COMBINED MATRIX-ISOLATION FT-IR AND AB-INITIO 6-31-ASTERISK-ASTERISK STUDY OF H-BONDED COMPLEXES BETWEEN WATER AND MOLECULES MODELING CYTOSINE OR ISOCYTOSINE(+G)

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