Adenine and Some of Its Analogues in DMSO-d6 Solution: an NMR and GIAO-DFT Study

The results of GIAO-DFT calculations [at B3LYP/6-311++G(2d,p) PCM and PBE1PBE/6-311+G(2fd,p) PCM levels] of 13C NMR and 15N NMR parameters have been employed to reinvestigate tautomerism of adenine in DMSO solution. The separate 13C NMR sig nals of N7-H and N9-H adenine tautomers have been observed for the first time. The ex erimental 13C chemical shifts, 3J(C,H) coupling constants and 15N chemical shifts of adenine and three of its methyl derivatives (9MeA, 7MeA and 3MeA) have been quantitatively analyzed using the calculated parameter values. This procedure has provided the solution populations of the predominant adenine tautomers being in agreement with the available literature data. The apparent dependence between the amino group geometry and the chemical shift of the adjacent carbon in 2-aminopyridine system has also been discussed.