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The Proton Affinity of Some Extended pi-Systems

The absolute proton affinity (APA) of some extended pi-electron systems involving guanidine, cyclopropeneimine and quinoneimine subunits is explored by theoretical MP2 and scaled Hartree-Fock (HFSC) models. It is shown that some of the studied model molecules should exhibit higher gas-phase basicity than the Schwesinger proton sponge (SPSG) and Schwesinger phosphazene (SPH), which are considered to be the most powerful neutral superbases. The origin of the increased basicity is identified as a dramatic resonance effect in cations triggered by the protonation. It is interesting to note that the examined model compounds possess higher PAs than their polyguanide counterparts. The reason behind this is the well-established fact that three-membered (cyclopropene) and six-membered (quinoid) rings undergo aromatization in the conjugate acid forms. The important role of substituents varying in their electron donor/acceptor properties is underscored. Special features like the intramolecular hydrogen bond (IHB) corona effect are briefly discussed.

proton affinity; basicity; neutral organic superbases

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