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].
As a [beta]-diketone, curcumin also exists as a keto-enol tautomer
that chelates transition metal ions with high affinity.
Using Pipeline Pilot of SciTegic to enumerate the tautomer
, stereoisomers and protonation/deprotonation form at pH 7.
Principle: Creatinine in alkaline medium reacts with picric acid to form a red tautomer
of creatinine picrate, the intensity of which is measured by colorimeter at 520nm.
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.
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.
1]H NMR) and UV-vis spectroscopy, X-ray analysis (20), (21) or fluorescence spectroscopy (22), (23), the latter arguing the formation of keto tautomer
accompanied of a large Stokes shift (100 nm).
In gas phase, one tautomer
is energetically favored over the other by a difference of 1.
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.