Tautomery and H-bonding characteristics of : 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 and complexes of with water. occurs in Ar predominantly as the amino–N9H tautomer, but small amounts of the amino–N7H tautomer are also present. An approximate KT value for this tautomeric equilibrium is found to be 0.016 (RHF) and 0.015 (DFT) using the infrared intensity measurement. Four 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$\cdots$H–O$\cdots$H–N9, N1$\cdots$H–O$\cdots$H–NH, N3$\cdots$H–O$\cdots$H–NH and the open complex N7$\cdots$H–OH. From the experimental results, the proton affinity of the N7 atom in can be estimated. The dependence of the strength on the linearity is demonstrated by a correlation between the N$\cdots$H distance and the N$\cdots$H–O angle in closed N$\cdots$H–O$\cdots$H–N complexes