EFFECT OF THE H-BONDING ON AROMATICITY OF PURINE TAUTOMERS
Presented by Dr. Halina SZATYLOWICZ
Purine, as a mother template of important nucleobases, is the good model system to study such properties of these systems as pi-electron delocalization and interactions with acidic or basic partners. For this purpose, modeling of equilibrium H-bond formation of the types N…HF and NH…F– was used . H-bonded complexes of purine (four complexes for each tautomer: 1-H, 3-H, 7-H and 9-H purines) were studied . In the case of NH…F– interactions, a proton is transferred to fluoride and hydrofluoric acid is formed. In consequence, two types of H-bonds were considered: conventional, N…HF, and charge-assisted, N–…HF. For all complexes optimized structural parameters, relative energies and strengths of H-bonds were calculated at the B3LYP/6-311++G** level theory. Detection and classification of the observed hydrogen bonds were performed by means the Quantum Theory of Atoms in Molecules. The Popelier’s criteria were applied to the critical point (CP) describing H-bond formation. These criteria were fulfilled by all H-bonds for the studied complexes. Moreover, the negative value of the total energy density at CP suggested partially covalent character of all intermolecular interactions. The ionic H-bonds, N–…HF, are stronger by ~10 kcal/mol than the neutral ones. For all N…HF interactions in a particular purine tautomer, the greatest basicity (and, thus, the strongest H-bonding) is observed if basic and acidic centers belong to different rings. To study the effect of H-bond formation on pi-electron delocalization two aromaticity indices (HOMA and NICS) were applied. The H-bonded complexes for 7-H and 9-H tautomers are characterized by higher aromaticity (HOMAtot) and a much lower range of HOMA variability. The 6-membered rings without NH group (in 7-H and 9-H tautomers) exhibit higher aromaticity (mean HOMA6 is equal to 0.971), whereas for 1-H and 3-H tautomers, the aromaticity is lower (mean HOMA6 = 0.742). To the contrary, in the 5-membered rings, the presence of NH favors aromaticity. The HOMA index calculated for 7-H and 9-H tautomers (mean HOMA5 = 0.833) is higher than the corresponding HOMA value obtained for 1-H and 3-H (mean HOMA5 = 0.720). Similar trends are observed for NICS values, since there is an acceptable linear regression between NICS and HOMA. This work was performed under MPD/2010/4 project which is realized within the MPD programme of Foundation for Polish Science, cofinanced from European Union, Regional Development Fund.  T.M. Krygowski, J.E. Zachara, H. Szatyłowicz J. Phys. Org. Chem. 2005, 18, 110–114.  O.A. Stasyuk, H. Szatylowicz, and T.M. Krygowski J. Org. Chem. 2012, 77, 4035–4045.
Location: Portorož, Slovenia
Address: University of Primorska, Faculty of Tourism Studies, Obala 11a, SI-6320 Portorož - Portorose, Slovenia