Tautomeric equilibria of 2(4)-monooxopyrimidines in the gas phase,in low-temperature matrices and in solution

IR absorption spectra, including the NH, OH and CO stretching regions, have been recorded for 4-oxo-6-methyl- and 2-oxo-4,6-dimethyl pyrimidines and several related derivatives, in the gas phase, in low-temperature inert matrices, and in several liquid solvents.All the 4-oxopyrimidines in the gas phase, and 4-oxo-6-methylpyrimidine in low-temperature matrices, exhibit comparable populations of the keto and enol forms. By contrast the 2-oxopyrimidines are predominantly in the enol forms. Both classes of com pounds are predominantly in the keto form in liquid solvent systems. The tautomeric equilibrium constant (K_T) in the vapour phase for 4-oxo-2,6-dimethylpyrimidine is about 2, and for the other 4-oxopyrimidines is about 1. For 4-oxo-6-methylpyrimidine, the equilibrium constant in inert matrices varies slightly with the activity of the matrix gas, with the keto tautomer favoured in the more active matrix. From the temperature-dependence of K_T the free energy difference between the two tautomeric forms of 4-oxo-6-methylpyrimidine in the vapour phase has been calculated. Heats of vaporization have also been calculated for the various compounds and related to their abilities to associate by hydrogen bonding in the condensed phase.The UV absorption spectra of some of the foregoing have also been recorded in the gas phase, but these were of only limited value in studies of tautomeric equilibria, as connpared to the IR spectra.