Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers

Some free radicals
are stable enough to be isolated, but most are
either unstable transient species or exist as metastable species in
equilibrium with a dimeric form, usually a spin-paired sigma dimer
or a pi dimer (pimer). To gain insight into the different modes of
dimerization, we synthesized and evaluated a library of 15 aryl dicyanomethyl
radicals in order to probe what structural and molecular parameters
lead to σ- versus π-dimerization. We evaluated the divergent
dimerization behavior by measuring the strength of each radical association
by variable-temperature electron paramagnetic resonance spectroscopy,
determining the mode of dimerization (σ- or π-dimer) by
UV–vis spectroscopy and X-ray crystallography, and performing
computational analysis. We evaluated three different hypotheses to
explain the difference in the dimerization behavior: (1) that the
dimerization behavior is dictated by radical spin densities; (2) that
it is dictated by radical polarizability; (3) that it is dictated
by London dispersion stabilization of the pimer. However, no single
parameter model in itself was predictive. Two-parameter models incorporating
either the computed degree of spin delocalization or the radical polarizability
as well as computed estimates for the attractive London dispersion
forces in the π-dimers lead to improved forecasts of σ-
vs π-dimerization mode, and suggest that a balance of spin delocalization
of the isolated radical as well as attractive forces between the stacked
radicals, govern the formation of diradical pimers.