INVESTIGATION OF THE STRUCTURE AND THE PROPERTIES OF AMMELINE, MELAMINE, AND 2,4-DIAMINO-1,3,5-TRIAZINE BY ABINITIO CALCULATIONS

Ammeline, melamine, and 2,4-diamino-1,3,5-triazine have been studied by ab initio calculations at the SCF-level using several different basis sets. It is predicted that melamine has pyramidal amino groups (one up, and two down) with angles between the C-N bond and the NH2-plane of 12.2-degrees (one) and 9.9-degrees (two) using the 6-31G** basis set. It is also predicted that the amino groups of ammeline and 2,4-diamino-1,3,5-triazine were planar. The hydroxy tautomer of ammeline, 1, is predicted to be 4.82 kcal/mol more stable than the carbonyl tautomer, 2, and 23.8 kcal/mol more stable than the quinoid carbonyl tautomer, 11. The experimental literature is discussed in this context. Both protonated ammeline and deprotonated ammeline were also studied. The calculations predicted that the protonation of ammeline occurred on the ring nitrogens. Even triprotonated ammeline is protonated only at the ring nitrogens. The hydroxyl group of ammeline was more acidic than that of phenol. The energy change of the isodesmic proton transfer from ammeline to phenoxide was -10.11 kcal/mol (6-31G** basis set). The amino groups are significantly pyramidal in the optimized geometry of the ammeline anion. The angle between the C-N bond and the NH2 plane was 43.9-degrees. This causes the amino groups of the anion to be far more nucleophilic toward formaldehyde than those of ammeline itself