From owner-chemistry@ccl.net Wed May  2 14:46:00 2007
From: "Fedor t.goumans{:}ucl.ac.uk" <owner-chemistry|*|server.ccl.net>
To: CCL
Subject: CCL:G: Summary: large imaginary frequency in coronene when diffuse functions are used
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X-Original-From: Fedor <t.goumans-.-ucl.ac.uk>
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Date: Wed, 02 May 2007 17:07:32 +0100
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Sent to CCL by: Fedor [t.goumans-.-ucl.ac.uk]
Dear CCL-ers,

A summary of what I found out thusfar about the imaginary frequency in 
coronene
for certain approaches.

Cory Pye and Marcel Swart helped to find the stable structures 
corresponding to
the saddle point. The stable structure is of D3D-symmetry, which is found by
manually displacing the structure along (bits of) the B2G-imaginary mode.
Gaussian's IRC apparently has sufficient numerical noise to misjudge the
symmetry after stepping along the imaginary mode (C2H). The
symmetry-determining algorithms are of course very sensitive to slight
numerical differences in cartesian coordinates.
The resulting optimised stable structure is 'ruffled' with alternative upward
and downward displacement out of the plane by ~0.003A for the central carbon
atoms and less for the other atoms. It is very comparable in energy to 
the flat
structure (~0.3 kJ/mol).

Thanks to Errol Lewars, Stefan Grimme, Steven Bachrach and Paul Schleyer for
pointing out the paper
J. Am. Chem. Soc., 2006, 128(9), 9342
where imaginary frequencies are observed for different 
methodologies/basis sets
in benzene. These imaginary frequencies are also reported for large 
Pople basis
sets by Steve Williams. This basis-set dependent effect is very likely related
to that in coronene and, as Petar Todorov pointed out, in other PAHs as well.
In this paper, this basis set effect is attributed to a an 
intramolecular basis
set incompleteness error.
To illustrate how sensitive the basis set effect might be: with MPWB1K the
ruffled structure is preferred if a 6-31+G* basis is used as well as when a
split 6-31+G*(central)/6-31G*(peripheral) basis is used. However, as Grzegorz
Mazur pointed out, at the HF/6-31+G* level the flat D6H structure is 
preferred,
while for the split basis it is unstable and the ruffled D3D structure is
preferred instead.
In both my and previous (JACS-paper cited) calculations the aromatics 
are planar
when Dunning's basis (aug-cc-pVnZ) are used, but not always when Pople basis
sets with diffuse function are used.

I have not fully pinned down yet what the underlying reason for the basis set
dependent nonplanarity is, although I do feel that Isaac Bersuker's suggestion
of a basis-set dependent pseudo Jahn-Teller effect is very plausible.

A question that comes to mind is if this tendency for puckering may be "real",
especially for the larger PAHs in view of the structure of graphene recently
determined to be corrugated (Nature, 2007, 446, 60)?

Thanks for all your help,
Kind regards,
Fedor