Matrix isolation FT-IR studies and ab initio calculations of hydrogen-bonded complexes of molecules modeling cytosine or isocytosine tautomers. 1. Pyridine and pyrimidine complexes with H₂O in Ar matrices

An experimental method is described for a vibrational spectroscopic study of the H-bonding properties of cytosines and isocytosines in low-temperature inert matrices. The method comprises a detailed experimental and theoretical ab initio study of several molecules of increasing complexity, each modeling one or more interaction sites of cytosine tautomers which have been demonstrated to occur in low-temperature Ar matrices. A first series of results is presented for the simple N-bases pyridine and pyrimidine complexed with H₂O in Ar. Pyridine forms the stronger N⋯H₂O complex, which is manifested in relatively far displaced H₂O absorptions and in stronger perturbations of the base ring fundamentals. The experimental results are supported by the SCF/6-31++G** predicted shifts. Two different 1:2 complexes are identified for pyrimidine/H₂O in annealed matrices. The presence of N⋯H-O and H-O⋯H-O H-bonds in the first and of N₁⋯H-O and N₃⋯H-O H-bonds in the second complex is deduced from strongly different H-bond cooperativities in these complexes.