Tautomery and H-bonding characteristics of 2-aminopurine: A combined experimental and theoretical study

An experimental and theoretical RHF, MP2 and DFT/6-31++G** study is described of the matrix FT-IR spectra of monomer 2-aminopurine and H-bonded complexes of 2-aminopurine with water. 2-aminopurine occurs in Ar predominantly as the amino-N9H tautomer, but small amounts of the amino-NTH tautomer are also present. An approximate K_T value for this tautomeric equilibrium is found to be 0.016 (RHF) and 0.015 (DFT) using the infrared intensity measurement. Four H-bonded complexes of the abundant amino-N9H form with water are detected in the experimental FT-IR spectrum by their characteristic predicted absorptions, i.e. the three closed complexes N3⋯ H O⋯-H-N9, N1⋯ H-O⋯ H NH, N3⋯ H-O⋯ H-NH and the open complex NT⋯ H OH. From the experimental results, the proton affinity of the N7 atom in 2-aminopurine can be estimated. The dependence of the H-bond strength on the H-bond linearity is demonstrated by a correlation between the N⋯ H distance and the N⋯ H O angle in closed N⋯ H-O⋯ H-N complexes.