Crystal structure of and solvent effect on tautomeric equilibrium in Schiff base derived from 2-hydroxy-1-naphthaldehyde and methylamine studied by X-ray diffraction, DFT, NMR and IR methods

The structure of the Schiff base derived from 2-hydroxy-1-naphthaldehyde and methylamine (1) has been studied by X-ray diffraction, B3LYP/6-31G(d,p) calculations, NMR and FTIR spectroscopy. The crystal is monoclinic, space group P2(1)/c,with symmetry-independent two molecules forming a dimer(I)via two short intramolecular (2.592(2) and 2.579(2) angstrom) and two longer intermolecular (2.957(2) and 3.053(2) angstrom) N-H center dot center dot center dot O hydrogen bonds. The structures of dinner (2) and two monomers (3 and 4) have been analyzed by B3LYP/6-31G(d,p) level of theory. Tautomeric equilibrium due to the intramolecular hydrogen transfer between enol and ketol forms (4 reversible arrow 3) has been studied in five solvents on the basis of C-13 NMR chemical shifts and equilibrium constants, K-eq, determined. The equilibrium constants, K-eq, have been found to increase linearly with the solvent parameter, E-T. Linear correlations have also been established between the experimental H-1 and C-13 NMR chemical shifts (delta(exp)) of keto (3) and enol (4) units in five solvents and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors (sigma(calc)) using COSMO screening solvation model, delta(exp) = a + b sigma(calc). (c) 2009 Elsevier B.V. All rights reserved.