the solid phase because it was exclusively crystallized, as proven
by single crystal X-ray structure. However, in the solution phase
probably Form B binds better to acetate as suggested by NMR titration spectral analysis. The peak at slightly downfield (d 12.57 ppm)
probably belongs to –OH of Form A and the other at d 12.52 due to
–OH of Form B existing in solution. However the other possible
tautomer having vinylic –NH is probably not present as the vinylic
–NH should not appear at such down field (i.e. around d 12 ppm).
So possibly there is an energy barrier between the two hydrogen
bonded conformers A and B showing two peaks. The –OH proton
of L itself appeared at d 12.5 (H a ) ppm, downfield from its normal
value (d 10 ppm), due to its intramolecular H-bonding between
imine nitrogen and naphthol –OH group which is shown in scheme
2. However, in case of the titration experiments for the receptor L
and tetrabutylammonium acetate in DMSO-d 6 solvent, the proton
signal of the naphthol hydroxyl disappeared after the addition of
0.5 equiv of acetate and there was no appearance of proton signal
by naphthol hydroxyl group even after the addition of 1, 2, and
5 equiv of acetate. This indicates that there is a new complex formation between naphthol hydroxyl group and acetate ion.