UO+ and UO2+, to activate the O-H bond of H2O was studied by using two different approaches of the density functional theory. First, relativistic small-core pseudopotentials were used together with B3LYP hybrid functional. In addition, frozen-core PW91-PW91 calculations were performed within the ZORA approximation. A close description of the reaction mechanisms leading to two different reaction products is presented, including all the involved minima and transition states. Different possible spin states were considered as well as the effect of spin-orbit interactions on the transition state barrier heights. The nature of the chemical bonding of the key minima and transition states was studied by using topological methodologies (ELF, AIM). The obtained results are compared with experimental data, as well as with previous studies on the reaction of the bare uranium cations with water, to analyze the influence of the oxo-ligand in reactivity.

76 citations

Journal ArticleDOI
TL;DR: The results obtained show that rotational excitation decreases the reaction probability even when comparing reaction probabilities at equivalent collision energies, and can nicely be explained by a transition state view which considers the vibrational states of the activated complex.
Abstract: Full-dimensional quantum dynamics calculations studying all initial state-selected reaction probabilities of the H+CH4→H2+CH3 reaction relevant at total energies below 0.58 eV are presented. The calculations employ a flux correlation function based approach to obtain the initial state-selected reaction probabilities: A complete set of wavepackets is generated at the top of the reaction barrier and propagated into the reactant asymptotic region. The results obtained show that rotational excitation decreases the reaction probability even when comparing reaction probabilities at equivalent collision energies. The efficiency of different types of reactant vibrational energy in supporting the reaction processes can nicely be explained by a transition state view which considers the vibrational states of the activated complex.

76 citations

Journal ArticleDOI
TL;DR: The solvent dependence of the free energy barrier and of the (13)C and (18)O kinetic isotope effects using a quantum mechanical solvation model based on class IV charges and semiempirical atomic surface tensions provides a consistent interpretation of the experimental results and provides a striking confirmation of the soundness of the solvation modeling.
Abstract: The rate constants and kinetic isotope effects for decarboxylation of 4-pyridylacetic acid depend strongly on whether the solvent is water or dioxane, and the present paper interprets this finding. We calculate the solvent dependence of the free energy barrier and of the 13C and 18O kinetic isotope effects using a quantum mechanical solvation model based on class IV charges and semiempirical atomic surface tensions. The calculations provide a consistent interpretation of the experimental results, which provides a striking confirmation of the soundness of the solvation modeling. Even more significantly, the agreement of theory and experiment gives us confidence in the physical picture of the reaction provided by the model. This indicates that the location of the transition state, as measured by the length of the breaking C−C bond, is 0.24 A later than the gas phase in dioxane and 0.37 A later than the gas phase in water. Charge development at the transition state also depends strongly on the solvent; in pa...

76 citations

Journal ArticleDOI
TL;DR: In this article, it was shown that if a hydride ion is split off from the paraffin, then the transition state resembles the adsorbed carbonium ion and the reaction results in molecular hydrogen and in formation of the surface alkoxy group.
Abstract: HF-21G quantum-chemical analysis of the protolytic attack of acid protons in zeolites at the C-H bonds in methane and ethane indicated that the resulting transition states depend on the sign of the bond polarization. If a hydride ion is split off from the paraffin, then the transition state resembles the adsorbed carbonium ion and the reaction results in molecular hydrogen and in formation of the surface alkoxy group. The case, when a proton tends to split off from the paraffin, corresponds to the hetero-isotope exchange of paraffins with surface OH groups. This is a concerted acid-base reaction with a transition state different from adsorbed carbonium ion.

75 citations

Journal ArticleDOI
TL;DR: The rationale for a change in mechanism in the more basic EtO(-)/EtOH nucleophile/solvent system by a stepwise mechanism instead of a concerted one in aqueous base is established.
Abstract: We report on a spectrophotometric kinetic study of the effect of Li+ and K+ cations on the ethanolysis of 4-nitrophenyl dimethylphosphinate (4a) in ethanol at 25 °C. The nucleophilic displacement reaction of 4a with LiOEt and KOEt in the absence and presence of 18-crown-6 ether (18-C-6) furnished observed first-order rate constants which increase in the order EtO− δGip for Li+ and δGts ∼ δGip for K+. These results indicate moderate catalysis by Li+, with 4a manifesting lesser susceptibility to catalysis than other substrates previously studied. Second-order rate constants for the reaction of the aryl dimethylphosphinates 4a–f with free EtO− were obtained from plots of log kobsvs. [KOEt], measured in the presence of excess 18-C-6. Hammett plots with σ and σ° substituent constants give significantly better correlation of rates than σ− and yield a moderately large ρ(ρ°) value; this is interpreted in terms of a stepwise mechanism involving rate-limiting formation of a pentacoordinate intermediate. Comparison of the present results with those of Williams on the aqueous alkaline hydrolysis of Me2P(O)–OPhX and Ph2P(O)–OPhX esters, establishes the rationale for a change in mechanism in the more basic EtO−/EtOH nucleophile/solvent system by a stepwise mechanism instead of a concerted one in aqueous base. Structure–reactivity correlations following Jencks show that the change in mechanism is accounted for by cross interactions between the nucleophile and the leaving group in the transition state. The observed duality of mechanism is rationalized on the basis of the More O'Ferrall–Jencks diagram, as a spectrum of transition states covering a wide range of nucleophile and leaving group basicities.

75 citations


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