ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2011, том 85, № 6, с. 1092-1099
СТРОЕНИЕ ВЕЩЕСТВА ^^^^^^^^^^^^ И КВАНТОВАЯ ХИМИЯ
УДК 539.192
THE AB INITIO STUDY AND NBO ANALYSIS OF THE SOLVENT DIELECTRIC CONSTANT EFFECTS AND THE IMPLICIT WATER MOLECULES ON THE STRUCTURAL STABILITY OF THE 1-(6-CHLOROQUINOXALIN-2-YL)-2-(4-(TRIFLUOROMETHYL)-2,
6-DINITROPHENYL) HYDRAZINE
© 2011 M. Noei*, S. Suzangarzadeh**, S. Golshani***, A. Tahan****
*Department of Chemistry, Mahshahr branch, Islamic Azad University, Mahshahr, Iran ** Department of Chemistry, ShahreRey branch, Islamic Azad University, Tehran, Iran *** Department of Chemistry, ShahreRey branch, Islamic Azad University, Young Researchers Club, Tehran, Iran **** Semnan branch, Islamic Azad University, Semnan, Iran E-mail: Arezoo.Tahan@gmail.com Received July 25, 2010
Abstract—Ab initio molecular orbital (MO) and density functional theory (DFT) methods were used to analyze the structure and the relative stability of 1-(6-chloroquinoxalin-2-yl)-2-(4-(trifluoromethyl)-2,6-dini-trophenyl) hydrazine in gas phase and the different solvent media. The effects of solvent dielectric constant and the implicit water molecules were investigated on the structural stability and intra-molecular interactions. All used methods revealed that by the increase of solvent dielectric constant, the relative stability of the considered compound increase. Hence, the most stable structure is perceived in aqueous solution. Furthermore, natural bond orbital (NBO) analysis demonstrated that in the presence of implicit water molecules, the lone pair electrons of nitrogen have the most contribution in the resonance interactions of the aromatic rings and their stability. These facts may be the probable reasons behind the structural stability of the considered structure in the water solution based on energetic data and NBO analysis at the microscopic level.
Keywords: 1-(6-chloroquinoxalin-2-yl)-2-(4-(trifluoromethyl)-2,6-dinitrophenyl) hydrazine, solvent effect, NBO analysis.
INTRODUCTION
Quinoxalines are the heterocyclic compounds containing a ring complex made up of a benzene ring and a pyrazine ring. They are isomeric with quinazolines. Among the various classes of nitrogen-containing heterocyclic compounds, quinoxalines display a broad spectrum of the biological activities including antiviral and antibacterial properties and also act as the kinase inhibitors [1—4]. Quinoxalines play an important role as a basic skeleton for the design of a number of antibiotics such as echinomycin, actinomycin, and lero-mycin. It has been reported that these compounds inhibit the growth of gram-positive bacteria, and are active against the various transplantable tumors. The quinoxaline ring is also a constituent of many pharmacologically and biologically active compounds such as insecticides, fungicides, herbicides, and anthelmi-nitics. Quinoxaline derivatives have found application in dyes, electron luminescent materials, organic semiconductors, chemically controllable switches, as building blocks for the synthesis of anion receptors, cavitands, dehydroannulenes, DNA cleaving agents, and also serve as useful rigid subunits in macrocyclic receptors or in molecular recognition [5—13]. They are formed by the condensing ortho-diamines with
1,2-diketone; the parent substance of the group, qui-noxaline, resulting when glyoxal is so condensed, while substitution derivatives arise when a-ketonic acids, a-chlorketones, a-aldehyde alcohols and a-ke-tone alcohols are used in place of diketones [14].
Recently, Moustafa and Abbady reported the synthesis of various quinoxaline derivatives of a new type claiming that they will likely show interesting biological properties [15]. The authors recorded the IR and 1H NMR spectra of the new species as part of their characterization studies, but no reference to structural parameters is found in that study. In this paper, with the objective of describing further the chemistry of these relatively new compounds, we modeled one of the new quinoxaline derivatives, 1-(6-chloroquinoxa-lin-2-yl)-2-(4-(trifluoromethyl)-2,6-dinitrophenyl) hydrazine, using Hartree-Fock and DFT methods in the different solvent media (see Fig. 1). The basic interest of the present study is to analyze the effects of solvent dielectric constant and the implicit water molecules on the structural stability and the effective factors on it by NBO analysis and NMR parameters.
COMPUTATIONAL DETAILS
Geometry optimizations were performed by using Hartree—Fock (HF) and Becke's exchange and Lee— Yang—Parr's correlation junctionals (BLYP) and Beck's three parameter hybrid with the Lee—Yang—Parr correlation functional, B3LYP [16, 17], methods along with the 3-21G, 3-21G(d), 6-31G, 6-31G(d) basis sets on 1-(6-chloroquinoxalin-2-yl)-2-(4-(trifluorometh-yl)-2,6-dinitrophenyl)hydrazine. Energy minimum molecular geometries were located by minimizing energy, with respect to all geometrical coordinates without imposing any constraints. The nature of the stationary points for the interested structure has been fixed by the imaginary frequencies. For minimum state structures, only real frequency values were accepted. To model the bulk effects on the interested structure, self-consistent reaction field (SCRF) method is based on a continuum model with uniform dielectric constant (s).The simplest SCRF model is the Onsager reaction field model. We first performed a volume calculation to determine cavity radii required for the use of the Onsager solvent model in our calculations. The Onsager model as implemented in Gaussian 98 is a method to predict the solvation effect on properties of the solute without considering explicit solvent molecules. This model has been used to predict structural and spectral changes due to the solvent with implicitly accounting for the solvent molecules [18, 19]. Application of the Onsager continuum model requires a cavity radius and a dielectric constant.
NBO analysis was then performed at the B3LYP/6-311+G** level of theory on the optimized structure at the B3LYP/6-31G* level by using the NBO 3.1 program in the different media [20, 21]. The bonding and lone pair orbital occupancies in the optimized structure of the considered compound, the stabilization energies associated with LP (1) N7 ^ CT*11-m5, LP (1)
N16 ^ ctN17-H20 and LP N17 ^ ctN16-H18 delocaliza-tions and also natural hybrid of nitrogen lone pair orbital (LP N) were calculated by using NBO analysis.
GIAO nuclear magnetic shielding calculations were carried out on optimized structures using the HF/6-311+G**//B3LYP/6-31G*method and the values of symmetric shielding (Act) and asymmetry shielding (n) were calculated by below formulas [22, 23]:
1) the isotropic value CTiso:
CTiso _
( 1/3)(CTu + CT22 + CT33) , (1)
2) the anisotropy shielding (Act):
ACT = CT33 - (1/2)(CTu + CT22), (2)
and
3) the asymmetry parameter (n):
n = |CT22 -an|/ICT33 -ctiSo|. (3)
All calculations in present work were performed by using the GAUSSIN 98 program [24].
Fig. 1. The optimized structure of 1-(6-chloroquinoxalin-
2-yl)-2-(4-(trifluoromethyl)-2,6-dinitrophenyl)hydra-
zine.
RESULTS AND DISCUSSION
Hartree—Fock and density functional theory methods along with the 3-21G, 3-21G(d), 6-31G, 6-31G(d) basis sets were used for geometry optimizations of the 1-(6-chloroquinoxalin-2-yl)-2-(4-(trifluoromethyl)-2,6-dini-trophenyl)hydrazine structure. The effects of solvent dielectric constant and polarized basis sets on the structural stability and parameters were investigated. All used methods reveal that by the increase of solvent dielectric constant, the relative stability of the considered compound increases in most cases (see table 1). Hence, the most stable structure is perceived in aqueous solution. Furthermore, Adding polarized functions to heavy atoms (6-31G (d) vs. 6-31G and 3-21G (d) vs. 3-21G results) and electron correlations in DFT level (B3LYP/6-31G(d) and HF/6-31G(d) results) in the vacuum and solvent media again increases the relative stability in most occasions. However, it can be concluded that by the improvement of basis set and including electron correlations, the relative stability of the interested structure increases. Hence, the most stable structure is perceived in the aqueous solution at the B3LYP/6-31G* level of theory. Unfortunately, be-
Table 1. Calculated relative energy values (in kcal mol-1) for the 1-(6-chloroquinoxalin-2-yl)-2-(4-(trifluorometh-yl)-2,6-dinitrophenyl) hydrazine structure in the different levels of theory and solvent dielectric constants (s)
Methods Б -Eel AEel
BLYP/3-21G 1 1954.460888 -
BLYP/6-31G 1 1964.559994 -
B3LYP/3-21G 1 1954.786632 -
B3LYP/6-31G 1 1964.840906 -
B3LYP/6-31G* 1 1965.3145431 0.057292
F/3-21G 1 1945.779480 2.27156
HF/3-21G* 1 1945.886606 2.17285
HF/6-31G 1 1955.669272 3.31415
HF/6-31G* 1 1956.348257 1.92444
B3LYP/6-31G* 4.9 1965.3146048 0.01857
HF/3-21G 4.9 1945.779892 2.01326
HF/3-21G* 4.9 1945.862568 17.25675
HF/6-31G 4.9 1955.669742 3.01875
HF/6-31G* 4.9 1956.3487 1.62944
B3LYP/6-31G* 20.7 1965.314628 0.004016
HF/3-21G 20.7 1945.779876 2.02326
HF/3-21G* 20.7 1945.862743 17.14675
HF/6-31G 20.7 1955.669933 2.93875
HF/6-31G* 20.7 1956.3489 1.50944
B3LYP/6-31G* 32.6 1965.3146311 0.002071
HF/3-21G 32.6 1945.7831413 0.025728
HF/3-21G* 32.6 1945.8899996 0.042922
HF/6-31G 32.6 1955.6745429 0.006338
HF/6-31G* 32.6 1956.3513428 -0.011797
B3LYP/6-31G* 46.7 1965.3145348 0.062499
HF/3-21G 46.7 1945.779892 2.01326
HF/3-21G* 46.7 1945.862775 17.12675
HF/6-31G 46.7 1955.669965 2.87875
HF/6-31G* 46.7 1956.348951 1.48944
B3LYP/6-31G* 78.39 1965.3146344 0.0000
HF/3-21G 78.39 1945.7831003 0.0000
HF/3-21G* 78.39 1945.890068 0.0000
HF/6-31G 78.39 1955.674553 0.0000
HF/6-31G* 78.39 1956.351324 0.0000
Notes. The mentioned values are dielectric constant for "Vkcuum, Chloroform, Acetone, Methanol, dimethyl sulfoxide(DMSO) and water media, respectively.
cause of limitation of computational resources
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