Absolute proton affinity of some polyguanides

The problem of the absolute proton affinity (APA) of some polyguanides is a ddressed by the MP2(fc)/G-311+G**//HF/6-31G* theoretical model. It is shown that the linear chain polyguanides exhibit increased basicity as a functio n of the number of guanide subunits. However, the saturation effect yields an asymptotic APA value of 254 kcal/mol. Branched polyguanides on the other hand have higher APAs than their linear counterparts. The largest proton a ffinity is found in a doubly bifurcated heptaguanide, being as high as 285 kcal/mol, thus potentially representing one of the strongest organic bases. Finally, it is found that all polyguanides protonate at imino nitrogen ato ms, since they are apparently susceptible the most to the proton attack. Th e origin of their very high intrinsic basicity is traced down to a dramatic increase in the resonance interaction of the corresponding conjugate bases . For instance, the increase in the resonance energy in the protonated guan idine is estimated to be in a range of 24-27 kcal/mol, which is higher than the aromatic stabilization in benzene. The proton affinity of some polycyc lic guanides including Schwesinger proton sponge and porphine is briefly di scussed.