RELATIVE ENTHALPIES OF 1,3-DIMETHYL-2,4-PYRIMIDINEDIONE, 2,4-DIMETHOXYPYRIMIDINE, AND 4-METHOXY-1-METHYL-2-PYRIMIDINONE - ESTIMATION OF THE RELATIVE STABILITIES OF 2 PROTOMERS OF URACIL

The relative gas-phase enthalpies of 1,3-dimethyl-2,4-pyrimidinedione (5), 2,4-dimethoxypyrimidine (6) and 4-methoxy-1-methyl-2-pyrimidinone (7) were determined by calorimetric measurements of the heats of isomerization in the liquid and estimates of the heats of vaporization. The values of .DELTA.Hg.degree. are -38 .+-. 4.7 kcal/mol for 5-6 and -27 .+-. 4.1 kcal/mol for 5-7. The relative energies of the amide-imidate functions in a heteroaromatic system can be quite different. The relative enthaplies for 5, 6 and 7 are used to provide estimates of the enthalpy differences between uracil (11) and 2,4-dihydroxypyrimidine as -22 .+-. 10 kcal/mol and between 11 and 4-hydroxy-2-pyrimidinone as -19 .+-. 6 kcal/mol. Although the errors are large, this is the first experimentally based estimate for the relative stabilities of these tautomers in the vapor. The relative energies of uracil protomers in solution may be affected more by H- bonding than by reaction field effects.