Matrix-isolation FT-IR studies and ab initio calculations of hydrogen-bonded complexes of molecules modeling cytosine or isocytosine tautomers. 6. Experimental observation of a water-induced tautomeric shift for 2-hydroxypyrimidine and 5-bromo-2-hydroxypyrimidine

The H-bond interaction of the cytosine model compound 2-hydroxypyrimidine and its 5-bromo derivative with water is investigated using the combined matrix-isolation FT-IR and theoretical ab initio method. As predicted by the ab initio calculations, both compounds occur dominantly in the hydroxy tautomeric forms. The estimated K_T(h/o) values are 60 and 184, respectively. When water is added to the Ar matrix, a noticeable shift of the tautomeric equilibrium towards the oxo form is observed. The theoretical results indicate that the closed N⋯H-O(H)⋯H-O and C=O⋯H-O(H)⋯H-N H-bonded water complexes are the most stable systems for the hydroxy and the oxo tautomers, respectively. The experimental spectra are consistent with this prediction, but additional structures, such as an open N⋯H-OH complex of the hydroxy tautomer, are also identified. The frequency shift of the stretching mode of doubly H-bonded water in the two closed complexes is larger, and the ratio between the calculated and measured frequencies smaller than expected from the correlation established before for open, singly H-bonded complexes involving similar molecules. Although some cooperativity exists between the two H bonds in each of the closed complexes, this effect is limited because the geometrical structures of both H bonds are noticeably perturbed from the perfect alignment due to the cyclic arrangement of the complex. A possible mechanism of the proton transfer process leading from the hydroxy to the oxo tautomeric form is discussed in terms of proton tunneling and in relation to recent literature data.