The proton affinity of some extended pi-systems involving guanidine and cyclopropenimine subunits

The proton affinity (PA) of some extended pi -electron systems with guanidi ne and cyclopropenimine structural motifs is explored by theoretical MP2 an d HFSC models. It is shown that some of the studied molecules should exhibi t higher gas-phase basicity than the Schwesinger proton sponge II, which is considered to be one of the most powerful organic superbases. The origin o f the increased basicity is traced to a dramatic resonance effect triggered by the protonation. It is interesting to note that the examined compounds possess higher PAs than their polyguanide counterparts. The reason behind t his is the well-established fact that three-membered rings undergo aromatiz ation in the conjugate acids form. The important role of substituents in de termining high inherent basicities is underscored.