Table 1: DFT/B3LYP (3-21G) parameters over all proposed tautomers
(12-18 and 21A-C).
The compound also exists as a keto-enol tautomer
in aqueous solution that can form stable complexes stoichiometrically with transition metals including copper (II) (Kanhathaisong et al.
The NMR spectra reported below refer to the major tautomer
The model of tautomer
state of PSTM-3T (Figure 3(c), Scheme 2) was investigated using the density functional B3LYP method with 6-311G(d, p) level.
In one suggested conformer (18,19), the enol tautomer
of l-methadone forms an intramolecular hydrogen bond with the nitrogen producing a seven-membered ring (Figure 2.2a-d).
In this case we thought that 49b-3, an enehydrazine tautomer
of 49b-2, was stable by hydrogen bonding, and that is the reason why 49b-2 was predominant.
In gas phase, one tautomer
is energetically favored over the other by a difference of 1.7 kcal/mol.
A freeware version is also available, which contains tools for tautomer
prediction; 2-D structure cleaning; 3-D optimization and viewing; and drawing of polymers, organometallics and Markush structures.
Grande-Allen adds, "My students appreciate ACD/ChemSketch not only because it is free for academic use, but also because of the added features such as tools for tautomer
prediction, 2D structure cleaning, and 3D viewing."
In all but one instance involving methyl 2(aminosulfonyl)benzoate, only a single tautomer
resulted after recrystallization from ethanol.
This positive-ion formation for dicarboxylic acid diesters can be explained through resonance of the tautomer
structures promoted by the delocalization of charge within the molecule (Fig.
The band attributed to the N-H stretching of thione tautomer
appeard at low frequency due to the intramolecular hydrogen bond N-H--S, but it is difficult to differentiate between the N-H and C-H stretching vibrations with certainty [11, 35].