The IR spectra of a series of 2-substituted-6-hydroxy-4-pyrimidine carboxylic
acids (substituent = OH, SH, CH3, CH3S and NH2) were studied from the aspect
of the influence of the subsitutent on the polarizability of some bonds,
keto-enol tautomerism and hydrogen bond formation. The spectra were taken
using solids due to the low solubility of the acids. Theoretical
calculations were done using the MNDO-AM1 semiempirical molecular-orbital
method. The stabilities of various tautomers were calculated simulating the
dielectric continuum using the COSMO facility of the MOPAC program package.
Theoretical calculations were made for all the possible tautomers of the
2-substituted-6-hydroxy-4-pyrimidine carboxylic acids. For the most stable
isomers, the vibrational spectra were calculated. For the majority of the
compounds the most stable isomer was identified having the structure
2-Y-6-oxo-4-carboxy-3H-pyrimidine. Besides this structure, for the 2-amino-,
and 2-methyl- derivatives the zwitterionic forms have very similar
stability. The 2-hydroxy compound is most stable as the 2,6-dioxo-1H, 3H
isomer. The calculated vibrations for the compounds with a single stable
structure correlate very well with the experimental frequencies. For the
2-methyl- and 2-amino- compounds the correlation is considerably less
satisfactory. The most probable reason for this deviation is the existence
of two or more tautomets in equilibrium. The correlation of the measured
frequencies and the pKa values of the acids, indicate that the same
tautomers exist in the solid state and in the solution.
Infrared study of some 2-substituted-6-hydroxy-4-pyrimidine carboxylic acids: Correlation with MO-calculations
