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 SHIFTFOR 2-HYDROXYPYRIMIDINE AND 5-BROMO-2-HYDROXYPYRIMIDINE

The H-bond interaction of the cytosine model compound 2-hydroxypyrimid ine and its 5-bromo derivative with water is investigated using the co mbined matrix-isolation FT-IR and theoretical ab initio method. As pre dicted 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 noticea ble shift of the tautomeric equilibrium towards the oxo form is observ ed. 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 sy stems for the hydroxy and the oxo tautomers, respectively. The experim ental spectra are consistent with this prediction, but additional stru ctures, 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 be tween the calculated and measured frequencies smaller than expected fr om the correlation established before for open, singly H-bonded comple xes involving similar molecules. Although some cooperativity exists be tween the two H bonds in each of the closed complexes, this effect is limited because the geometrical structures of both H bonds are noticea bly perturbed from the perfect alignment due to the cyclic arrangement of the complex. A possible mechanism of the proton transfer process l eading from the hydroxy to the oxo tautomeric form is discussed in ter ms of proton tunneling and in relation to recent literature data.