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Matrix-Isolation FT-IR Studies and ab Initio Calculations of Hydrogen-Bonded Complexes of Molecules Modeling Cytosine or Isocytosine Tautomers. 5. 1-CH 3 -Cytosine Complexes with H 2 O in Ar Matrices

1996; American Chemical Society; Volume: 100; Issue: 16 Linguagem: Inglês

10.1021/jp951167v

1541-5740

Johan Smets, Ludwik Adamowicz, Guido Maes,

Spectroscopy and Quantum Chemical Studies

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 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 CO group is presented for the open CO···HOH complexes of 1-CH3-cytosine and some related compounds.