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 T. Dziembowska a , M. Szafran b, * , A. Katrusiak b , Z. Rozwadowski a a West Pomeranian University of Technology, Al Powstan ´ców 42, 70065 Szczecin, Poland b Faculty of Chemistry, Adam Mickiewicz University, ul. Grunwaldzka 6, 60780 Poznan ´, Poland article info Article history: Received 1 April 2009 Accepted 1 April 2009 Available online 10 April 2009 Keywords: Schiff base X-ray diffraction NMR and FTIR spectra B3LYP calculations abstract 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 (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 13 C 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 1 H and 13 C NMR chemical shifts (d exp ) of keto (3) and enol (4) units in five solvents and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors (r calc ) using COSMO screen- ing solvation model, d exp = a + br calc . Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Schiff bases derived from aromatic o-hydroxyaldehydes have re- ceived special attention due to their interesting termochromism and/or photochromism [1,2], biological properties [3,4], as well as a variety of potential applications, e.g. for optical data storage [1,2,5], as nonlinear optical materials [6,7], anticorrosive materials [8] or anticancer medicines [9,10]. One of the most characteristic structural features of these Schiff bases is the presence of the intra- molecular hydrogen bonding [11–13]. Intramolecular proton trans- fer equilibrium has been recognized in a number of Schiff bases of aromatic orto-hydroxyaldehydes. Since the pioneer studies by Du- dek [14,15], a large number of experimental UV–vis [14,16–20], 1 H, 13 C, 15 N NMR [16,21–30] and theoretical [19,29,31–34] works on tautomerism in Schiff bases have been published (Scheme 1). In this work crystal structure of a Schiff base derived from 2-hydro- xy-1-naphthaldehyde and methylamine and solvent effect on its tautomeric equilibrium are studied by X-ray diffraction, DFT, NMR and IR methods. 2. Experimental The title Schiff base was prepared as described previously [26]. A solid product obtained was recrystallized from hexane, m.p. 132–133 °C. Deuterated analogues were obtained by repeated crystallization from CH 3 OD. The X-ray diffraction measurements were carried out on a KUMA KM4-CCD diffractometer [35]. The structure was solved by direct methods with the SHELXS-97 program [36] and refined by the full-matrix least-squares method on F 2 using the SHELXL- 97 program [37]. The H-atoms were located from the molecular geometry, except for the N–H atoms that were found in the dif- ference Fourier map and refined with isotropic thermal parame- ters. Electron-density peaks about the methyl hydrogens indicated the disorder of the methyl groups and therefore the methyl hydrogens were assumed to be regularly disordered in two orientations and refined as rigid groups in two orientations. The crystal data, together with the details on the data collection and structure refinement, are given in Table 1. The molecular structure and atom numbering of 1-(N-methylaminomethylene)- 2-naphthalenone dimer (1) are shown in Fig. 1, while the crystal packing in Fig. 2. The atomic coordinates, bond lengths (Å), bond and torsion angles (deg) are listed in Tables 2 and 3. The param- eters in the CIF form are available as Electronic Supplementary 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.04.001 * Corresponding author. E-mail address: szafran@amu.edu.pl (M. Szafran). Journal of Molecular Structure 929 (2009) 32–42 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc