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 reaction electronic flux: a new descriptor of the electronic activity taking place during a chemical reaction. Application to the characterization of the mechanism of the schiff's base formation in the maillard reaction

Abstract: Within the framework defined by the reaction force analysis, the reaction electronic flux, a new descriptor proposed to characterize the electronic activity that takes place during a chemical reaction, is used to elucidate the mechanism of the Schiff’s base formation in the Maillard reaction The electronic activity is identified as being due to electronic polarization and transfer effects that show up at specific regions along the reaction coordinate It is found that the Schiff base formation step of the Maillard reaction proceeds through consecutive electron polarization and transfer processes, the energy involved at each step of the reaction is quantified through the reaction works obtained from the reaction force profile

Computational study of the reactions of methanol with the hydroperoxyl and methyl radicals. 2. Accurate thermal rate constants.

Abstract: Multistructural canonical variational-transition-state theory with multidimensional tunneling (MS-CVT/MT) is employed to calculate thermal rate constants for the abstraction of hydrogen atoms from both positions of methanol by the hydroperoxyl and methyl radicals over the temperature range 100–3000 K. The M08-HX hybrid meta-generalized gradient approximation density functional and M08-HX with specific reaction parameters, both with the maug-cc-pVTZ basis set, were validated in part 1 of this study (Alecu, I. M.; Truhlar, D. G. J. Phys. Chem. A2011, 115, 2811) against highly accurate CCSDT(2)Q/CBS calculations for the energetics of these reactions, and they are used here to compute the properties of all stationary points and the energies, gradients, and Hessians of nonstationary points along each considered reaction path. The internal rotations in some of the transition states are found to be highly anharmonic and strongly coupled to each other, and they generate multiple structures (conformations) whose c...

Matrix-Isolation Fourier Transform Infrared and Theoretical Studies of Laser-Ablated Sc Atom Reactions with Water Molecules

Abstract: Laser-ablated Sc atoms have been reacted with water molecules during condensation with argon at 11 K. In agreement with previous thermal atom reactions, absorptions at 1482.6 and 713.0 cm-1 are assigned to the HScOH molecule formed via an insertion reaction. Photolysis of the HScOH produces the metal monoxide ScO. In addition, new absorption at 765.6 cm-1 is also observed and is assigned to the ScOH molecule. The ScOH molecule undergoes photoinduced rearrangement to the HScO molecule, which is characterized by Sc−H and Sc−O stretching vibrations at 1391.1 and 922.3 cm-1. Density functional theoretical calculations have been performed for the aforementioned species, and important transition states on the reaction paths have been obtained.

Studies on the biosynthesis of paraherquamide A and VM99955. A theoretical study of intramolecular Diels-Alder cycloaddition.

Abstract: Intramolecular Diels-Alder reactions of 2-azadiene models have been studied quantum chemically at the B3LYP/6-31G level in order to elucidate the stereochemical features of the cyclization step involved in the biosynthesis of paraherquamide A and VM99955. These cycloadditions take place through concerted transition states associated with [4 + 2] processes. Analysis of the energies along the competitive paths reveals that while the cycloadditions of the oxindoles present a large anti selectivity, the indoles show a low syn selectivity for the formation of the C20 stereogenic center that is larger for the reduced tertiary amide form. The presence of the C14 methyl of the beta-methylproline ring produces a low hindrance along the reaction coordinate for the syn approach of the isoprene framework, in agreement with the low facial selectivity found experimentally. An analysis of the electrophilicity and activation parameters for experimental models of the inter- and intramolecular Diels-Alder reactions reveals several significant factors controlling these biosynthetic cyclizations. The results are in reasonable agreement with the available experimental data.

Palladium-catalysed [3+2] cycloaddition of alk-5-ynylidenecyclopropanes to alkynes: A mechanistic DFT study

Abstract: The mechanism of the palladium-catalysed [3+2] intramolecular cycloaddition of alkylidenecyclopropanes to alkynes has been computationally explored at DFT level The energies of the reaction intermediates and transition states for different possible pathways have been calculated in a model system that involves the use of PH3 as a ligand The results obtained suggest that the most favourable reaction pathway involves the initial C--C oxidative addition of the cyclopropane to a Pd0 complex to give an alkylidenepalladacyclobutane, which isomerises to a methylenepalladacyclobutane intermediate Subsequent cyclisation by alkyne carbometallation, followed by reductive elimination affords the final product An alternative mechanism consisting of a palladaene-type rearrangement is less probable in terms of Gibbs energy, but cannot be fully discarded because it is competitive if one considers electronic energies For substrates that present an ester group at the terminal position of the triple bond we have found an alternative, more favourable mechanistic route that explains why the [3+2] cycloaddition of these types of systems does not lead to the expected cycloadducts