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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: In this paper, the AM1 hamiltonian was used to estimate aqueous pKa for 16 carboxylic acids and 15 phenols with experimental data taken from literature.
Abstract: Estimation of aqueous pKa through quantum chemical gas-phase and solution-phase calculations is investigated for 16 carboxylic acids and 15 phenols with experimental data taken from literature. Parameters based on the AM1 hamiltonian include enthalpy and free energy differences between the ground state of the ionizable compounds and their anion counterparts as well as respective transition states and intermediates along the reaction path of the aqueous proton transfer. With carboxylic acids, additional ab initio calculations are performed to evaluate the semiempirical level of theory. Aqueous solvation is modelled in three different ways: Application of continuum-solvation methods AM1-COSMO and AM1-SM2, microsolvation of the solutes through formation of clusters with three water molecules, and combination of both approaches to include both bulk water polarization and solute-solvent coupling effects. Regression equations with r values up to 0.93 for carboxylic acids and 0.96 for phenols suggest, that continuum-solvation models can be recommended to estimate aqueous pKa through electronic structure calculations.

50 citations

Journal ArticleDOI
TL;DR: Distortion/interaction and absolutely localized energy decomposition analyses provide new insights into why the (3 + 2) pathway is highly preferred over the (2 + 1) pathway, the origin of rate enhancement from ligated base, and reactivity differences between OsO (4), ReO(4)(-), TcO(-), and MnO( 4)(-).
Abstract: The reaction pathways (including the transition states) of ethylene addition to osmium tetroxide (OsO4, and amine ligated), rhenate (ReO4−), technetate (TcO4−), and permanganate (MnO4−) have been studied by qualitative and quantitative analyses. Distortion/interaction and absolutely localized energy decomposition analyses provide new insights into why the (3 + 2) pathway is highly preferred over the (2 + 2) pathway, the origin of rate enhancement from ligated base, and reactivity differences between OsO4, ReO4−, TcO4−, and MnO4−. The (2 + 2) transition state has a much larger barrier than the (3 + 2) transition state because (1) the Os−O bond is stretched significantly resulting in a larger distortion energy (ΔEd⧧) value and (2) the transition state interaction energy (ΔEi⧧) is destabilizing due to large exchange repulsions overwhelming stabilizing charge-transfer terms. Base ligation lowers osmium tetroxide and ethylene distortion energies due to the ground-state O−Os−O angle being predistorted from 110°...

50 citations

Journal ArticleDOI
TL;DR: In this paper, the formation of DMC from MeOH and CO2 under [Nb(OMe)5]2 catalysis follows a different route (acid-plus-base activation of methanol) with respect to other known catalytic systems such as Sn(IV) and dicyclohexyl carbodiimide (DCC) that promote a double base activation.
Abstract: The formation of dimethylcarbonate (DMC) from MeOH and CO2 under [Nb(OMe)5]2 catalysis follows a different route (“acid-plus-base activation” of methanol) with respect to other known catalytic systems such as Sn(IV) and dicyclohexylcarbodiimide (DCC) that promote a “double-base activation”. The reaction intermediates and related transition states obtained from density functional (DFT) calculations are presented. Experimental data also feature a different reaction mechanism.

50 citations

Journal ArticleDOI
TL;DR: In this article, the authors analyzed the thermal properties of the CH3C•(O)) + O2 reaction system with density functional and ab initio calculations to evaluate reaction paths and kinetics in both oxidation and pyrolysis.
Abstract: Thermochemical properties for reactants, intermediates, products, and transition states important in the acetyl radical (CH3C•(O)) + O2 reaction system are analyzed with density functional and ab initio calculations, to evaluate reaction paths and kinetics in both oxidation and pyrolysis. Enthalpies of formation (Δ ) are determined using isodesmic reaction analysis at the CBSQ composite and density functional levels. Entropies ( ) and heat capacities ( (T)) are determined using geometric parameters and vibrational frequencies obtained at the HF/6-31G(d‘) level of theory. Internal rotor contributions are included in S and Cp(T) values. The acetyl radical adds to O2 to form a CH3C(O)OO• peroxy radical with a 35 kcal/mol well depth. The peroxy radical can undergo dissociation back to reactants, decompose to products, CH2CO + HO2 via concerted HO2 elimination (Ea = 34.58 kcal/mol), or isomerize via hydrogen shift (Ea = 26.42) to form a C•H2C(O)OOH isomer. This C•H2C(O)OOH isomer can undergo β scission to prod...

49 citations

Journal ArticleDOI
TL;DR: Calculation of the reaction path for the enzymatic hydrolysis of the substrate by dUTPase for an active-site model with 85 atoms, including several loosely bound water molecules, indicates that the method is feasible for the study of enzyme mechanisms.
Abstract: We present a generalization of the reaction coordinate driven method to find reaction paths and transition states for complicated chemical processes, especially enzymatic reactions. The method is based on the definition of a subset of chemical coordinates; it is simple, robust, and suitable to calculate one or more alternative pathways, intermediate minima, and transition-state geometries. Though the results are approximate and the computational cost is relatively high, the method works for large systems, where others often fail. It also works when a certain reaction path competes with others having a lower energy barrier. Accordingly, the procedure is appropriate to test hypothetical reaction mechanisms for complicated systems and provides good initial guesses for more accurate methods. We present tests on a number of simple reactions and on several complicated chemical transformations and compare the results with those obtained by other methods. Calculation of the reaction path for the enzymatic hydrolysis of the substrate by dUTPase for an active-site model with 85 atoms, including several loosely bound water molecules, indicates that the method is feasible for the study of enzyme mechanisms.

49 citations


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