Use of Buckingham potentials in engineering approximations for chemical kinetics

“…According to our ab initio calculations ,,− and work of previous investigators, ,, the Pauli repulsions between the electrons in the reactants play a significant role in the barrier to reaction. For example, Figure shows a plot of the orbital distortions that occur during the reaction H + H 2 → H 2 + H calculated as described in our recent work. , Notice that, as the reaction occurs, the orbitals on the hydrogen and H 2 distort.…”

Section: The Potentialmentioning

confidence: 67%

An Extension of the Marcus Equation for Atom Transfer Reactions

Blowers

Masel

1999

J. Phys. Chem. A

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The Marcus equation for electron transfer has been widely applied to atom transfer reactions, but the equation does not seem to work well for very endothermic or very exothermic reactions. In this paper, a modified model is proposed. The modified model assumes that the potential energy surface can be written as a sum of the potentials for the individual molecules and an intermolecular potential that keeps the reactants apart. The activation barrier predicted by the model is within 3 kcal/mol of that predicted by the Marcus electron transfer equation when −1 ≤ ΔH r/4E °≤ 1, where ΔH r is the heat of reaction and E ° is the intrinsic barrier. However, there are significant deviations when ΔH r/4E ° < −1 and when ΔH r/4E ° > 1. The modified model predicts that the activation barrier should equal ΔH r/4E ° in the very endothermic limit, (i.e., ΔH r/4E ° > 1), while the Marcus electron transfer equation predicts that the activation energy, E a, should diverge from ΔH r. Data shows that E a approaches ΔH r. The modified model predicts that the activation barrier goes to zero for very exothermic reactions, (i.e., ΔH r/E ° < −1) while the Marcus electron transfer equation predicts large barriers. Data shows, though, that the barriers approach zero. We also compare to the Marcus hyperbolic cosine expression and find that the modified model is within 3 kcal/mol of the Marcus hyperbolic cosine expression over the entire energy range. The modified model predicts that the barriers to reaction are associated with Pauli repulsions and not with bond stretching. That prediction agrees with recent ab initio calculations, and the VB model but not with the intersecting parabola model. Overall, the modified model seems to extend the original Marcus equation to very endothermic and very exothermic reactions. Also, it gives predictions similar to the Marcus hyperbolic cosine expression over the entire energy range.

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“…Figure 2 compares the values of r t B and r t F calculated at the MP2/6-31G* level to those predicted by Eqs. (9) and (10). In this case, the errors are slightly larger.…”

Section: Computational Resultsmentioning

confidence: 88%

“…Still, the transition state geometries predicted from Eqs. (9) and (10) are always within 0.15 A Ê of the MP2/6-31G* geometries for all of the reactions in Table 1.…”

Section: Computational Resultsmentioning

confidence: 96%

“…A comparison of r t B and r t F for a number of reactions calculated at the MP2/6-31G* level to those estimated from Eqs. (9) and (10) 6-311++G(d,p) calculations is 0.031 A Ê . By comparison, the average dierence between r t B and r t F predicted from the MP2/6-31g* calculations and those predicted by the CCSD(T)/6-311++G(d,p) calculations is 0.031 A Ê .…”

Section: Computational Resultsmentioning

confidence: 98%

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Extensions of the Marcus equation for the prediction of approximate transition state geometries in hydrogen transfer and methyl transfer reactions

Blowers

Masel

2000

Theoretical Chemistry Accounts: Theory, Computation, and Modeli

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with an average error of 0.04 A Ê . In the last two equations above, DU is the heat of reaction, E 0 A is the intrinsic barrier, and C A is a constant that comes from the model of Blowers and Masel [8].It is proposed that the above three equations are useful in generating initial guesses for transition state geometries in ab initio calculations. In the cases that were tried, rapid convergence was found when these guesses were used.

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Use of Buckingham potentials in engineering approximations for chemical kinetics

Abstract: Pre®ious in®estigators ha®e often used the

Cited by 8 publications

References 24 publications

Engineering approximations for activation energies in hydrogen transfer reactions

Engineering approximations for activation energies in hydrogen transfer reactions

An Extension of the Marcus Equation for Atom Transfer Reactions

Extensions of the Marcus equation for the prediction of approximate transition state geometries in hydrogen transfer and methyl transfer reactions

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