Matrix-Isolation FT-IR Studies and ab Initio Calculations of Hydrogen-Bonded Complexes of Molecules Modeling Cytosine or Isocytosine Tautomers. 5. 1-CH3-Cytosine Complexes with H2O in Ar Matrices

The combined experimental and theoretical ab initio method for investigation of hydrogen-bonded complexes between water and molecules modeling cytosines has been applied to 1-CH3-cytosine. This compound occurs in Ar predominantly in the amino−oxo form, but traces of the imino−oxo E tautomer are also present in accordance with the ab initio prediction. An approximate value of 7.2 is obtained for the tautomerization constant KT(a−o/i−o) using the infrared intensity measurement. The most stable H-bonded complex of the abundant a−o tautomer with water is the closed N3···H−O···H−N14 structure, which shows cooperativity between the two H-bonds. Less stable complexes are the closed N3···H−O−H···O7 complex and the open N14−H···OH2 and N14···H−OH structures. In the experimental FT-IR spectra, the closed N3···H−O···H−N14 and open O7···H−OH complexes are identified based on characteristic absorptions predicted by ab initio calculations or based on earlier obtained experimental results for similar structures. Despite...