CCL: Limitations of DFT

 Sent to CCL by: "Gary W  Breton" [gbreton a berry.edu]
 I recently submitted a manuscript for publication revolving around the
 decomposition of a strained class of heterocyclic compounds.  Three potential
 decomposition mechanisms were proposed.  The first mechanism can be ruled out
 for reasons not important here.  The second mechanism (B) is a quasi-concerted
 elimination (this *could* be symmetrical or asymmetrical...no telling as of
 yet).  The final mechanism (C) occurs via cleavage of one of the strained bonds
 to form a transient diradical (of unknown lifetime) followed by further bond
 cleavage to ultimately lead to the same products as would be observed by
 mechanism A.  The experimental results were not conclusive in differentiating
 beteen the mechanisms, so I  proposed in the manuscript that a purely
 computationally-based follow-up paper on the intermediates, transition states
 (with IRC calculations), etc.  at the DFT B3LYP/6-311+G(d,p) level of theory(a
 paper I am currently working on) would help to distiguish between the
 mechanisms.  I had used DFT calculations at the same level of theory on a
 similar class of compounds before (a JOC paper that has been published) and had
 no problems with reviewers.  However, one of the reviewers on *this* manuscript
 objected to the use of the DFT calculations with the following:
 "With regards to using DFT calculations to distinguish between mechanisms B
 and C, I would put no confidence in such a result.  It is well known that DFT
 calculations greatly overemphasize the stabilization provided by delocalization,
 so they have a built-in bias towards concerted reactions, which is useful if the
 reaction actually is concerted, but a disaster if it is not."
 Is this a true statement? And, if so, where can I find more about this
 limitation of DFT theory? I have searched but come up empty so far.  Plus, I'm
 not sure exactly what the reviewer means, since the diradical intermediate of
 mechanism C would also be subject to delocalization effects...possibly even
 greater than those expected by a "concerted" reaction!
 Also (and I hate the thought of this because I have put so much time into these
 calculations already) if DFT calculations are NOT sufficient, what should I be
 looking at?
 Thanks again for any advice,
 Gary Breton
 Berry College
 (btw...I went 2 years without submitting a question to CCL and now I find myself
 submitting 2 questions in a week's time!)