СПЕКТРОСКОПИЯ ^^^^^^^^
КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
УДК 539.19
DFT STUDY OF SOLVENT EFFECTS ON 3-HYDROXY-2-QUINOXALINECARBOXYLIC ACID TAUTOMERS
© 2014 г. S. Badoglu and $. Yurdakul
Department of Physics, Faculty of Science, Gazi University, Teknikokullar, 06500Ankara, Turkey
E-mail: senayy@gazi.edu.tr Received December 4, 2013
By using the combination of density functional theory (DFT) B3LYP/6-311++G(d,p) within polarized continuum (PCM) model, solvent effects of 3-hydroxy-2-quinoxalinecarboxylic acid (3HQC) tautomers were investigated. Geometrical parameters, vibrational frequencies, electronic and energetic properties, and the nucleus-independent chemical shifts (NICS) were calculated in benzene, diethyl ether, and water. It has been seen that these molecular features of 3HQC are mostly solvent dependent.
DOI: 10.7868/S0030403414080042
INTRODUCTION
Tautomerism refers to equilibrium between different structures of the same compound. Usually the tautomers differ in the point of attachment of a hydrogen atom. In recent years the investigation on tautomerism has been one of the major topics in theoretical chemistry. By reviewing the literature, many papers about the study of tautomerism have been found.
It is well known that the solvent effects play an important role in determining the tautomeric equilibria. Tautomers interconvert each other by either intra- or inter-molecular proton interchange mechanisms.
These mechanisms are susceptible to solute—solvent interactions, and thus tautomeric equilibria may shift or completely change depending on the solvent effects. A good example for such a case is the tautomer-ization between 2-hydroxypyridine (2HP) and 2-pyri-done (2PY), in which the predominant form is 2HP in polar solvents but 2PY in the gas phase [1].
Solvent effects are a group of effects that a solvent influence on the chemical and physical features of molecular systems. There are several basic approaches available for modeling molecular systems in solution. One of them is the implicit treatment of solvent mole-
Table 1. Energies (hartree) and relative Gibbs energy differences (kcal/mol) of 3HQC tautomeris in solvents and in the gas phase
3HQC-1 Vacuum Benzene Ether Water
-681.825858 -681.834786 -681.840384 -681.850310
E3 Hc Gd AG(gas-soln) -681.815701 -681.814757 -681.861919 0.00 -681.824569 -681.823624 -681.870906 5.64 -681.830121 -681.829177 -681.876541 9.18 -681.839951 -681.839006 -681.886571 15.47
3HQC-2 Vacuum Benzene Ether Water
E0 -681.824578 -681.834156 -681.839392 -681.847085
E3 Hc G* AG(gas-soln) -681.814220 -681.813276 -681.861254 0.00 -681.823897 -681.822952 -681.870630 5.88 -681.829172 -681.828227 -681.875783 9.12 -681.836910 -681.835966 -681.883347 13.86
a Eq. Sum of electronic and zero-point energies (Eelec + ZPE). b E. Sum of electronic and thermal energies (Eq + E^ + Etot + Etrans). c H. Sum of electronic and thermal enthalies (E + RT). d G. Sum of electronic and thermal freee energies (H — TS).
Fig. 1. Geometries of 3HQC tautomers in vacuum [6]: (a) 3HQC-1, (b) 3HQC-2.
cules [2]. Self-consistent reaction-field (SCRF) models employ this approach [3]. The first proposed SCRF method was the polarizable continuum model (PCM) by Miertug, Scrocco, and Tomasi [4]. This method has proven itself in recent years. Employing PCM model in the density functional theory (DFT) is a good method while investigating solvent effects [5].
In our previous work, we have been reported the tautomerism of the free 3-hydroxy-2-quinoxalinecar-boxylic acid (3HQC) [6]. 3HQC consists of quinoxa-line skeleton and carboxylic acid group. Although 3HQC and its derivatives have been studied as inhibitors of neurotransmitter molecules [7—9], solvent effects on their electronic and spectroscopic properties are unknown. In this paper, we present a theoretical study of the geometrical parameters, vibrational frequencies, and energetics of 3HQC. We have employed PCM in the DFT to calculate those properties in three different solvents, and compared the obtained data with the same properties in the gas phase. The solvents chosen were benzene (s = 2.2706), ether (s = 4.24), and water (s = 78.3553). Nucleus independent chem-
ical shifts (NICS) were also calculated as aromaticity indices.
COMPUTATIONAL METHODS
All calculations were performed by using the Gaussian 09W suite of programs [10] at B3LYP/6-311++G(d,p) level. The PED (potential energy distribution) values obtained by using the VEDA 4 program were used to characterize the fundamental vibrational modes [11]. While investigating the solvent effects, the PCM was employed in the calculations. Geometries were fully optimized without symmetry constraints. Harmonic vibrational frequencies, total and Gibbs energies, frontier molecular orbitals properties, natural bond orbital (NBO) charges, and dipole moments are calculated. The aromaticity index NICS values were calculated within the gauge independent atomic orbital (GIAO) method at B3LYP/6-311++G(d,p) level. The NICS probes (Bq) were placed at the geometric center of the benzene and pyrazine rings, and 1 A above the ring center perpendicular to the ring plane.
Table 2. Geometrical parameters R (A) and A (deg) of 3HQC-1
Parameter Vacuum* Benzene Ether Water
R(1,2) 1.3722 1.3725 1.3728 1.3734
R(1,6) 1.4188 1.4191 1.4192 1.4193
R(1,7) 1.0835 1.0848 1.0856 1.0869
R(2,3) 1.4175 1.4180 1.4182 1.4185
R(2,8) 1.0835 1.0849 1.0858 1.0872
R(3,4) 1.4332 1.4334 1.4333 1.4325
R(3,16) 1.3539 1.3534 1.3531 1.3527
R(4,5) 1.4174 1.4166 1.4162 1.4155
R(4,15) 1.3554 1.3569 1.3578 1.3593
R(5,6) 1.3741 1.3749 1.3754 1.3764
R(5,9) 1.0830 1.0846 1.0855 1.0869
R(6,10) 1.0841 1.0854 1.0863 1.0876
R(11,12) 1.4401 1.4394 1.4392 1.4399
R(11,13) 1.3320 1.3347 1.3362 1.3379
R(11,15) 1.3100 1.3093 1.3089 1.3087
R(12,14) 1.4956 1.4931 1.4917 1.4907
R(12,16) 1.3141 1.3138 1.3134 1.3121
R(13,20) 0.9826 0.9825 0.9827 0.9836
R(14,17) 1.3296 1.3280 1.3267 1.3239
R(14,18) 1.2159 1.2182 1.2199 1.2229
R(17,19) 0.9777 0.9794 0.9809 0.9877
R(18,20) 1.7994 1.8068 1.8080 1.8067
A(2,1,6) 120.3742 120.3912 120.3943 120.3810
A(2,1,7) 120.1405 120.1485 120.1573 120.1795
A(6,1,7) 119.4853 119.4603 119.4484 119.4395
A(1,2,3) 119.6653 119.6338 119.6202 119.5887
A(1,2,8) 121.9127 121.8724 121.8535 121.8553
A(3,2,8) 118.4220 118.4938 118.5263 118.5560
A(2,3,4) 120.1672 120.1347 120.1179 120.1316
A(2,3,16) 120.4442 120.3986 120.3567 120.2399
A(4,3,16) 119.3887 119.4667 119.5254 119.6286
A(3,4,5) 118.6173 118.7194 118.7780 118.8551
A(3,4,15) 121.6057 121.3931 121.2594 121.0144
A(5,4,15) 119.7769 119.8875 119.9626 120.1305
A(4,5,6) 119.9755 119.9048 119.8654 119.7995
A(4,5,9) 118.0300 118.2370 118.3813 118.6390
A(6,5,9) 121.9944 121.8582 121.7533 121.5615
A(1,6,5) 121.2005 121.2162 121.2242 121.2441
A(1,6,10) 119.1483 119.1489 119.1484 119.1384
A(5,6,10) 119.6512 119.6349 119.6274 119.6174
A(12,11,13) 121.2222 121.4588 121.6001 121.8992
A(12,11,15) 120.8472 120.9568 121.0215 121.1206
A(13,11,15) 117.9306 117.5844 117.3784 116.9802
A(11,12,14) 121.4016 121.3918 121.3421 121.2031
A(11,12,16) 121.9960 121.8402 121.7450 121.4882
A(14,12,16) 116.6024 116.7680 116.9129 117.3087
A(11,13,20) 108.1849 108.4604 108.6027 108.8676
A(12,14,17) 115.1421 115.7966 116.3025 117.4599
A(12,14,18) 122.3942 122.3827 122.3053 121.9522
A(17,14,18) 122.4636 121.8207 121.3922 120.5879
A(4,15,11) 117.8536 117.9244 117.9626 118.0572
A(3,16,12) 118.3088 118.4188 118.4861 118.6910
A(14,17,19) 106.9298 108.2483 109.2941 111.5664
RESULTS AND DISCUSSION
Energetics of 3HQC
In Table 1 energies of the DFT-PCM (B3LYP) calculations are reported for 3HQC in benzene, ether, and water together with the theoretical gas phase data. The relative Gibbs energy differences are also supplied. The relative abundance ofpossible 3HQC taute-mers was calculated using equation AG = — RT lnK, 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 species 3HQC-1 and 3HQC-2 are calculated as 90 and 10% at 298 K.
From the analysis of the results in Table 1 the following observations and conclusions can be drawn.
1) Among the two tautomers of 3HQC, 3HQC-1 is the most stable one regardless of the selected medium.
2) Energies of both tautomers are continuously decreased with increasing dielectic constant of the medium. The stabilities of the tautomers are increased due to the interactions between the solvent and the solute.
3) The lowest energy values of 3HQC tautomers are obtained from aqueous solution calculations.
The rate of change in relative Gibbs energy differences, AG(gas-soln), of 3HQC tautomers are different for two tautomers. The increase in AG(gas-soln) differences of 3HQC-1 tautomer are larger than of 3HQC-2 tautomer. This means that tautomeric equilibrium is shifted in favor of 3HQC-1 with increasing dielectric constant of the medium.
Geometrical Parameters
Tautomers of 3HQC in vacuum are depicted in Fig. 1. Computed geometrical parameters of two tau-tomers are collected in Tables 2 and 3. Those parameters are given in the increasing order of dielectric constants of the media (vacuum s = 1, benzene s = = 2.2706, ether s = 4.24, water s = 78.3553). The results of solvent effects on the molecular geometries of 3HQC tautomers are estimated by the DFT-PCM method.
On going from gas phase to solution, the geometrical parameters of 3HQC tautomers exhibited quiet small changes. In 3HQC tautomers the geometrical parameters gradually changed with increasing dielectric constant of medium. The most significant changes for 3HQC-1 are determined at the carboxylic and hy-droxylic sites of the molecule. 14C-18O bond length is calculated as 1.2159 A for the gas phase. For benzene, ether, and water solutions the same bond's length is obtained as 1.2182, 1.2199, and 1.2229 A, respectively. In addition, we can propose an intramolecular hydrogen bond betwee
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