CCL:NBO Analysis

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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