1 H NMR, IR and UV/VIS Spectroscopic Studies of Some Schiff Bases Derived From 2-Aminobenzothiazole and 2-Amino-3-hydroxypyridine Raafat M. Issa, Abdalla M. Khedr* and Helen Rizk Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt By condensing 2-aminobenzothiazole with 2-hydroxy-1-naphthaldehyde, 2-hydroxybenzaldehyde, 4-methoxybenzaldehyde, 4-hydroxybenzal-dehyde, benzaldehyde and 4-dimethylaminobenzaldehyde, and five Schiff bases Ia-Ie are prepared. Also, two Schiff bases IIa and IIb are prepared by condensation of 2-amino-3-hydroxypyridine with 2-hydroxy-1-naphthaldehyde and 2-hydroxybenzaldehyde. The 1 H NMR, IR and UV/Vis spectra of these seven Schiff bases are investigated. The signals of the 1 H NMR spectra as well as the important bands in the IR spectra are considered and discussed in relation to molecu- lar structure. The UV/Vis absorption bands in ethanol are assigned to the corresponding electronic transi- tions and the electronic absorption spectra of Schiff bases Ib and IIb are studied in organic solvents of dif- ferent polarities. The UV/Vis absorption spectra of 2-amino-3-hydroxypyridine Schiff bases IIa and IIb are investigated in buffer solutions of different pH values containing 5% (v/v) methanol, and the results are utilized for the determination of pK a and DG* of the ionization of the phenolic OH-groups. The fluo- rescence spectra of IIa and IIb are studied in organic solvents of different polarities. The obtained spectral results are confirmed by some molecular calculations using the atom super posi- tion and electron delocalization molecular orbital theory for the Schiff base IIb. Keywords: 2-Aminobenzothiazole; 2-Amino-3-hydroxypyridine; Benzaldehyde derivatives; Schiff bases; Spectral studies; Molecular orbital calculations. 1. INTRODUCTION Although there are wide applications of Schiff bases and their metal chelates in biological systems, 1,2 cataly- sis, 3,4 dying processes, 5,6 and analytical applications, 7 the spectral studies of the Schiff bases containing a hetero- cyclic ring are comparatively minor. 7-9 In the present article, the 1 H NMR, IR and UV/Vis spectra of seven Schiff bases derived from 2-aminobenzo- thiazole and 2-amino-3-hydroxypyridine (Scheme I) are considered. The UV/Vis absorption spectra of Schiff bases Ib and IIb as well as the fluorescence spectra of IIa and IIb are investigated in organic solvents of different polarities and discussed with respect to different solvent parameters. The ionization of phenolic OH-groups of 2-amino-3-hy- droxypyridine Schiff bases IIa and IIb is studied in aqueous buffer solutions of varying pH values. The results of spec- tral studies are supported by some molecular orbital calcu- lations using the ASED-MO theory for the Schiff base IIb. 2. EXPERIMENTAL All compounds used in the present study were of pure grade available from BDH, Aldrich or Sigma. The solvents used for the spectral study were spectroscopic grade from Aldrich. The Schiff bases were obtained by refluxing equi- molar quantities of the aromatic amine and the aldehyde (0.01 mole of each in 25 mL ethanol) on a water bath for 2-5 hrs. 10 The Schiff base compounds precipitated on cool- ing, were then filtered and purified by recrystallisation Journal of the Chinese Chemical Society, 2008, 55, 875-884 875 * Corresponding author. E-mail: abkhedr2001@yahoo.com; Tanta, P. O. 31527. Scheme I General formulae of the Schiff bases under investigation