Computational and experimental studies on the IR spectral and structural changes caused by the conversion of 5,5-diethyl(1H, 3H, 5H)-pyrimidinetrione into azanion and diazanion

The IR spectral and structural changes, caused by the conversion of 5,5-diethyl(1H, 3H, 5H)-pyrimidinetrione (Veronal, 1) into azanion and diazanion have been studied by spectroscopic experiments, HF and B3LYP computations. The triketoform is by 65.54 (HF) or 61.51 (B3LYP) kJ mol−1 more stable than the 6-hydroxy tautomer. The conversion of molecule into azanion causes strong frequency decreases of the three carbonyl stretching bands νCO: computed 270 cm−1 (B3LYP/6-31++G∗∗), measured 243 cm−1 (solid) or 229 cm−1 (in DMSO-d6) (total values), and other IR changes. In a good agreement between our computational and literature X-ray diffraction data, the molecule → azanion conversion results in strong CN bond shortenings at the azanionic center N1 and strong lengthenings of the CO bonds next to it. The formation of Veronal-diazanion has been assumed to take place in DMSO-d6/CD3ONa solutions. The total νCO decrease, caused by the molecule → diazanion conversion is: predicted 580 cm−1 (B3LYP/6-31++G∗∗), measured 603 cm−1 (in DMSO-d6). According to the computations, the azanion → diazanion conversion causes CN bond shortenings at the second azanionic center N3 and CO bond lengthenings next to it.