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Theories of Molecular Reaction DynamicsThe Microscopic Foundation of Chemical Kinetics$
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Niels E. Henriksen and Flemming Y. Hansen

Print publication date: 2008

Print ISBN-13: 9780199203864

Published to Oxford Scholarship Online: January 2010

DOI: 10.1093/acprof:oso/9780199203864.001.0001

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Microscopic interpretation of Arrhenius parameters

Microscopic interpretation of Arrhenius parameters

Chapter:
(p.211) 8 Microscopic interpretation of Arrhenius parameters
Source:
Theories of Molecular Reaction Dynamics
Author(s):

Niels E. Henriksen

Flemming Y. Hansen

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780199203864.003.0008

This chapter reviews the microscopic interpretation of the pre-exponential factor and the activation energy in rate constant expressions of the Arrhenius form. The pre-exponential factor of apparent unimolecular reactions is, roughly, expected to be of the order of a vibrational frequency, whereas the pre-exponential factor of bimolecular reactions, roughly, is related to the number of collisions per unit time and per unit volume. The activation energy of an elementary reaction can be interpreted as the average energy of the molecules that react minus the average energy of the reactants. Specializing to conventional transition-state theory, the activation energy is related to the classical barrier height of the potential energy surface plus the difference in zero-point energies and average internal energies between the activated complex and the reactants. When quantum tunnelling is included in transition-state theory, the activation energy is reduced, compared to the interpretation given in conventional transition-state theory.

Keywords:   pre-exponential factor, activation energy, transition-state theory, zero-point energy, quantum tunnelling

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