Some structural aspects of ammeline - Keto preference and dimerization

Subtle tautomerism in ammeline is clarified by means of IR- and UV-spectroscopic measurements and DFT calculations. While the enol form is narrowly preferred as an isolated molecule, dimerization of the keto form is plausible in solid phase due to double hydrogen bonds. In aqueous solutions, the keto form is proved to be more stable than the enol form due to the large electric dipole moment of the peptide group. The keto preference is also consistent with the acidity and basicity of the related heterocycles.