Principal Research Interests
My group of undergraduate research students
is interested in the synthesis and study of organic molecules
with unusual electronic structures. In particular, we are interested
in probing the limits of classical structure theory by designing
and building molecules that choose to leave two π-electrons
unpaired, thereby violating the principle of maximum bonding.
These "Kekulé biradicals" are both highly
colored and magnetic, suggesting the potential for future applications
in materials science. But even more important, they constitute
a fundamentally new class of organic molecules, and new types
of molecules are guaranteed to teach us new things.
We use a combination of simple resonance
arguments and more sophisticated molecular orbital concepts
to identify potential Kekulé biradicals. For example, isobenzofulvene (below,
n = 1) has a fully covalent conjugated polyene form and biradical
form whose resonance energy partly offsets the cost of its "missing" π-bond.
The same structural features that make the biradical competitive
with the covalent form produce an unusually small gap and strong
exchange coupling between the HOMO and LUMO. The result is
a triplet (pure biradical) state that is not much higher in
energy than the (mostly covalent) "biradicaloid" singlet
state. Electronic structure calculations (B3LYP/6-31G*) predict
that as n increases, the relative energies of the two states
switch — i.e., the biradical becomes the lower energy
structure!
We can generate these highly reactive molecules by matrix-isolation
photochemistry at cryogenic temperatures. Under these conditions
they can be observed directly by electronic spectroscopy (UV-vis
and fluorescence) and by EPR (electron paramagnetic resonance).
At higher temperatures their reactivity can be studied by isolating
and identifying their reaction products.
Our goal is ultimately to learn enough
about these unusual species that we can design new members
of this class and confidently predict their magnetic and
electronic properties as well as their chemical behavior.
Reaching this goal requires a synergy between theory and
experiment — theory aids the interpretation
of the experiments and suggests new directions, and the experimental
results test the validity and scope of various theoretical
approaches.
Representative Publications
"Experimental Determination of the Antiaromaticity of
Cyclobutadiene" Ashok A. Deniz, Kevin S. Peters, Gary
J. Snyder; Science 1999 (Nov 5), 286, 1119-1122.
"2,2-Dimethyl-2H-dibenzo[cd,k]fluoranthene, the First
Kekulé Hydrocarbon with a Triplet Ground State" Daniel
R. McMasters, Jakob Wirz, and Gary J.
Snyder; J. Am. Chem.
Soc. 1997, 119, 8568-8569. |
