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.

Rate constants for the reactions of hydroxyl radicals and chlorine atoms with halogenated aldehydes

Abstract: Rate constants for the reactions of OH radicals and chlorine atoms with halogenated aldehydes of the type CX 3 CHO (X=H, Cl, or F) have been determined at 298±2 K. The OH radical rate data were determined using a pulsed laser photolysis resonance fluorescence technique at total pressures in the region 15-100 Torr and a conventional photolytic relative rate technique at atmospheric pressure. The chlorine atom rate data were determined using the relative rate method only. The rate data for reaction with both OH radicals and chlorine atoms indicate that increasing halogen atom substitution decreases the rate constant for reaction. Evidence is presented which suggests that the deactivation of these compounds is due mainly to polar effects in the transition states of the reactions induced by the halomethyl substituents, rather than decreassed overall reaction enthalpy

Reaction kinetics of ethylene glycol reforming over platinum in the vapor versus aqueous phases

Abstract: First-principles, periodic, density functional theory (DFT) calculations are carried out on Pt(111) to investigate the structure and energetics of dehydrogenated ethylene glycol species and transition states for the cleavage of C−H/O−H and C−C bonds. Additionally, reaction kinetics studies are carried out for the vapor phase reforming of ethylene glycol (C2H6O2) over Pt/Al2O3 at various temperatures, pressures, and feed concentrations. These results are compared to data for aqueous phase reforming of ethylene glycol on this Pt catalyst, as reported in a previous publication (Shabaker, J. W.; et al. J. Catal. 2003, 215, 344). Microkinetic models were developed to describe the reaction kinetics data obtained for both the vapor-phase and aqueous-phase reforming processes. The results suggest that C−C bond scission in ethylene glycol occurs at an intermediate value of x (3 or 4) in C2HxO2. It is also found that similar values of kinetic parameters can be used to describe the vapor and aqueous phase reforming ...

Diels-Alder reactions of allene with benzene and butadiene: concerted, stepwise, and ambimodal transition states.

Abstract: Multiconfigurational complete active space methods (CASSCF and CASPT2) have been used to investigate the (4 + 2) cycloadditions of allene with butadiene and with benzene. Both concerted and stepwise radical pathways were examined to determine the mechanism of the Diels–Alder reactions with an allene dienophile. Reaction with butadiene occurs via a single ambimodal transition state that can lead to either the concerted or stepwise trajectories along the potential energy surface, while reaction with benzene involves two separate transition states and favors the concerted mechanism relative to the stepwise mechanism via a diradical intermediate.

Lewis acid catalysis alters the shapes and products of bis-pericyclic Diels-Alder transition states.

Abstract: The reactions of cyclopentadiene with α-keto-β,γ-unsaturated phosphonates or with nitroalkenes proceed through an unsymmetrical bis-pericyclic transition state to give both Diels−Alder and hetero-Diels−Alder cycloadducts. The change in periselectivity of the Lewis acid catalyzed reactions can be qualitatively rationalized by the change in the potential energy surface (PES) landscapes, which ultimately affects the branching ratio of these bis-pericyclic reactions.

Experimental Proof of the Non-Least-Motion Cycloadditions of Dichlorocarbene to Alkenes: Kinetic Isotope Effects and Quantum Mechanical Transition States

Abstract: High-precision experimental kinetic isotope effects were measured at natural abundance for the cycloaddition of dichlorocarbene to 1-pentene. These values were compared to isotope effects predicted for the addition of dichlorocarbene to propene at the B3LYP/6-31G* and B3LYP/6-311+G* levels. The results provide unambiguous evidence for a nonlinear attack in which bond formation is more advanced at the unsubstituted carbon of the terminal alkene in the transition state of the cycloaddition. This is the first experimental confirmation of the unsymmetrical, non-least-motion approach of a carbene to an alkene first proposed by Skell over 40 years ago and predicted by quantum mechanical calculations.