From chemistry-request@server.ccl.net Thu Mar 6 02:37:21 2003 Received: from smtp2.ruc.dk ([130.225.220.70]) by server.ccl.net (8.11.6/8.11.0) with ESMTP id h267bLa24785 for ; Thu, 6 Mar 2003 02:37:21 -0500 Received: by smtp2.ruc.dk (Postfix, from userid 504) id 6A9E0954EF2; Thu, 6 Mar 2003 08:37:20 +0100 (CET) Received: from virgil.ruc.dk (virgil.ruc.dk [130.225.220.110]) by smtp2.ruc.dk (Postfix) with ESMTP id 6A63E954EEF; Thu, 6 Mar 2003 08:37:19 +0100 (CET) Received: from VIRGIL/SpoolDir by virgil.ruc.dk (Mercury 1.47); 6 Mar 03 08:37:19 +0100 Received: from SpoolDir by VIRGIL (Mercury 1.47); 6 Mar 03 08:37:16 +0100 From: "Jens Spanget-Larsen" Organization: Roskilde Universitetscenter To: "Silva Lopez; Carlos" Date: Thu, 6 Mar 2003 08:37:03 +0100 Subject: CCL:NBO Analysis Reply-To: spanget@ruc.dk Cc: Chemistry@ccl.net X-Confirm-Reading-To: spanget@ruc.dk X-pmrqc: 1 Priority: normal X-mailer: Pegasus Mail for Windows (v2.23) Message-ID: X-Spam-Status: No, hits=-3.2 required=5.0 tests=QUOTED_EMAIL_TEXT version=2.50 X-Spam-Level: X-Spam-Checker-Version: SpamAssassin 2.50 (1.173-2003-02-20-exp) 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 >--< =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= JENS SPANGET-LARSEN Office: +45 4674 2710 Department of Chemistry Fax: +45 4674 3011 Roskilde University (RUC) Mobile: +45 2320 6246 P.O.Box 260 E-Mail: spanget@ruc.dk DK-4000 Roskilde, Denmark http://virgil.ruc.dk/~spanget =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=