Combined matrix-isolation FT-IR and ab initio 6-31++G** study of H-bonded complexes between water and molecules modelling cytosine or isocytosine

A step-by-step experimental and ab initio SCF/6-31++G** procedure is described for the interpretation of the matrix FT-IR spectra of H-bonded complexes of cytosine or isocytosine with water. The method involves a detailed study of several compounds, each modelling one or a few cytosine sites and/or tautomers in order of increasing complexity. Results are presented for tautomerization processes in 2-OH-pyridine, 4-OH-pyrimidine and isocytosine, and for H-bonded complexes of water with pyrimidine, 4-OH-pyridine and 4-NH₂-pyridine. The whole series of obtained results has allowed the setting up of a large-scale vibration correlation diagram for water complexed with N- and O-bases. It also allows some new, important conclusions to be drawn about the major problem of scaling ab initio predicted frequencies for anharmonic proton-donor vibrational modes.