Subject: infinite polyradicals

From: <[email-masked]>
Date: Tue, 15 Feb 1994 15:22:12 -0500 (EST)

Dear co-netters:
I have been working on a quantum chemistry problem, very interesting, but with
unexpected difficulties.  Actually, Jimmy Stewart tried his hand on it too, and
he could not solve it either.  I am sending this message, "urbe et orbe", in
the
hope that somebody will get interested and think of something that will work,
which could then easily lead to a very nice paper in collaboration.

It all started with an article that appeared in C&EN, issue of August 31, 1992,
page 8.  There, three guys from Europe reported having measured the polymers
with the lowest bandgap ever: approx. 0.5 ev.  Since the subject of intrinsic
conducting polymers interested me for some time, I got the full article, that
appeared in Pol. Bull. 29, 119-126 (1992).  The polymers measured are
essentially polycroconaine and polysquaraine, and the repeat units are
biradicals.

The measurements are very interesting, but the authors offer no theoretical
explanation for the low band gap, other than saying that the repeat units
consist of alternating strong donor and acceptor moieties.  They also suggest
the possibility of soliton-like behavior of the charge-carriers.

It occurred to me that the explanation for the low bandgap should rather lie in
the existence of almost degenerate singlet and triplet states, and decided to
give it a try.  I used MOPAC, the AM1 parametrization, and the cluster
approximation to represent the infinite quasi-1D polymer.  the UHF calculation
*
rather easy, but as expected, the spin contamination was very high, and I
thought the calculation was not reliable enough.  So I decided to try the
representation of the biradical provided by the keyword OPEN(2,2); that is a
ROHF hamiltonian with a limited CI.  (I have done this before, and it worked).
To my surprise, the optimization of the geometry became totally impossible;
the gradient will not decrease with any of the optimization algorithms provided
in MOPAC93.  If anyone is interested in getting involved, I have all the
details, and the input files.  I think this could lead, on the one hand to the
capability to predict a whole new class of intrinsic conductors: the
polyradicals; and on the other to substantial progress in the theory of organic
(ferro)-magnetic materials.  Is there anyone out there?  Thank you for your
attention.
        Dr. Jorge Medrano
        Department of Chemistry
        University of Cincinnati