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 K_T(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 Na₃⋯H-O⋯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 some trace amounts of the i-o E tautomer in the Ar matrix, H-bonding of this rare form with water was detected in the form of the closed N₁₄⋯H-O⋯H-N₃ structure. A preliminary correlation between the frequency shifts of the bonded water stretching mode and the proton affinities of the C=O group is presented for the open C=O⋯H-OH complexes of 1-CH₃-cytosine and some related compounds.