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.

Theoretical Study of Bimolecular Reactions between Carbenium Ions and Paraffins: The Proposal of a Common Intermediate for Hydride Transfer, Disproportionation, Dehydrogenation, and Alkylation

Abstract: The mechanism of the bimolecular reactions of ethyl cation with ethane and propane and of s-propyl cation with ethane, propane, and isopentane is theoretically investigated by means of the B3PW91 density functional method. The study includes complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved, calculation of the reaction enthalpies and activation energies for the different elemental steps, and obtainment of the equilibrium constants and relative reaction rate constants by means of transition state theory. It is found that the interaction of a carbenium ion with a saturated alkane always results in formation of a stable carbonium ion intermediate and that different intramolecular rearrangements of this common intermediate can explain the mechanism of acid-catalyzed hydrocarbon reactions, such as hydride transfer, disproportionation, dehydrogenation, and alkylation.

SN2 reaction at neutral nitrogen: transition state geometries and intrinsic barriers.

Abstract: The reactants, ion-dipole complexes, transition states, and products for the nucleophilic displacement reactions X - +H 2 NY→H 2 NX+Y - have been optimized at the ab initio DZP+/SCF level of theory for X, Y=F, Cl, OH, CN, and H. The intrinsic barriers for the degenerate reactions (25.3, 22.9, 38.8, and 75.3 kcal/mol for X=Y=F, Cl, OH, and CN, respectively) are larger than the corresponding values for carbon species. The intrinsic barriers ΔE X,Y correlate with the degree of the N-X and N-Y bond elongations in the transition structures. Both intrinsic and overall barriers can be interpreted with the aid of Marcus theory

Experimental and Theoretical Multiple Kinetic Isotope Effects for an SN2 Reaction. An Attempt to Determine Transition‐State Structure and the Ability of Theoretical Methods to Predict Experimental Kinetic Isotope Effects

Abstract: The secondary alpha-deuterium, the secondary beta-deuterium, the chlorine leaving-group, the nucleophile secondary nitrogen, the nucleophile (12)C/(13)C carbon, and the (11)C/(14)C alpha-carbon kinetic isotope effects (KIEs) and activation parameters have been measured for the S(N)2 reaction between tetrabutylammonium cyanide and ethyl chloride in DMSO at 30 degrees C. Then, thirty-nine readily available different theoretical methods, both including and excluding solvent, were used to calculate the structure of the transition state, the activation energy, and the kinetic isotope effects for the reaction. A comparison of the experimental and theoretical results by using semiempirical, ab initio, and density functional theory methods has shown that the density functional methods are most successful in calculating the experimental isotope effects. With two exceptions, including solvent in the calculation does not improve the fit with the experimental KIEs. Finally, none of the transition states and force constants obtained from the theoretical methods was able to predict all six of the KIEs found by experiment. Moreover, none of the calculated transition structures, which are all early and loose, agree with the late (product-like) transition-state structure suggested by interpreting the experimental KIEs.

Ab initio studies of the radical cation diels-alder reaction

Abstract: The radical cation Diels−Alder reaction of the 1,3-butadiene radical cation with ethylene, yielding the cyclohexene radical cation, was studied by B3LYP hybrid functional and QCISD(T)//QCISD calculations using the 6-31G* basis set. The intermediates and transition states involved in three different mechanisms, a concerted Cs-symmetric and a stepwise unsymmetric anti [4 + 2] pathway and a stepwise unsymmetric out-gauche pathway leading to vinylcyclobutane, have been considered. The synchronous Cs-symmetric pathway is prevented by a pseudo-Jahn−Teller distortion and is 19 kcal/mol higher in energy than the stepwise pathways. The stepwise anti pathway was found to be the lowest-energy pathway with an activation energy of 0.3 kcal/mol relative to the initially formed ion−molecule complex. The gauche-out pathway, leading to vinylcyclobutane, is 3.5 kcal/mol higher in energy than the anti pathway, leading to cyclohexene. In contrast to earlier calculations by Bauld at the MP2/6-31G*//3-21G level of theory, an i...

Theoretical study on the detailed reaction mechanisms of carbonyl oxide with formic acid

Abstract: The reaction mechanisms of carbonyl oxide with formic acid are investigated using the B3LYP/6-311G(d,p) and CBS-QB3 theoretical methods. The investigation encompasses the eight complexes formed between carbonyl oxide and formic acid, the initial transition states responsible for the formation of these transitory products including hydroxylated ozonide and hydroperoxymethyl formate, the cleavages of the transitory intermediates and the interconversion between the transitory products and between syn-formic acid anhydride and anti-formic acid anhydride. The calculated results predict that the binding energy of the most stable complex in the eight complexes is −11.0 kcal/mol, which indicates that the formed pre-complexes are of special important for the reaction carbonyl oxide with formic acid. In addition, the barrier heights of the transition states that lead to the hydroxylated ozonide and hydroperoxymethyl formate are −0.5 kcal/mol, −1.3 kcal/mol, respectively, at the CBS-QB3 level of theory, which shows that the two reaction channels contribute to the transitory product formation. In addition, under some circumstances, the cleavages of transitory products result in the formation of the anti-formic acid anhydride, which is in well agreement with experimental predictions. It is noted that splits of hydroxylated ozonide are responsible for the formation of formic acid.