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 P21/c, with symmetry-independent two molecules forming a dimer (1) via two short intramolecular (2.592(2) and 2.579(2) Å) and two longer intermolecular (2.957(2) and 3.053(2) Å) N–H···O hydrogen bonds. The structures of dimer (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 ⇌ 3) has been studied in five solvents on the basis of 13C NMR chemical shifts and equilibrium constants, Keq, determined. The equilibrium constants, Keq, have been found to increase linearly with the solvent parameter, ET. Linear correlations have also been established between the experimental 1H and 13C NMR chemical shifts (δexp) of keto (3) and enol (4) units in five solvents and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors (σcalc) using COSMO screening solvation model, δexp = a + bσcalc.