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Showing papers on "Transition state published in 1985"


Journal ArticleDOI
TL;DR: In this paper, the implementation of RRKM theory for unimolecular dissociations having transition states of any degree of looseness is described for reactions involving dissociation into two fragments.
Abstract: Implementation of RRKM theory for unimolecular dissociations having transition states of any degree of looseness is described for reactions involving dissociation into two fragments. The fragments may be atomic, diatomic, or polyatomic species. Action-angle and internal coordinates for the transitional modes of the reaction, transformations to Cartesian coordinates, and other calculational aspects are described. Results for the NO2-->NO+O reaction are presented, including the dependence of the microcanonical rate constant on the bond fission and bending potentials for model potential energy surfaces. Illustrative calculations for the C2H6-->2CH3 reaction are also given.

211 citations


Journal ArticleDOI
TL;DR: In this paper, a comprehensive theoretical study has been made of the energetics of the important pathways involved in the reaction of hydrogen atoms with hydrogen cyanide, including the hydrogen abstraction reaction and the hydrogen addition reaction.
Abstract: A comprehensive theoretical study has been made of the energetics of the important pathways involved in the reaction of hydrogen atoms with hydrogen cyanide. For each reaction ab initio GVB‐CI calculations were carried out to determine the structures and vibrational frequencies of the reactants, transition states, and products; then POL‐CI calculations were carried out to more accurately estimate the electronic contribution to the energetics of the reactions. The hydrogen abstraction reaction is calculated to be endoergic by 24 kcal/mol [expt. ΔH (0 K)=16–19 kcal/mol] with a barrier of 31 kcal/mol in the forward direction and 6 kcal/mol in the reverse direction. For the hydrogen addition reactions, addition to the carbon atom is calculated to be exoergic by 19 kcal/mol with a barrier of 11 kcal/mol, while addition to the nitrogen center is essentially thermoneutral with a barrier of 17 kcal/mol. Calculations were also carried out on the isomerization reactions of the addition products. The cis→trans isome...

98 citations


Journal ArticleDOI
TL;DR: In this paper, a theoretical study has been made of acetylene and ethylene adsorption and rearrangement reactions on Pt(111) and the structures, energies, and transition states are determined for the reaction scheme and compared with results of numerous experimental studies.

72 citations


Journal ArticleDOI
TL;DR: Emphasis has been placed on the geometry of the resulting potential-energy hypersurface for the system as a whole to explain temperature dependence of short-range order, and to characterize bistable degrees of freedom that dominate low-temperature properties in this, and other, amorphous solids.
Abstract: Molecular-dynamics computer simulations have been carried out to study the liquid and amorphous solid states for a system comprising 120 Ni and 30 P atoms. This study utilized additive central pair potentials to model the interactions. Emphasis has been placed on the geometry of the resulting potential-energy hypersurface for the system as a whole to explain temperature dependence of short-range order, and to characterize bistable degrees of freedom that dominate low-temperature properties in this, and other, amorphous solids. A small collection of transition states and associated reaction coordinates for such degrees of freedom has been numerically constructed. The corresponding atomic motions tend to be localized mainly on a small subset of the atoms, but are diverse insofar as which chemical species are involved, in barrier height and asymmetry, and in the nonlinearity of the collective reaction coordinate.

70 citations



Journal ArticleDOI
TL;DR: In this article, the authors applied improved canonical and microcanonical variational transition state theories to the calculation of the rate constants for the polyatomic reaction OH+H2→H2O+H over the temperature range 200-2400 K using the Schatz-Elgersma fit to the Walch-Dunning ab initio potential energy surface.
Abstract: We have applied improved canonical and microcanonical variational transition state theories to the calculation of the rate constants for the polyatomic reaction OH+H2→H2O+H over the temperature range 200–2400 K using the Schatz–Elgersma fit to the Walch–Dunning ab initio potential energy surface. The results are compared to canonical variational transition state theory calculations that employed the same potential energy surface and to experiment. We find that the new results, which are in good agreement with experiment, differ very slightly from those obtained with canonical variational theory. One explanation for this agreement is that the microcanonical variational transition states have a rather weak energy dependence and lie within 0.04a0 of the ground‐state adiabatic barrier maximum. We also find that quantum mechanical effects and the inclusion of reaction‐path curvature are important at lower temperatures, and that the transition state theory treatment of this reaction breaks down for temperatures higher than about 2400 K.

41 citations


Journal ArticleDOI
TL;DR: In this article, a kinetic formalism based on the concept of steric or strain volume of activation is proposed for the calculation of structure effects on the position of the transition state along the reaction coordinate.
Abstract: This paper considers the description of the pressure effect on strained transition states and proposes a kinetic formalism based on the concept of steric or strain volume of activation with a view to devising models allowing the calculation of structure effects on the position of the transition state along the reaction coordinate. The applicability of the equation is examined for several reactions (Menshutkin, solvolysis and hydrogen transfer). When steric or strain factors operate in a given reaction the pressure effect is more marked than in an unhindered analogue. Possible consequences of this phenomenon are considered, with special emphasis on mechanistic and synthetic implications at high pressure.

36 citations


Journal ArticleDOI
22 Feb 1985-Science
TL;DR: There is increasing excitement concerning the potential of laser methods to interrogate the transition states of molecular reactions.
Abstract: Lasers are used in increasingly sophisticated ways to carry out reactions between molecules in selected vibrational, rotational, and electronic states and to probe the product states of chemical reactions. Such investigations are providing unprecedented insights into chemical reaction dynamics, the study of the detailed motions that molecules undergo in simple chemical reactions. In many cases it is possible to describe the influence that specific types of molecular excitation have on reactive events. Experiments are also being carried out to leam about chemical reactivity as a function of the alignment of reagents. There is increasing excitement concerning the potential of laser methods to interrogate the transition states of molecular reactions.

29 citations



Journal ArticleDOI
TL;DR: In this paper, the Renner-Teller effect was applied to linear electronically degenerate transition states in the context of transition state theory and the magnitude of the effect was shown to be significant for the O(3P)+H2→OH+H reaction at room temperature.
Abstract: The inclusion of the Renner–Teller effect in the treatment of linear electronically degenerate transition states in the context of transition state theory is indicated. The magnitude of the effect is shown to be significant for the O(3P)+H2→OH+H reaction at room temperature. Various approximations to the Renner–Teller effect which employ decoupled potential energy surfaces are evaluated over a range of values likely to be encountered in transition states.

16 citations


Journal ArticleDOI
TL;DR: In this article, the authors derived a model free energy surface, which includes solvent polarization as one of the coordinates, and showed how that surface can account for the observed behavior of these methyl-transfers.
Abstract: Many experimental observations imply that transition states for methyl-transfers to water have little or no water—methyl covalent bonding and that the rate-determining process is predominantly or entirely a fluctuation in solvent structure. Currently accepted descriptions of methyl-transfer mechanisms cannot easily account for such a transition state structure or for the anomalous selectivities observed in these reactions. This paper derives a model free energy surface, which includes solvent polarization as one of the coordinates, and shows how that surface can account for the observed behavior of these methyl-transfers. The transition state on that surface has a structure in which the solvent polarization is not in equilibrium with the internal charge distribution.

Journal ArticleDOI
TL;DR: In this article, the mechanism of the Cannizzaro reaction has been studied using the MNDO SCF-MO method and the results discussed in terms of the symmetry and properties of the transition states.
Abstract: The mechanism of the Cannizzaro reaction has been studied using the MNDO SCF-MO method. A hydride transfer between the intermediate (1) to the aldehyde (2)(R =R1=H) was calculated to proceed via a symmetrical transition state with a barrier of 72 kJ mol–1. Alternative mechanisms involving a single-electron transfer between (1) and (2) are predicted to be less favourable, particularly for aliphatic aldehydes. A radical chain mechanism involving a single-electron-transfer step is proposed as an alternative. Mechanisms involving dianions are discussed, in which single-electron transfers are particularly favourable, and radicals such as H˙ should be formed at high pH. Primary hydrogen kinetic isotope effects are calculated for all the steps involving hydrogen transfers and the results discussed in terms of the symmetry and properties of the transition states.

Journal ArticleDOI
TL;DR: In this paper, the morphology of the dynamic electron transfer characteristic of chemical reaction dynamics has been studied by introducing the new criterion of an isomorphism of the manifold of electron orbitals at different points of the reaction coordinate.
Abstract: The morphology of the dynamic electron transfer characteristic of chemical reaction dynamics has been studied by introducing the new criterion of an isomorphism of the manifold of electron orbitals at the different points of the reaction coordinate. This criterion is a natural development of the famous Amos–Hall corresponding orbital. Natural orbitals are obtained from the dynamic Fock equation, then it is possible to describe the electronic process of a chemical reaction by the way that the occupation number of an electron orbital changes within the electron orbital manifold which is constructed to have isomorphic equivalency along the reaction coordinate. The electronic process accompanying the chemical reaction CH3+HF → CH4+F has been studied as a demonstration of the present theory. The electron orbital which is transformed by the new criterion is strongly localized, though the transformation operator itself does not contain any explicit localization terms, such as self‐energy integrals. The transitio...

Journal ArticleDOI
TL;DR: In this article, the first-order solvolysis of 1,2-chlorothiocyanatobis(1, 2-diaminoethane)cobalt(III) ions have been measured for a range of temperatures in water and in water + propan-2-ol.
Abstract: Rates of the first-order solvolysis of 1,2-chlorothiocyanatobis(1,2-diaminoethane)cobalt(III) ions have been measured for a range of temperatures in water and in water + propan-2-ol. A plot of log (rate constant) against the Y factor is linear, but a plot of log (rate constant) against the reciprocal of the dielectric constant is curved. From a comparison of the variation of the enthalpy and entropy of activation with the physical properties of the solvent mixture it is concluded that solvent structure is an important factor determining solvolytic reactivity. The evidence currently available concerning the nature of the transition state in substitution reactions and, in particular, the solvolytic transition states of CoIII complexes is assessed. The influence of values for the free energy of transfer of ions on the assignment of the contributions of solvent structure to the initial and transition states for substitution reactions is discussed. As a result of the application of a free-energy cycle relating the process initial state → transition state in water to that in the mixture, it is concluded that the influence of solvent structure predominates on the pentacoordinated cation in the transition state over its influence on the hexacoordinated cation in the initial state.

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the dehydrogenation reaction paths of methanethiol CH3SH by abinitio MO calculations and determined the geometries and energies of the reactants, transition states, and products on the singlet potential energy surface of the ground state.
Abstract: The dehydrogenation reaction paths of methanethiol CH3SH were analyzed by abinitio MO calculations. The geometries and energies of the reactants, transition states, and products have been determined on the singlet potential energy surface of the ground state. We also analyze the similar dehydrogenation reactions of methanol (CH3OH) and ethanol (CH3CH2OH) for the purpose of comparison. The activation energy and the heat of reaction calculated by incorporating the effect of electronic correlation shows that the product H2C=S is chemically more unstable than the product H2C=O. "Reaction ergodography" along the intrinsic reaction coordinate (IRC) for these dehydrogenation paths clarifies the distinct differences in atomic movement. The path of methanethiol has two characteristic steps before the transition state, while neither paths of the two carbinols have such multisteps. The electronic intramolecular interaction between subsystems of the reactant molecule is investigated in terms of Interaction Hybrid Mol...

Journal ArticleDOI
TL;DR: In this article, the solvent dependence of proton abstraction from various nitrophenylmethanes has been examined for aqueous dimethyl sulphoxide and methanolic dimethyl sulfoxide solutions.
Abstract: The solvent dependence of proton abstraction from various nitrophenylmethanes has been examined for aqueous dimethyl sulphoxide and methanolic dimethyl sulphoxide solutions. Even though the compounds studied vary considerably in thermodynamic acidity (pKa), their proton-abstraction rates all show the same solvent dependence. It is suggested that the transition states for these reactions all occur at similar positions on the reaction pathway, and that transition state imbalances exist for these proton transfers.

Journal ArticleDOI
TL;DR: In this article, photolysis of 4-(N,N-dibenzylamino)-2-phenylbut-1-ene and 5(N, N-Dibenzenylamino) 2 -phenylpent-1 -ene gave cyclisation products via 1,6-and 1,7-hydrogen shifts respectively.
Abstract: Photolysis 4-(N,N-dibenzylamino)-2-phenylbut-1-ene and 5-(N,N-dibenzylamino)-2-phenylpent-1-ene gave cyclisation products via 1,6- and 1,7-hydrogen shifts respectively.

Journal ArticleDOI
TL;DR: In this paper, two probable pathways for the gas-phase hydrolysis of hydrogen isocyanate have been determined using the MINDO/3 method, and activation barriers obtained showed that the one-step mechanism is preferred to the two-step process involving an intermediate.
Abstract: Transition states for the two probable pathways for the gas-phase hydrolysis of hydrogen isocyanate have been determined using the MINDO/3 method. Activation barriers obtained showed that the one-step mechanism is preferred to the two-step mechanism involving an intermediate. It was shown that the reaction of polymeric water has lower activation barrier than the reaction of monomeric water. Energy and charge decomposition analysis showed that in the former less energy is required in the deformation of molecules for the transition state (TS) formation due to the cyclic flow of electronic charges in the TS.

Journal ArticleDOI
TL;DR: In this paper, a structurally stable model of the standard adiabatic gradient field of the potential energy surface for certain pericyclic reactions is derived, which are not subjected to the principles of orbital isomerism or to the Woodward-Hoffmann rules.
Abstract: A structurally stable model of the standard adiabatic gradient field of the potential energy surface for certain pericyclic reactions is derived. These reactions are not subjected to the principles of orbital isomerism or to the Woodward-Hoffmann rules. Use is made of a principle established by Ariel Fernandez and Oktay Sinanoglu which precludes direct meta-IRC connections between transition states. It is shown that Jahn-Teller isomers of the singlet biradicals involved in the process are not interconvertible since the biradical configuration is not a transition state but a critical point with Hessian matrix with two negative eigenvalues. The topological features of the PES obtained by combinatorial methods are in full agreement with earlier results obtained from MINDO calculations.

Journal ArticleDOI
TL;DR: In this article, the authors assume that the branching into the different product states is independent of the initial quantum states of the reactants, and derive an expression for the state-to-state cross section in terms of the stateto-all one, as well as microcanonical rate constants.
Abstract: We assume that, having arrived at the transition state, the branching into the different product states is independent of the initial quantum states of the reactants. This assumption plus the familiar transition state approximation (that the reaction rate is the rate of the passage across the barrier) yields an expression for the state‐to‐state cross section in terms of the state‐to‐all one, as well as microcanonical rate constants. Models, adiabatic correlations, purely statistical considerations, or collinear computations can provide the required input for the theory. Exact quantal computations on the 3D H + H2 reaction are found to satisfy the assumed factorization quite well. Furthemore, reaction probabilities derived from a line‐of‐centers model, with a barrier height dependent on the approach angle, account for the probabilities derived from the exact quantal computation.

Journal ArticleDOI
TL;DR: In this article, it was shown that the ring closure rule for such systems is not incompatible with recent theoretical predictions of an obtuse approach angle for a nucleophile attacking a carbon-carbon triple bond.
Abstract: Exothermic reactions can have late transition states if the reactant molecule is subject to facile distortion. This is exemplified by the ring closure reaction of the 4-oxo-butynylate ion. Ab initio calculations on this system are reported. It is shown that the ring closure rule for such systems is not incompatible with recent theoretical predictions of an obtuse approach angle for a nucleophile attacking a carboncarbon triple bond.

Journal ArticleDOI
TL;DR: In this article, the authors studied the SN2 reaction between glycine and ammonia molecules with magnesium cation Mg2+ as a catalyst, and the stationary points of each reaction including intermediate and transition states have been identified and free energies calculated for all geometry-optimized reaction species to determine the thermodynamics and kinetics.
Abstract: The SN2 reaction between glycine and ammonia molecules with magnesium cation Mg2+ as a catalyst has been studied as a model reaction for Mg(2+)-catalyzed peptide bond formation using the ab initio Hartree-Fock molecular orbital method. As in previous studies of the uncatalyzed and amine-catalyzed reactions between glycine and ammonia, two reaction mechanisms have been examined, i.e., a two-step and a concerted reaction. The stationary points of each reaction including intermediate and transition states have been identified and free energies calculated for all geometry-optimized reaction species to determine the thermodynamics and kinetics of each reaction. Substantial decreases in free energies of activation were found for both reaction mechanisms in the Mg(2+)-catalyzed amide bond formation compared with those in the uncatalyzed and amine-catalyzed amide bond formation. The catalytic effect of the Mg2+ cation is to stabilize both the transition states and intermediate, and it is attributed to the neutralization of the developing negative charge on the electrophile and formation of a conformationally flexible nonplanar five-membered chelate ring structure.

Journal ArticleDOI
TL;DR: In this article, a tricentric interaction has been found in the STO-3G basis set, and the appearing of such structure is discussed and justified, by means of ab initio calculations.
Abstract: Ethylene dimerization has been studied by means of ab initio calculations using the STO-3G basis set. A transition state that presents a tricentric interaction has been found. The appearing of such structure is discussed and justified.

Journal ArticleDOI
TL;DR: In this paper, the transition states for singlet methylene to hydroxyethylene and acrolein were determined by using the MINDO/3 method, and the results led to the conclusions that the simultaneous use of reduced surfaces and of direct location is the best strategy to obtain the true transition state of a reaction.
Abstract: The transition states for the addition of singlet methylene to hydroxyethylene and acrolein have been determined by using the MINDO/3 method. It is found that the ‘inward’ approach of methylene to the unsubstituted carbon atom of the alkene is the most favourable one. The results obtained lead to the conclusions that the simultaneous use of reduced surfaces and of direct location is the best strategy to obtain the true transition state of a reaction.