Solvents Effect on Geometries and Electronic Properties of Picolinic Acid : A theoretical Study

The DFT(B3LYP) with five basis sets (6–31G*, 631G**, 6-311G**, 6-311+G**, 6-311++G**) was used to study the effect of basis sets on geometry, vibration frequencies, electronic properties and chemical shifts. The largest differences in geometry were observed in ethanol with 0. 006Å, 0. 014Å, -0. 015Å, -0. 005Å and 0. 004Å for N-C1, C6-O1, C6-O2, C1-C6 and O2-H bonds respectively which show that solvent-molecule interactions are more prominent around the heteroatoms especially the polar solvents. The magnitude of solvation energies and dipole moments and solvents effect factor (þ) calculated for both C NMR and H NMR was in agreement with the experimental degree of polarity of the solvents. 1.0 INTRODUCTION Picolinic acid is a six-membered ring structure and an isomer of nicotinic acid and isonicotinic acid containing a five member carbon atoms, a nitrogen and a carbonyl at position 2, for picolinic acid and position 3, and 4 respectively for nicotinic and isonicotinic acid. They bind metal ions in a bidentate fashion with nitrogen and oxygen atoms donor forming a stable five member chelate ring [1-3]. Picolinic acid and its derivatives have continued to attract attention from various researchers mainly for its structural, biological and industrial applications . Picolinic acid is the body’s prime chelator and is effective with minerals ions such as Cr, Zn, Mn, Cu, Fe and perhaps Mo. It is generated in liver and kidneys during the catabolism of the essential amino acid tryptophan and, subsequently, transported to the pancreas. During digestion it is secreted to the intestine and is used as a complexing agent in and cosmetic the absorption of essential metals [4,5]. They are also useful as food preservatives [6] as well as antiseptic agents in food, pharmaceutical and textile industry [7-11]. They are used as building block in photovoltaic devices [12], and as model systems in environmental studies [13]. They are found to exhibit intermolecular hydrogen bonds in the solid state to form polymeric chains [14-16]. The reactivity of three isomeric pyridine monocarboxylic acids with diazodiphenylmethane in twenty aprotic solvents has been studied. It was reported that the reactivity was highly dependent on the relative positions of the ring nitrogen and the carboxylic group [17,18], which was also observed at the very beginning of the study of heterocyclic carboxylic acids in their reaction with ethanol but the order of reactivity was reversed in some of the solvents, particularly in aprotic ones [19]. The vibrational frequencies of picolinic, nicotinic, isonicotinic acids have been studied [20,21], analyzed and the designed experimental bands were compared to that scaled theoretical calculated at B3PW91/6-311++G** level. The wavenumbers and intensities for the three isomeric acids were compared and discussed in terms of location of the carboxylic group [20]. The vabritional frequencies, UVVisible, 1 H and 13 C NMR spectra of piconilic acid with its metal complexes of Fe(III), Ni(II), Cu(II), Zn(II) and Ag(I) was recently reported [22,23]. Despite a lot of research work on picolinic acid, there is a dearth of information on the solvents effect on the molecular structure and electronic properties of picolinic acid. Therefore, the aim of this work is to theoretically study the solvents effect on the molecule and also to show the effect of basis sets on geometry, vibrational frequencies and NMR. The knowledge of the solvents effect both on the geometry and electronic properties will be helpful when making predictions about the reactivity and interactions of picolinic acid in