TOWARD ORGANIC SUPERBASES - THE ELECTRONIC-STRUCTURE AND THE ABSOLUTEPROTON AFFINITY OF QUINODIIMINES AND SOME RELATED-COMPOUNDS

Several molecular systems that may well serve as potent organic superb ases are examined by using ab initio and semiempirical theoretical mod els. It is found that the imino group attached to the semiquinoid frag ment or to a backbone of several quinoid six-membered rings exhibits a very high proton affinity (PA). It is found that the reason behind th eir amplified basicity is appreciable aromatization of the quinoid bui lding blocks upon protonation. The underlying mechanism in extended sy stems is that of the aromaticity spin-off effect, triggered by the pro ton attack at the imino N atom and spread along the quinoid ribbon in a typical domino fashion. It yields an increase in the PA as high as r oughly 20 kcal/mol per the quinoid ring. Susceptibility toward the pro ton attack is further amplified by the alkyl substitution at the imino nitrogen atom and by additional substitution of the amino groups at s pecific positions within the framework of the quinoid building block(s ). It is stressed that synthesis of the studied systems might provide very patent organic superbases.