Subject: second post: JT vs. doublet instability

From: "DR. DOUGLAS A. SMITH, UNIVERSITY OF TOLEDO" <[email-masked]>
Date: Fri, 27 Aug 1993 10:01:04 -0500 (EST)

A while back I posted this to the net; suprisingly and disappointingly, I
received absolutely no replies.  So, I would like to try again.

In a recent paper (J. Org. Chem. 1993, 58, 6) we reported on the geomietric
distortion of the triplet nitrenium ion derived from azepine (C6H6N+), 
although we could not distinguish between Hartree-Fock doublet instability
and vibronic coupling-induced pseudo Jahn Teller distortion as the cause.
In trying to sort this out, I have several questions to put to the readership
of this mailing list.

1.	In other azaaromatic systems such as pyridine and pyrimidine, 
vibronic coupling of low lying triplet states is invoked as the reason
for geometric distortions.  These distortions are out-of-plane deformations,
as opposed to the in-plane distortion we observed in the azepine system.  Is
this because the two former systems are six-membered rings while azepine
is a seven atom heterocycle?

2.	Are there any examples of vibronic coupling-induced geometric 
distortions for other homoaromatic systems, e.g. tropylium?  If so, are
these in-plane or out-of-plane distortions?

3.	Are there any examples of vibronic coupling-induced geometric 
distortions for three-membered ring aromatic systems, e.g. cyclopropenyl
cation?  This system would necessarily be planar, so any distortion should
be (IMHO) in-plane.

4.	In the absence of experimental data, how does one computationally
determine the existence of vibronic coupling?  From my reading of the 
literature (e.g. Buma, et al., J. Am. Chem. Soc. 1992, 114, 9544) I cannot
see how coupling is unambiguously determined.  In the paper cited, the 
authors calculated several low lying excited triplet states and examined
the normal mode frequencies,then they correlated the direction of the 
geometric distortions in each state.  This somehow led them to the conclusion
that vibronic coupling was occuring.  (I am only an organic chemist, their
arguments just don't make enough sense to me.)  Can someone help explain
this to me?

5.	Furthermore, the explanation by these authors based on electronic
structure ("We argue that an azabenzene in an npi* state may well be
susceptible to out-of-plane distortions because the pi-electron system in 
this kind of molecules [sic] becomes in first approximation a 7-pi-electron
system and thus is no longer aromatic.") seems to me to be just handwaving
of the type an organic chemist would use.  The explanation seems empirically
valid but does not really explain anything, especially in light of our 
results for the azepine system.  On the other hand, if this explanation 
really is valid, then perhaps our azepine results are due to HF doublet 
instability rather than vibronic coupling.  Help!

6.	These authors also use energetic arguments that don't seem to me to
"prove" vibronic coupling.  For example, "The excitation to the 3A2(npi*)
state... results in a large out-of-plane distortion of the C2-H7 fragment
with a large stabilization energy, indicating that the vibronic coupling 
between the 3A2(npi*) and 3B2(pipi*) states is considerably stronger than
the vibronic coupling between the 3B1(npi*) and 3A1(pipi*) states [where
the stabilization energy was small]."  This sounds like circular reasoning
to me, i.e. vibronic coupling exists because there is a large stabilization
energy due to vibronic coupling.  Or am I missing the point?

	On a related topic, another question.  When doing a CASSCF calculation
on a triplet state of any molecule, is it absolutely necessary to include
both unpaired electrons and both singly occupied orbitals (and their 
corresponding antibonding orbitals)?  In particular, when calculating the
triplet state of a nitrenium ion, should the two unpaired electrons formally
on the nitrogen be tightly coupled or correlated?

	Please direct replies to the net, in order to foster discussion and
further insights.

Doug

Douglas A. Smith
Assistant Professor of Chemistry
  and of Medicinal and Biological Chemistry
The University of Toledo
Toledo, OH  43606-3390

voice    419-537-2116
fax      419-537-4033
email    [email-masked]

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From: [email-masked] (Ryszard Czerminski X 285)
Date: Fri, 27 Aug 1993 17:17:01 -0400

Doug:

Probably this reference will not answer your questions
directly, but you might find it usefull...

A. Sobolewski, R. Czerminski
"The Potential Energy Surfaces and the Radiationless Dynamics of
Excited States of Benzene and Pyridazine"
Chem. Phys. 130, 123 (1989)

Ryszard Czerminski
Molecular Simulations
16 New England Executive Park,
Burlington, MA 01803-5297
phone: (617)229-8875 x 217
fax: (617)229-9899
e-mail: [email-masked]