CCL: Case Studies of QM Computational Chemistry in Reactivity

 Sent to CCL by: Jean-Pierre DJUKIC [djukic^_^]
 Tom is right.
I wonder how this very example that Tom quotes would be treated with modern DFT methods. The rotational barrier of Cr(CO)3 is in benzene[Cr(CO)3] around 1 kcal/mol. In substitued systems, say anilineCr(CO)3 it rises a bit for various reasons, but not that much and nonetheless there exist rotamers of very close energy the statistical weight of which is thought to be different for each rotamer. This statistical distribution is indeed temperature dependent, it explains why a nucleophile will prefer attacking at the meta position relative to some nucleofuge and not ortho like intuition could suggest at a first glance under "some" conditions. We are talking about kinetic discrimination of states close in energy. The same stands for enantioselective processes where the differences of TS energies may challenge common accuracy in DFT, then add solvation issues ... and the fact that in reality molecules are animated and flexible indeed and not static like we generally see them in most published studies.
 Le 10/09/2015 18:58, Tom Albright talbright1234-$ a écrit :
 Furthermore the orientation of the Cr(CO)3 group directs attack by
 nucleophiles on the arene. All this at the (almost, but I am sure in
 your mind, totally) unreliable EHT level. The energy differences are
 small but one can formulate a coherent explanation of the
 regioselectivity. This is useful.
 Dr Jean-Pierre DJUKIC (Đukić)(DR CNRS)
 Laboratoire de Chimie et Systémique OrganoMétalliques (LCSOM)
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