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Abstract: (Chemical Abstract 2000, 133:281331)
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Accurate ab initio calculations at the B3LYP/6-311+G** level have been performed on
anionic and neutral structures of 4,5-dihydro-5-oxo-[1,2,4]triazolo[1,5-a]pyrimidine,
4,7-dihydro-7-oxo-[1,2,4]triazolo[1,5-a]pyrimidine,
and 4,5-dihydro-7-amino-5-oxo-[1,2,4]triazolo[1,5-a]pyrimidine compounds.
The quality of the theoretical results has been tested against the experimental
X-ray structures for the three most stable neutral forms, yielding very good agreement.
The electronic properties of the anionic forms have been evaluated by means of the atoms
in molecules theory (AIM) and electrostatic potential analyses, allowing the
rationalization of the coordination mode preferences observed experimentally for the anions.
Moreover, a theoretical study of the tautomerism for the neutral forms has also been made,
by energetic, geometrical and NMR chemical shift (GIAO method) calculations.
The results gave the protonated forms at the N4 atom as the most stable ones,
corroborated by the experimental geometries and the NMR data. In addition,
the preferred coordination mode at the N3 atom, against N1 or the carbonyl oxygen,
is also discussed.
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