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 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry