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-N7H 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 N7 ^... 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. Received 10 December 2001 Published online 13 September 2002