Effect of Para Substituents on the Molecular and Electronic Structures of Sterically Congested Triplet Diphenylcarbenes

摘要

A series of triplet di(2,6-dimethylphenyl)carbenes (~32) bearing nine symmetrical para di-substituents with well-distributed electronic properties have been generated by the irradiation of the corresponding diazo precursors and studied using electron paramagnetic resonance spectroscopy. The zero field splitting parameters, D and E, were measured in matrices of different viscosities and are analyzed in terms of a sigma-dot (#sigma#~.) scale of spin-delocalization substituent constants. Fairly good correlation (r > 0.9) with #sigma#~. based on the spin delocalization in the absence of polar effects was found for the D values of ~32 in its minimum energy geometry, but not for those in the metastable state. The magnitude of #rho# for ~32 was larger than that for 0-unsubstituted diphenylcarbenes and is discussed in terms of the difference in the geometry between the two carbene systems.