СПЕКТРОСКОПИЯ ^^^^^^^^^^
КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
УДК 539.19
STUDY ON MOLECULAR STRUCTURE AND VIBRATIONAL SPECTRA OF 5,7-DIMETHOXYCOUMARIN USING DFT: A COMBINED EXPERIMENTAL AND QUANTUM CHEMICAL APPROACH © 2014 г. E. Karakas Sarikaya*, ** and O. Dereli*
* A. Kelesoglu Education Faculty, Department of Physics, Necmettin Erbakan University, Meram, 42090 Konya, Turkey ** Department of Physics, Science Faculty, Selcuk University, 42090 Konya, Turkey E-mail: ebrukarakas_84@hotmail.com.tr Received July 5, 2013
Conformational analysis of 5,7-dimethoxycoumarin was performed and two stable conformers were obtained. The difference between the total energies of these conformers was 1.4698 kcal/mol and the difference between the zero point corrected energies was nearly zero. Vibrational frequences of these conformers were calculated by B3LYP method using 6-311++G(d,p) basis sets and compared with experimentally recorded FT-IR and Raman spectra. Vibrational assignments were made by calculated total energy distributions. Time dependent density functional theory calculations were done by the same level of theory in order to investigate low-lying excited state and obtained results were compared with the maximum absorbtion peak value of experimental UV-visible spectrum.
DOI: 10.7868/S0030403414070101
INTRODUCTION
Coumarins are derivatives of a-pyrone [1] and they are prevalent in plants such as tonka bean, vanilla grass, woodruff, mullein, sweet grass, lavender, sweet clover grass, licorice, and also occur in food plants such as strawberries, apricots, cherries, and cinnamon [2—4]. In addition, coumarins show low toxicity in the human body [5]. They show antimutagenic, anticancer and antibacterial properties. Moreover, they possess several biological activities such as antiinflamma-tory, antibacterial, vasorelaxant, antihepatitis-C virus agent, antiproliferative activity, xanthine oxidase inhibitor, antidiabetic, anticancer, anti-HIV, anticoagulant, spasmolytic [6—13] and have wide range of therapeutic properties. As coumarins are biologically active substances, they can protect against reactive oxygen species-mediated damage [14—16]. Likewise, coumarins and their synthetic derivatives have been used in clinical experiments aiming to evaluate their therapeutic efficiency against many malignancies, including kidney and lung cancer and malignant melanoma.
One of the most interesting substances from this group is our title compound 5,7-Dimethoxycoumarin (DMC). Also DMC is present in such plants as Citrus limon and Caripa papaya. This compound shows antiproliferative properties, as it blocks cell cycle in malignant melanoma cells. Although malignant melanoma is one of the most dangerous types of tumor, DMC reduces the metabolic activity of murine and human cell lines [3] and induces processes associated with melan-ogenesis in these cells [1, 17]. Furthermore, DMC in-
hibits the growth of several cell lines of melanoma and carcinoma origin, in a comparable manner.
Because the most of physical and chemical properties of a molecule depend on the molecular structure and several medicinal effects, especially for biologically active molecules, depend on conformational behavior, informations about molecular structure and con-formational behavior of a compound is very important. For this reason, combined experimental and theoretical infrared and Raman spectral studies have been widely used in molecular structural studies nowadays. It is known that B3LYP [18, 19] functionals of DFT exhibit good performance in theoretical part of these studies [20—26]. DMC is an important cou-marin derivative, but to the best of our literature survey neither molecular structure nor conformational behavior of DMC and many of coumarin derivatives are not known.
In this study, conformational search of the DMC has been performed. Geometry parameters and vibrational frequencies of the title compound have been calculated by B3LYP method using 6-311++G(d,p). FT-IR and Raman spectra of DMC have been recorded and the calculated vibrational frequencies have been analyzed and compared with obtained experimental results. Low-lying excited state of DMC has been calculated by the same method and compared with the experimental counterpart.
EXPERIMENTAL
The DMC powder was purchased from ABCR Chemicals Company. FT-IR spectrum of solid DMC
STUDY ON MOLECULAR STRUCTURE AND VIBRATIONAL SPECTRA
253
nT o ö
CS
CS
3000 2000 1000 0
Wavenumber, cm-1
Fig. 1. Experimental and theoretical IR spectra of DMC conformers.
"A
|: . - t ------Wv-*-»! Experimental i i ,7 i fl
Conformer 1
Conformer 2 1 1 y y Y 1 1
Experimental
w
Conformer 1
Conformer 2
3000 2000 1000 0
Wavenumber, cm-1
Fig. 2. Experimental and theoretical Raman spectra of DMC conformers.
was recorded in the range 4000—400 cm 1 on Brucker IFS 66/S with PIKE Gladi ATR (Diamond) spectrometer at room temperature with 2 cm-1 resolution. The FT-Raman spectrum was recorded on a Brucker FRA 106/S spectrometer using 1064 nm excitation from a Nd:YAG laser. The detector was a Gediode cooled to liquid nitrogen temperature. The upper limit for wave numbers was 3500 cm-1 and the lower wave number was around 50 cm-1. The measured FT-IR and FT-Raman spectra are shown in Figs. 1 and 2. The maximum absorbtion peak value of UV-vis spectrum of title compound was obtained from previously performed experimental study in literature [27].
COMPUTATIONAL DETAILS
The conformational analysis of DMC was performed by conformational distribution option of Spar-
tan 08 program [28]. After the conformers determined, using Gaussian 03W program package [29], geometry optimizations, harmonic vibrational frequencies and the other calculations of these conformers were performed by B3LYP/6-311++G(d,p) level of DFT. The total energy distributions (TED)s were calculated for the assignments of the calculated wave-numbers. TED calculations were performed by SQM method of PQS (parallel quantum solutions) program [30, 31]. Time DFT calculations were performed in order to investigate low-lying excited state of the title compound.
RESULTS AND DISCUSSION
Conformational Stability and Molecular Structure
Conformational analysis results of the title compound showed that DMC has two conformers as given
Conformer 1
Fig. 3. Stable conformers of DMC.
J
Conformer 2
in Fig. 3. Ground state energies, zero point corrected energies (Select + ZPE), relative energies and dipole moments of conformers were presented in Table 1. Zero point corrections have not caused any significant changes in the stability order. From both the calculated energies of two conformers, given in Table 1, the conformer 1 is the most stable. Since the difference between the total energies of the most stable two conformers is 1.4698 kcal/mol and the difference between the zero point corrected energies of these two con-formers is nearly zero, both forms were used in the future arguments in this study. The optimized geometric parameters (bond lengths, bond angles, and dihedral angles) of the DMC conformers were given in Table 2 in accordance with the atom numbering scheme given in Fig. 4.
Vibrational Assignments
The DMC molecule has 25 atoms, which possess 69 normal modes of vibrations. All the vibrations are active in the infrared and Raman spectra. Usually the calculated harmonic vibrational wave numbers are higher than the experimental ones, because of the an-harmonicity of the incomplete treatment of electron correlation and of the use of finite one-particle basis set. For this reason, calculated frequencies are scaled by a proper scale factor. Erdogdu et al. [32, 33] used 0.967 (for wave numbers under 1800 cm-1) and 0.955 (for those over 1800 cm-1) for B3LYP/6-311++G(d,p) calculations of organic molecules and obtained good results. For this reason we used the same scale factors in this study. Experimentally ob-
served and theoretically calculated harmonic vibra-tional frequencies are given in Table 3. According to the our calculations, the computed values are in good agreement with the observed values.
CH vibrations. Two bands for each aromatic C-H stretching, bending and out-of-plane bending vibrations were observed in the experimental FT-IR and Raman spectra. The first group was observed at 3086 and 3050 cm-1 in the FT-IR spectrum and at 3085 and 3048 cm-1 in the Raman spectrum, the second group was observed at 1362 and 1204 cm-1 in the FT-IR spectrum and at 1365 and 1203 cm-1 in the Raman spectrum, and the third group was observed at 988 and 810 cm-1 in the FT-IR spectrum and at 988 and 802 cm-1 in the Raman spectrum. They were assigned as stretching, bending and out-of-plane bending vibrations of aromatic C-H group, respectively. In addition to this, according to the TED analysis given in the Table 3, the band observed at 638 cm-1 in Raman spectrum was assigned as C-H out-of-plane bending vibration.
Four bands observed at 3019, 2890, 2949, and 2845 cm-1 in the FT-IR spectrum and three bands observed at 3019, 2949 and 2845 cm-1 in the Raman spectrum, two bands observed at 1453 and 1445 cm-1 in the FT-IR spectrum and two bands observed at 1460 and 1445 cm-1 in the Raman spectrum, a band observed at 1421 cm-1 in the FT-IR spectrum and a band observed at 1419 cm-1 in the Raman spectrum, two bands observed at 1194 and 1185 cm-1 in the FT-IR and Raman spectra, a band observed at 1114 cm-1 in the FT-IR and Raman spectra were assigned as
Table 1. Energetics of the conformers calculated at the B3LYP/6-311++G(d,p) level
Conformer E, Hartree AE, kcal/mol Eq, Hartree AEq, kcal/mol Dipole moment, D
1 -726.273682 0 -726.082282 0 6.3965
2 -726.271339 1.4698 -726.080068 0.0022 8.78
Eq - Zero-point corrected energy.
STUDY ON MOLECULAR STRUCTURE AND VIBRATIONAL SPECTRA 255
Table 2. The calculated geometric parameters of DMC by using B3LYP/6-311++G(d,p), bond lengths (A) and angles (degrees)
Parameters Conformer 1 Conformer 2 Parameters Conforme
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