Toward organic superbases: the electronic structure and the absolute proton affinity of quinodiimines and some related compounds

Several molecular systems which may well serve as potent organic superbases are examined by using ab initio and semiempirical theoretical models. It is found that the imino group attached to the semiquinoid fragment or to a backbone of several quinoid six membered rings exhibits a very high proton affinity (PA). It is found that the reason behind their amplified basicity is appreciable aromatization of the quinoid building blocks upon protonation. The underlying mechanism in extended systems is that of the aromaticity spin-off effect, triggered by the proton 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 roughly 20 kcal/mol per the quinoid ring. Susceptibility toward the proton attack is further amplified by the alkyl substitution at the imino nitrogen atom and by additional substitution of the amino groups at specific positions within the framework of the quinoid building block(s). It is stressed that synthesis of the studied systems might provide very potent organic superbases.