The Isomerization Barrier in Cyanocyclobutadienes: An ab Initio Multireference Average Quadratic Coupled Cluster Study

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Introduction
Cyclobutadiene (CBD) 1 is the second smallest highly
strained organic ring compound,1 which serves also as a
prototype of antiaromatic systems due to its 4π electrons.2,3 The
combination of these two facets leads to a number of unusual
properties, which have intrigued researchers for decades.4 A very
recent attempt of measuring the heat of formation of CBD5 and
some comments on the nature of its antiaromaticity6 witness
continuing interest on the topic illustrating the fact that some
controversies are still going on. Substituted CBDs represent also
good candidates for systems exhibiting bond-stretch isomerism,
i.e., that two stable structures differ only in their bond distances.
This is a point of considerable importance, in particular since
there is an ongoing debate whether bond-stretch isomers are
facts or artifacts.7-10 The experimental evidence is sparse, but
it is now unequivocally established that two 1,2- and 1,8-
dichloroperfluorocyclooctatetraene11 are true bond-stretched
isomers. The same holds for 1,3-diphosphabicyclobutane-2,4-
diyl- and 1,3-diphosphabicyclo[1.1.0]-butanes.12-15 Detailed
calculations performed in our group based on the multireference
average quadratic coupled cluster (MR-AQCC) method have
been performed on benzo[1,2:4,5]dicyclobutadiene.16 It was
found that the two bond-stretch isomers are minima on the
energy hypersurface, but they were separated by a barrier height
of only 5 kcal mol-1 implying that their existence in solution
at room temperature is questionable. They most likely form a
fluxional system with a permanent interconversion. Borden and
Davidson17 considered t-butyl substituted CBD and found that
the barrier was lowered relative to parent cyclobutadiene.
Clearly, this derivative is also a fluxional system just like the
parent CBD18 since it was shown by A
old and Michl that the
latter undergoes rapid bond flipping interconversions at 10 K.
It was argued that the reason behind a decreased barrier of
t-butyl derivative was increased steric repulsion between the
bulky substituents in the energy minimum relative to that in
the saddle point.17 In other words, the transition state is less
destabilized by substituents than the equilibrium structure. It is
of interest to examine influence of other substituents on the
barrier heights. Before doing that, it is necessary to obtain an
estimate of the barrier of automerization of CBD as accurately
as possible