Naslov
Absolute Proton Affinity of Some Polyguanidines

Autori
Kovačević, Borislav ; Maksić, Zvonimir B.

Izvornik
Journal of organic chemistry (0022-3263) 65 (2000); 3303-3309

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
organic superbases; proton affinity; resonance effect; polyguanidines; basicity

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

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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