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 investi gation of hydrogen-bonded complexes between water and molecules modeli ng 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 predicti on. An approximate value of 7.2 is obtained for the tautomerization co nstant 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 th e closed N-3...H-O...H-N-14 structure, which shows cooperativity betwe en the two H-bonds. Less stable complexes are the closed N-3...H-O-H.. .O-7 complex and the open N-14-H...OH2 and N-14...H-OH structures, In the experimental FT-IR spectra, the closed N-3...H-O...H-N-14 and open O-7...H-OH complexes are identified based on characteristic absorptio ns predicted by ab initio calculations or based on earlier obtained ex perimental results for similar structures, Despite some trace amounts of the i-o E tautomer in the Ar matrix, H-bonding of this rare form wi th water was detected in the form of the closed N-14...H-O...H-N-3 str ucture, A preliminary correlation between the frequency shifts of the bonded water stretching mode and the proton affinities of the C=O grou p is presented for the open C=O...H-OH complexes of 1-CH3-cytosine and some related compounds.