Matrix-isolation FT-IR studies and ab-initio calculations of hydrogen-bonded complexes of molecules modeling cytosine or isocytosine tautomers. 2. 4-Aminopyridine and 4-aminopyrimidine complexes with H₂O in Ar matrices

H-bonding interactions of the cytosine-modeling compounds 4-aminopyridine and 4-aminopyrimidine with water are investigated with the use of a combined experimental matrix-isolation FT-IR and theoretical ab-initio method. Both bases appear exclusively as the amino tautomers in low-temperature Ar matrices, in accordance with large ab-initio predicted energy differences with the rare imino forms. The most stable complex of water with 4-aminopyridine is the H-bonded N₁⋯HO-H structure, but the energy difference with the alternative N-H⋯OH₂ complex is calculated to be only 6.7 kJ/mol. Both complexes are observed in the Ar matrix. In the case of 4-aminopyrimidine, the closed N-H⋯O-H⋯N₃ structure containing two H-bonds is found to be significantly more stable than the singly H-bonded structures N₁⋯HO-H and N-H⋯OH₂ by 8.0 and 10.2 kJ/mol, respectively. The previously suggested relation between the scaling factor for the theoretically-calculated v^b_OH mode of the H-bonded water and the proton affinity value of the proton-acceptor site in the base is developed further.