научная статья по теме A COMBINED EXPERIMENTAL AND THEORETICAL STUDY ON 8-HYDROXY-2-QUINOLINECARBOXYLIC ACID Физика

Текст научной статьи на тему «A COMBINED EXPERIMENTAL AND THEORETICAL STUDY ON 8-HYDROXY-2-QUINOLINECARBOXYLIC ACID»

ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 116, № 2, с. 214-224

СТЕКТРОСКОПИЯ АТОМОВ И МОЛЕКУЛ

y%K 539.194

A COMBINED EXPERIMENTAL AND THEORETICAL STUDY ON 8-HYDROXY-2-QUINOLINECARBOXYLIC ACID

© 2014 n Serdar Badoglu and $enay Yurdakul

Department of Physics, Faculty of Science, Gazi University, Teknikokullar, 06500Ankara, Turkey

E-mail: senayy@gazi.edu.tr Received January 29, 2013

The mid-IR and Raman spectra of 8-hydroxy-2-quinolinecarboxylic acid (8HQC) were recorded. These spectra were interpreted with the help of B3LYP/6-311++G(d, p) calculations and potential energy distribution (PED) analysis. As a result of the calculations, seven tautomers were determined among many stable conformations. The experimental spectra were concordant with the theoretical data of one tautomer. In the functional group region overtone and combination bands were detected and assigned. In addition, because of several peaks in the IR spectrum, it was proposed that the 8HQC exhibits dimerization in condensed phase. Possible dimeric forms of 8HQC were evaluated at the same level of theory, and it has been seen that the calculation results confirm the above proposal. 1H and 13C NMR chemical shifts of 8HQC have been calculated, and compared with the experimental data. The frontier molecular orbital properties and the atomic charges were also theoretically obtained and presented.

DOI: 10.7868/s0030403414020202

1. INTRODUCTION

8-hydroxy-2-quinolinecarboxylic acid (8HQC) contains benzene and pyridine rings besides hydroxyl and carboxylic acid moieties. In his two published works, Hironori Oda has been investigated photochro-mic spyropans' complexes with 8HQC and its derivatives [1, 2]. As a result of his studies Oda has been found that 8HQC and its derivatives can be applied as effective stabilizers against the photo fading of indicator dyes in heat- (or pressure-) sensitive recording systems. Pearce et al. have synthesized an inhibitor of superoxide in four steps from 8HQC and investigated it as anti-inflammatory [3]. Mladenova et al. have synthesized new amides and esters from 8HQC and characterized them [4]. Bozoklu et al. synthesized Nd(II) complexes of 8HQC and investigated their structural and photo physical properties [5]. McDonald et al. reported the metal ion complexing properties of 8HQC by using X-ray diffraction and absorbance measurements [6]. In this study, the formation constants of the complexes were obtained and it was understood that 8HQC forms five-membered chelate rings while complex formation. There are several other crystallograph-ic studies on the metal complexes of 8HQC in the literature [7—10].

In this study, we present density functional calculations of the geometrical and vibrational properties of the ground state of the tautomeric forms of 8HQC. A complete assignment of the vibrational modes was performed on the basis of the potential energy distribution (PED) values. The theoretical data (molecular parameters, IR and Raman Spectra) are compared

with the results of the experimental studies performed in the solid state. Frontier molecular orbitals (FMO) properties and natural bond orbital (NBO) charges were performed at the same level of theory. The vibrational assignment of a dimeric form of 8HQC was used for the unassigned IR frequencies of the solid sample.

2. COMPUTATIONAL METHODS

The DFT calculations of the ground state geometries of 8HQC conformers were performed at B3LYP/6-311++G(d, p) level with the default convergence criteria without any constraint on the geometry. The stationary structures are found by ascertaining that all the calculated frequencies are real. All the calculations were carried out in Gaussian 03W program package [11]. The fundamental vibrational modes were characterized by their PED (potential energy distribution) obtained by using the VEDA 4 program [12]. NMR calculations were carried out at B3LYP/6-311++G(2d, p) level with GIAO/PCM approach, and the solvent was dimethyl sulfoxide (DMSO). The structures were re-optimized considering solvent effects, before the NMR calculations done. Chemical shifts were derived with respect to the isotropic shieldings of the TMS (tetramethylsilane).

3. EXPERIMENTAL

8-hydroxy-2-quinolinecarboxylic acid (8HQC) was purchased from Aldrich and used without further purification. Infrared spectrum of 8HQC was recorded between 3500 and 550 cm-1 on Bruker FTIR spec-

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4. RESULTS AND DISCUSSION

4.1. Tautomers of 8HQC

8HQC exhibits prototropic tautomerism. Besides, the hydroxyl and the carboxylic acid moieties of the molecule can rotate, and thus several conformations were supposed to occur in the condensed phase. The possible tautomeric forms of 8HQC are given in Fig. 1. The B3LYP/6-311++G(d, p) calculated total energy (E) and Gibbs free energy (G) predictions of seven tautomers are compared in Table 1 which shows that 8HQC-1 has the lowest energy value (E = —665.756 Hartrees) among seven tautomers. For those seven tautomers we have searched and generated a total of 40 conformers. All conformers were evaluated at the same level of calculation. Considered conformers are depicted in Fig. 2. Different isomers of the tautomers are distinguished as 'A', 'B', 'C', 'D', 'E', 'F', 'G', and 'H'. Table 2 shows their computed energetic data.

The relative abundance of possible 8HQC tautomers was calculated using equation: AG = —RTln K, where AG denotes the difference between Gibbs free energies of given two conformers and K is the equilibrium constant of these species. The abundance of the most stable species, 8HQC-1 equals 100% at 298 K. The remaining species have zero total population and are expected to be of no importance in relation to the experimental spectra. Hence, depending on the gas phase calculations only one tautomer has been considered in this work.

trometer, and the sample of the free ligand was examined by ATR apparatus. The Raman spectrum of 8HQC was recorded between 3500 and 550 cm-1 on Bruker FRA 106/S spectrometer, using 1064 nm excitation from an Nd:YAG laser. The detector is a liquid nitrogen-cooled Ge detector. 1H and 13C NMR spectra were recorded on Bruker Ultrashield 300 MHz spectrometer.

Table 1. Energies of 8HQC tautomers

Tautomer E (Hartree) G (Hartree) AE a, kcal/mol AG b, kcal/mol

8HQC-1 -665.756 -665.803 0.00 0.00

8HQC-2 -665.731 -665.779 15.67 15.16

8HQC-3 -665.731 -665.780 15.69 14.75

8HQC-4 -665.696 -665.743 37.80 37.64

8HQC-5 -665.687 -665.735 43.18 42.71

8HQC-6 -665.686 -665.736 43.86 42.46

8HQC-7 -665.684 -665.733 44.88 44.13

a AE = En — E1 are the total energy differences between the nth tautomer (n = 2, ..., 7) and the first tautomer. b AG = Gn — G1 are the Gibbs free energy differences between the nth tautomer (n = 2, ..., 7) and the first tautomer.

4.2. Molecular Parameters

The optimized bond lengths and bond angles obtained in the most stable structure of 8HQC using the B3LYP method with 6-311++G(d, p) basis set are given in Table 3; while the numbering of the atoms were plotted in Fig. 3. The optimized geometry is compared with the structural parameters obtained from crystal-lographic analysis of 8-hydroxyquinolinium-7-car-boxylate monohydrate [13].

It has been seen that the most stable tautomer (8HQC-1) has a nearly planar geometry. Some bond lengths like 12C—14O, 13N—3C are shorter than the experimental values. On the other hand, the bond lengths like 2C—3C, 5C—6C, 6C—1C, 11C—12C are overestimated. These deviations are attributable to the phase difference between the calculations and the experiment. The experimental data belong to a compound in the solid phase, while the calculations refer to an isolated 8HQC molecule in the gas phase. In general, the bond lengths are predicted very well. The maximum deviations of predicted values from the experimental data are 0.07 A for bond lengths and 3.87° for bond angles.

On the benzene ring the bond angles vary between 119.31°—121.66°, and on the pyridine ring the variation is between 116.71°—123.35°. The variations in the bond angles and the comparison of C=C, C—C, C—N,

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Fig. 2. Tautomeric/isomeric forms of 8HQC considered in this study (contd.).

ОПТИКА И СПЕКТРОСКОПИЯ том 116 № 2 2014

and C=N bond lengths of the rings shows that both benzene and pyridine rings are deformed. The 8HQC-1 tautomer exhibits O—H—N type intramolecular hydroge

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