conflict between energy and imaginary frequency

Dear CCL’ers,

When I perform a geometry optimization, with frequency calculation of a certain molecule with Gaussian03, I have the following problem:
Cs symmetry: imaginary frequency of -20 cm-1, so this is transition state structure.
C1 symmetry: no imaginary frequencies, so it is a minimum on the potential energy surface. It’s lowest frequency is 27 cm-1.
I checked the normal modes of both structures and the imaginary frequency of the Cs form and the lowest frequency of the C1 form correspond to the same normal mode.
The problem is that energy of the optimized structured shows that the Cs form has a lower energy (it is stabilized by 0.02 kcal/mol) than the C1 form. 

How can it be that a transition state has a lower energy than a minimum? 

The calculations were done with B3LYP/6-311+G*
opt=verytight int=ultrafine freq 

As the data conflict, I thought it might be a problem with some thresholds, so I tried to set better thresholds with:
Iop(3/27=15) and Iop(3/29=15) 

I also tried iop(3/32=x) in combination with the above set options, but for x=3,4,5 the scf did not converged (even not if I put MaxCycle on 500). 

Does anybody have any suggestions on how I can solve this? 

Best regards,


 Karla Tersago
 Structural Chemistry Group, Department of Chemistry
 University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium
 E-mail: /Phone:+32(03)8202366/Fax:+32(03)8202310