Subject: CCL:NBO Analysis

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Thu, 6 Mar 2003 08:37:03 +0100

Dear Carlos:

> I have done a NBO analysis of a transition structure were a sigma
> bond is being broken. The problem arises when check the orbital
> structure, there is still a pi bond where the sigma bond is already
> broken. Could it mean there is a certain aromaticity in that TS and
> pi electrons are still creating a "current" despite of the big bond
> distances. Is there any other point of view (with chemistry sense,
> not mathematical formulation of anything non graspable)? Thanks

You apply the NBO procedure on a "non-classical" chemical species, 
namely a transition state with a half-broken bond. The procedure 
tries to find a description of the electronic structure in terms of 
localized bonds and lone pairs, corresponding to a single classical 
Lewis formula, but this is obviously problematic for a species like 
the one you are investigating. The NBO procedure comes up with a 
compromize, and you should be careful not to overrate the chemical 
significance of the results of the analysis.

Yours, Jens >--<

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JENS SPANGET-LARSEN         Office:         +45 4674 2710
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