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 st udy of the H-bonding properties of cytosines and isocytosines in low-t emperature inert matrices. The method comprises a detailed experimenta l and theoretical ab initio study of several molecules of increasing c omplexity, each modeling one or more interaction sites of cytosine tau tomers which have been demonstrated to occur in low-temperature Ar mat rices. A first series of results is presented for the simple N-bases p yridine and pyrimidine complexed with H2O in Ar. Pyridine forms the st ronger N...H2O complex, which is manifested in relatively far displace d H2O absorptions and in stronger perturbations of the base ring funda mentals. The experimental results are supported by the SCF/6-31++G* p redicted shifts. Two different 1:2 complexes are identified for pyrimi dine/H2O in annealed matrices. The presence of N...H-O and H-O...H-O H -bonds in the first and of N-1...H-O and N-3...H-O H-bonds in the seco nd complex is deduced from strongly different H-bond cooperativities i n these complexes.