Substituent effects on the zero-field splitting parameters of diarylmethylene. Evidence for merostabilization in appropriately substituted diphenylmethylenes

An analysis of the temperature dependence (4–30 K) of the electron spin resonance (esr) spectra of substituted diphenylmethylenes (e.g. para-, para′-; methoxy, cyano, dimethylamino, and nitro) indicates all of these species have a triplet ground state. The variation in the zero-field splitting parameter D for this series of methylenes provides evidence that the average separation of the unpaired electrons is greatest for the unsymmetrically substituted derivatives, those having a strong para-electron-withdrawing substituent and a strong para′-electron-donating substituent. This result is explained in terms of a favourable contribution of charge-separated valence-bond structures, i.e., merostabilization.