Do High-Spin Topology Rules Apply to Charged Polyradicals? Theoretical and Experimental Evaluation of Pyridiniums as Magnetic Coupling Units

Ab initio calculations on pyridine and pyridinium
analogues of m-xylylene indicate that the
neutral
heterocycle is essentially equivalent to benzene as a ferromagnetic
coupling unit, while the cationic pyridiniums
behave much differently. Depending on the substitution pattern, a
protonated pyridine can serve as a ferromagnetic
coupling unit or an antiferromagnetic coupling unit. Both valence
bond and molecular orbital arguments provide
qualitative rationalizations of these results. In an effort to
test the theoretical predictions,
bis(trimethylenemethane)
analogues of the pyridine and pyridinium biradicals were synthesized
and analyzed by electron paramagnetic resonance
spectroscopy. General support for the theoretical predictions is
obtained.