Subject: CCL:Energy of Transition State is higher that ground state.

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Thu, 30 Nov 2000 16:40:45 +0100

Ali N Raschid:

> Hi, I user HF6-31+G* and 6-311+G+ to optimize the ground state of
> p-nitroamine. The amine come out pyramidal. When I hold the amine in a
> planar conformation I get one negative frequency. The energy of the
> pyramidal conformer is higher than that of the pyramidal one, but when I
> take into consideration the zero point energy correction the planar
> conformer (with the negative frequency) has lower energy than the
> pyramidal one (all positive frequencies).
> Does anyone have a clue as to why?

Dear Ali,

the ZPE corrections predicted for the equilibrium geometry and the transition 
state geometry are not strictly comparable. The transition state has one 
vibrational mode with a negative force constant and an imaginary frequency.  
This mode is not considered in the computation of the ZPE. - Anyway, the 
harmonic approximation is not reliable for this mode, that corresponds to the 
amine "inversion vibration".  The molecule is situated in a double minimum 
potential well with a relatively small barrier between the minima.  This 
situation is associated with large anharmonic effects, leading to tunnelling 
splitting (inversion doubling). 

Yours, Jens >--<
                             
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JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
Department of Chemistry             +45 4674 2710  (direct)
Roskilde University (RUC)   Fax:    +45 4674 3011 
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