Subject: Transition states

From: "Georg Schmeer" <[email-masked]>
Date: Wed, 5 Jan 2000 15:32:44 +0100

Dear CCLers,
during the calculation of the imidazole catalyzed ester hydrolysis in the gas
phase with GAUSSIAN 94/98 we get, going from the reactants to the products, 
two saddle points - characterized by one neative eigenvalue of the Hessian 
matrix of the SCF-energy -, and one intermediate (local minimum of the 
SCF-energy) in between. The reaction paths were determined by IRC-calculations 
>from the two transition states. The intermediate was found from both transition
states and has a lower SCF-energy than the two transition states. Structurally 
the three structures are very similar. 
If we calculate the Gibbs energies, however, the G-values of the intermediate 
is higher than that of the transition states. In this context I have some 
questions:
 - Which energy (E-SCF or Delta-G) should be taken into account for the 
   discussion of reaction energy barriers?
- It seems, that the positive change in Delta-G is caused by the special       
   degree of freedom, which has a negative Hessian eigenvalue in the transition 
   state, and which contributes with positive sign to the vibrational partition 
   function of the intermediate, having nearly the same strucuture. What are    
   the consequences of this difference for the determination of the Gibbs       
   energy of activation?
- Does anyone met this problem, too?

I would appreciate any comments and hints to this problem and will summarize 
the answers if possible.

With best regards

G.Schmeer
Prof. Dr. Georg Schmeer
Institut fuer Physikalische und Theoretische Chemie
Universitaet Regensburg
D-93040 Regensburg
[email-masked]
Tel: 0941-943-4745
Fax: 0941-943-4532