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 Two Protomers of Uracil

Article abstract of DOI:10.1021/ja00389a032

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) have been determined by calorimetric measurements of the heats of isomerisation in the liquid and estimates of the heats of vaporization.The values of ΔH_g⁰ are -38 +/- 4.7 kcal/mol for 5-6 and -27 +/- 4.1 kcal/mol for 5-7.These results show the relative energies of the amide-imidate functions in a heteroaromatic system can be quite different.The relative enthalpies for 5,6, and 7 are used to provide estimates of the enthalpy differences between uracil (11) and 2,4-dihydroxypyrimidine (16) as -22 +/- 10 kcal/mol and between 11 and 4-hydroxy-2-pyrimidinone (13) 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.It is suggested that the relative energies of uracil protomers in soluion are effected more by hydrogen bonding than by reaction field effects.