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-xylyl ene indicate that the neutral heterocycle is essentially equivalent to benzene as a ferromagnetic coupling unit, while the cationic pyridini ums behave much differently. Depending on the substitution pattern, a protonated pyridine can serve as a ferromagnetic coupling unit or an a ntiferromagnetic coupling unit. Both valence bond and molecular orbita l 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.