scispace - formally typeset
Search or ask a question

Showing papers on "Transition state published in 1975"


Journal ArticleDOI
TL;DR: In this paper, two different kinds of semiclassical approximations are used to evaluate a previously obtained quantum mechanical transition state theory rate expression for the collinear H+H2 reaction.
Abstract: Two different kinds of semiclassical approximations are used to evaluate a previously obtained quantum mechanical transition state theory rate expression. No assumptions, however, such as separability of the Hamiltonian, vibrationally adiabatic motion along a reaction coordinate, etc., are incorporated. Application is made to the collinear H+H2 reaction, and agreement with accurate quantum scattering calculations is found to be reasonably good. The results indicate that transition state theory—provided no assumptions of separability are included—is probably as accurate quantum mechanically as it has been found to be classically for describing the threshold of chemical reactions with an activation barrier.

142 citations


Journal ArticleDOI
TL;DR: Theoretical energy distributions of reaction products in molecular beam systems are described for reactions proceeding via transient complexes in this paper, where both loose and tight transition states are considered for the exit channel.
Abstract: Theoretical energy distributions of reaction products in molecular beam systems are described for reactions proceeding via transient complexes. Loose and tight transition states are considered for the exit channel. For a loose transition state and the case of l ≫ j, the result is the same as of Safron et al. For the case of a tight transition state exit channel effects are included analogous to steric effects for the reverse reaction. It is shown how, via one mechanism, bending vibrational energy of that transition state can contribute to the translational energy of the reaction products. Expressions are derived for the energy distributions of the products when l ≫ j and j ≫ l.

130 citations


Journal ArticleDOI
TL;DR: In this paper, an algorithm has been developed to calculate transition state geometries directly in the framework of ab initio single-determinant molecular orbital theory, which is applied to the rearrangement of ethenylidene to acetylene.

91 citations


Journal ArticleDOI
TL;DR: In this article, the authors examined two realistic potential energy surfaces for atom−diatomic molecule reactions for two reaction attributes: (1) vibrational energy of the products of a thermal energy exothermic reaction; (2) threshold energy for endothermic reactions of ground−state reagents.
Abstract: Collinear quasiclassical trajectories are examined for two realistic potential energy surfaces for atom−diatomic molecule reactions for two reaction attributes: (1) vibrational energy of the products of a thermal−energy exothermic reaction; (2) threshold energy for endothermic reaction of ground−state reagents. Eight different mass combinations are studied. The potential energy surfaces differ primarily in the amount of potential energy released in an exothermic reaction before and in the region of large curvature of the minimum−energy path and in the curvature of the repulsive potential energy contours when all three atoms are close. For attribute (1), we find the results are qualitatively correlated by the theory of Hofacker and Levine although, contrary to previous work, one potential energy surface shows high mixed energy release (in the language of Polanyi and co−workers) but low excitation to product vibration for five different mass combinations. For reaction attribute (2), we find one surface has a high translational threshold (or no reaction at any energy) for six mass combinations, while the other surface shows this behavior in only three cases. Thus, this type of surface provides an exception to previous generalizations that extra vibrational energy is required for very endothermic reactions with late barriers. This demonstrates the importance of the location of the curvature of the reaction channel for such reaction attributes. Very accurate determinations of potential energy surfaces will be required to make reliable predictions of reaction attributes such as (1) and (2) for real systems. Analysis of the details of the trajectories shows that the high threshold can generally be attributed to reflection before the saddle point of the surface rather than to recrossing the saddle point region. The vibrational excitation of reagents in nonreactive collisions is also strongly effected by curvature of the minimum−energy path.

86 citations




Journal ArticleDOI
TL;DR: The comparison of activated complex structures should be based on the evaluation of potential energies for initial, final, and transition states.
Abstract: The comparison of activated complex structures should be based on the evaluation of potential energies for initial, final, and transition states.

59 citations


Journal ArticleDOI
TL;DR: In this article, high pressure kinetics appears to be a valuable tool in investigating the mechanism of specific organic reactions, such as pericyclic, Mentshutkin, cage, and polymerization reactions.
Abstract: High pressure kinetics appears to be a valuable tool in investigating the mechanism of specific organic reactions. For instance, in pericyclic, Mentshutkin, cage, and polymerization reactions, such studies reveal various features of the transition state, in particular its localization along the reaction coordinate and its nature. However, precise conclusions require separation of the different effects (electrostatic, steric, orbital, etc.) which may contribute to the structure of the transition state.

46 citations


Journal ArticleDOI
TL;DR: In this article, the distribution of vibrational states at any point along the reaction path is obtained, in the classical path limit, for a model reactive collision problem, for reactants in the ground vibrational state, and hence the entropy can be analytically computed.

34 citations



Journal ArticleDOI
TL;DR: In this paper, the rate constants of the aniline and the N-methylaniline reactions (kA/kM) increase with increasing size of the 6-substituent.
Abstract: The kinetics of the SNAr reactions of aniline and N-methylaniline with a variety of substituted nitrochlorobenzenes in acetonitrile demonstrate that the formation of the intermediate σ-complex is rate determining. The ratio of the rate constants of the aniline and the N-methylaniline reactions (kA/kM) increases with increasing size of the 6-substituent; with picryl chloride kA/kM reaches a value of over 20 000. The reaction of aniline with 4-X-2,6-dinitrochlorobenzenes is subject to considerably larger para-substituent effects than the corresponding reactions with N-methylaniline. These results are interpreted in terms of two effects: (i) A primary steric effect, which renders the approach of N-methylaniline to the substrate difficult. (ii) A shift towards earlier, more reactant-like transition state structures caused by the primary steric effect. In early transition states the activating power of the electron-withdrawing substituents in the substrate is expected to be relatively small. An early transition state for the slow N-methylaniline reaction and a late transition state for the fast aniline reaction is in apparent contradiction to what would be expected on the basis of the Hammond postulate.



Journal ArticleDOI
TL;DR: In this article, it was shown that while a double front may occur for hydrogen-fluorine systems, in the case of the hydrogen-bromine reaction only a single front is obtained.
Abstract: Some theoretical computations are completed, illustrating possible influences of reaction mechanism on the structure of the reaction zone, for reactions which occur by a straight-chain process. The occurrence of single and of double reaction fronts is identified. It is demonstrated that while a double front may occur for hydrogen-fluorine systems, in the case of the hydrogen-bromine reaction only a single front is obtained.

Journal ArticleDOI
TL;DR: In this paper, the standard free energies of transfer from methanol to other alcohols and to water are reported for nearly 40 solutes, ranging from hydrocarbons to amino-acids.
Abstract: Standard free energies of transfer from methanol to other alcohols and to water are reported for nearly 40 solutes, ranging from hydrocarbons to amino-acids. These transfer free energies from methanol to alcohols are negative for nonpolar inert solutes, but quite positive for amino-acids and ion pairs; from the assembled data it is predicted that linear free energy relationships between ΔG‡ values for substitution reactions in alcohols will in general be approximate only. By combination of initial-state effects with values of ΔG‡ previously determined, free energies of transfer from 20 different substitution reactions involving electrically neutral reactants. The various values of ΔGot(Tr) are by no means linearly related, but by comparison with values for polar species such as amino-acids and ion pairs it is shown that the polarity of transition states increases in the order [R4Pb–I2]‡ < [R3N–RI]‡⩽[R4Sn–I2]‡ < [R4Sn–HgX2]‡ < [ButX]‡⩽[phCHMeCl]‡⩽[ph2CHCl]‡. The α-amino-acids are shown to be suitable model solutes for highly polar transition states, especially when any differences in molar volume between solute and transition states, especially when any differences in molar volume between solute and transition are taken into account.

Journal ArticleDOI
TL;DR: Deuterium isotope effects, operating in competitive losses of H2, HD, and D2 from some carbocations, are used to indicate that these reactions proceed through transition states in which two C-H bonds are synchronously stretched.
Abstract: Deuterium isotope effects, operating in competitive losses of H2, HD, and D2 from some carbocations, are used to indicate that these reactions proceed through transition states in which two C–H bonds are synchronously stretched

Journal ArticleDOI
TL;DR: The stereoselectivity of three kinds of electrophilic reactions, vis osmium tetroxide oxidation, bromohydrination and peracid epoxidation of 24-nor-5α-chol-22-en-3β-ol 4 was investigated by the analysis of the products as discussed by the authors.
Abstract: The stereoselectivity of three kinds of electrophilic reactions, vis. osmium tetroxide oxidation, bromohydrination and peracid epoxidation of 24-nor-5α-chol-22-en-3β-ol 4 was investigated by the analysis of the products. The conformations II and III are proposed as participating in the transition states in the course of these reactions. The stereochemistry of the Grignard reaction on the 22-aldehyde is also discussed.

Journal ArticleDOI
TL;DR: In this article, the transition state enthalpies of transfer from propanol and methanol for substitution on carbonyl carbon are calculated, and the transition states for substitution at sulphonyl sulphur are shown to have polar and polarizable structures with little localized negative charge, poor hydrogen bond acceptors, and well solvated by dipolar aprotic solvents.
Abstract: Rates, activation parameters, and heats of solution of the reactants have been measured for the reaction of imidazole with benzoyl and benzenesulphonyl chlorides in propanol and acetonitrile. Enthalpies of transfer from propanol to acetonitrile of the transition state for the two reactions have been calculated. Approximate values for transfer from methanol are also given. Transition state enthalpies of transfer from propanol and methanol for substitution on carbonyl carbon are ca.–4 to +7 kJ mol–1 whereas corresponding values for substitution at sulphonyl sulphur are ca.–15 to –25 kJ mol–1. This is taken to mean that transition states for substitution at sulphonyl sulphur are polar and polarizable structures with little localized negative charge, poor hydrogen bond acceptors, and well solvated by dipolar aprotic solvents. Corresponding transition states for substitution at carbonyl carbon resemble the tetrahedral intermediate, are good hydrogen bond acceptors, and well solvated by protic solvents.

Journal ArticleDOI
TL;DR: The absolute values of the rate constants of the reaction of cis-cycloolefins C5-C12 with O3 were determined in this article, and the greatest reactivity is possessed by C8 and C5, the least by C6, C10, and C12.
Abstract: 1. The absolute values of the rate constants of the reaction of cis-cycloolefins C5–C12 with O3 were determined. The greatest reactivity is possessed by C8 and C5, the least by C6, C10, and C12. 2. On the basis of a quantitative comparison of the change in the structures of the rings of the hydrocarbons in the initial and transition states, we calculated the relative values of the rate constants of the reaction of cycloolefins with ozone, which are satisfactorily correlated with the experimental data.


Journal ArticleDOI
TL;DR: A semi-empirical approach has been used to evaluate rate parameters for a three-center decomposition reaction from the point of view of transition state theory, with perfluorodiazirine serving as the prototype molecule.
Abstract: A semiempirical approach has been used to evaluate rate parameters for a three-center decomposition reaction from the point of view of transition state theory, with perfluorodiazirine serving as the prototype molecule. Several activated complex models are considered in which the reaction coordinate is chosen as the ∡ NCN bending mode. The constraints imposed include the principle of concerted bond-order conservation in passing from the initial to the final state, and use is made of empirical bond order–bond length and bond order–force constant relationships. The geometric configuration of the transition state sought is one which conforms with the lowest energy path and is also consistent with the observed entropy of activation. The potential energy of activation is taken as the optimum difference in binding energies (based on the INDO method) between the transition and initial states, and the critical energy is obtained by applying a correction for the zero-point energy difference, derived from normal coordinate analysis. Satisfactory agreement is found in the case of the activated complex model for which the total bond order is conserved and bonds undergoing rupture are assigned a fractional bond order (FBO) of 2/3, derived from the postulate (FBO) = α/β whe re α(=2) is the number of bonds breaking, and β(=3) is the number of bonds undergoing change in the ring opening.