Subject: Vibrational entropy of a transition state

From: Herve Toulhoat <[email-masked]>
Date: Fri, 25 Jun 1999 10:13:09 +0200

Hello,

The transition state theory (TST) allows to relate in principle the rate
constant of a reaction to the variation in free energy between
reactant(s) and the transition state. Of specific interest is the
variation of entropy between reactant(s) and TS, which relates to the
so-called pre-exponential factor of the rate equation.

The entropy change for a gas phase reaction will be dominated by the
vibrational component. The vibrational entropy of a molecular species
can be evaluated from the set of vibrational normal modes. A TS however
corresponds to a saddle point of the PES, and exhibits therefore one
imaginary frequency (one negative eigenvalue in the hessian matrix).
This imaginary frequency cannot be included in the statistical
mechanical calculation of the vibrational entropy. Is it legitimate to
simply forget about it?

More generally, is there a rigourous procedure for evaluating the
vibrational entropy of a TS?

I will summarize the replies.

Yours sincerely,

--
Dr Herve TOULHOAT

Directeur de Recherche Associe
Group Leader, Molecular Modeling and Computational Chemistry
Div. Computer Science and Applied Mathematics, IFP
Director, Groupement de Recherches CNRS G1209:
Dynamique Moleculaire Quantique Appliquee a la Catalyse
Research Group CNRS G1209
Ab Initio Molecular Dynamics Applied to Catalysis
(Scientific Partners: CNRS, IFP, Total, University of vienna, TU Eindhoven)

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