From chemistry-request: at :server.ccl.net Fri Jun 25 04:19:24 1999 Received: from ifp.ifp.fr (ifp.ifp.fr [156.118.212.2]) by server.ccl.net (8.8.7/8.8.7) with ESMTP id EAA07590 for ; Fri, 25 Jun 1999 04:19:23 -0400 Received: from irsil145.ifp.fr (irsil145.ifp.fr [156.118.20.20]) by ifp.ifp.fr (8.9.3/8.9.3) with ESMTP id KAA12042; Fri, 25 Jun 1999 10:15:15 +0200 (MET DST) Received: from ifp.fr (localhost [127.0.0.1]) by irsil145.ifp.fr (8.8.5/8.8.5) with ESMTP id KAA11315; Fri, 25 Jun 1999 10:13:10 +0200 (MDT) Sender: toulhoat(+ at +)ifp.fr Message-ID: <37733A15.A4590543 {*at*} ifp.fr> Date: Fri, 25 Jun 1999 10:13:09 +0200 From: Herve Toulhoat Organization: Institut Francais du Petrole X-Mailer: Mozilla 4.07C-SGI [en] (X11; I; IRIX64 6.5 IP30) MIME-Version: 1.0 To: chemistry[ AT ]ccl.net CC: Herve TOULHOAT , CSC-list-: at :-msi.com Subject: Vibrational entropy of a transition state Content-Type: multipart/alternative; boundary="------------265CF2C97919BCFE75CEA954" --------------265CF2C97919BCFE75CEA954 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit 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) INSTITUT FRANCAIS DU PETROLE Tel: +33-01-47-52-73-50 1 & 4 Avenue de Bois-Preau Fax: +33-01-47-52-70-22 92852 RUEIL-MALMAISON Cedex FRANCE E-mail: herve.toulhoat $#at#$ ifp.fr Visit IFP Website at http://www.ifp.fr --------------265CF2C97919BCFE75CEA954 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit 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)
        
INSTITUT FRANCAIS DU PETROLE                    Tel: +33-01-47-52-73-50
1 & 4 Avenue de Bois-Preau                      Fax: +33-01-47-52-70-22
92852 RUEIL-MALMAISON Cedex FRANCE       E-mail: herve.toulhoat -A_T- ifp.fr

Visit IFP Website at http://www.ifp.fr
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