ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 116, № 3, с. 376-387
СТЕКТРОСКОПИЯ АТОМОВ И МОЛЕКУЛ
y%K 539.194
MOLECULAR STRUCTURE, VIBRATIONAL SPECTRAL INVESTIGATION AND THE CONFIRMATION ANALYSIS OF 4-METHYLESCULETIN MOLECULE
© 2014 n Y. Erdogdu*, M. Guzel**, M. T. Gulluoglu*, M. Amalanathan***, S. Saglam****, and I. Hubert Joe***
*Department of Physics, Ahi Evran University, 40040, Kirsehir, Turkey **Dept. of Comp. Prog., Ahi Evran University Mucur Technical Vocational School, 40500, Mucur, Kir§ehir, Turkey ***Centre for Mol. and Biophys. Res., Dept. Phys., Mar Ivanios College, Thiruvananthapuram 695015, Kerala, India ****Department of Physics, Gazi University, Teknikokullar, Ankara, 06500 Turkey E-mail: yusuferdogdu@mail.com Received March 25, 2013
In this work, FT-IR, FT-Raman, and FT-NMR spectra of 4-Methylesculetin molecule are presented for the first time. FT-IR, FT-Raman and FT-NMR spectra of 4MEC have been recorded and analyzed. The FT-IR and FT-Raman spectra of this molecule are recorded at 4000—400 cm-1 and 50—3500 cm-1, respectively. The infrared vibrational frequencies, absolute intensities, potential energy profile, HOMO-LUMO plot and NBO analysis of the molecule have been also predicted using Becke's three-parameter hybrid B3LYP method in the density functional theory DFT method. Calculated and experimental data are in good agreement.
DOI: 10.7868/S0030403414030052
1. INTRODUCTION
The coumarin derivatives are known to have diverse applications as anticoagulants, spasmolytics, anticancer drugs, or as plant growth-regulating agents [1—4]. Their complexation ability in respect to different metal ions has been studied and discussed widely in a considerable number of investigations [5—9]. It has been found that the binding of a metal to the coumarin moiety retains or even enhances its biological activity [10—12]. Various types of coumarin substitutions in their skeletal structure can influence their biological activity. Therefore, a comprehensive structure—system—activity relationship study of coumarins with special respect to carcinogenicity, mutagenicity, and cancer-preventing activity would be of high interest. Recently considerable attention has been focused on biological activity displayed by the coumarins, especially byesculetin [13]. Esculetin and 4-methylescule-tin as coumarin derivatives have a diphenolic structure contained in many plants, such as Citrus limonia and Euphorbia lathyris. It has multiple biological activities, including of the inhibition of the xanthine oxidize activity [8], platelet aggregation [14], and the induction apoptosis. In addition, esculetin shows antioxida-tive activity [15, 16], an inhibitory effect on the growth of human breast cancer [17]. On the other hand the pharmaceuticals and agrochemicals containing cate-chol skeleton, represent one of the most ubiquitous families of natural antioxidants [18—20].
Modern FT-IR and the near infrared (NIR) FT-Raman spectroscopies have proven to be an excep-
tionally powerful technique for solving many drug molecules, biological molecules, and the natural products. It has been extensively employed both in the study of chemical kinetics and chemical analysis. Since fluorescence-free Raman spectra and computed results help in the unambiguous identification of vi-brational modes and provide deeper insight into the bonding and structural features of complex organic molecular systems [21, 22]. The advent of fast computers along with sophisticated computational methods makes the task of solving various structural chemical problems easily. Density functional theory (DFT) has become an efficient tool in the prediction of molecular structure, conjugation, hydrogen bonding harmonic force field, vibrational frequencies, and IR and Raman activities of the bioactive molecule [23—28]. The present work deals with the FT-IR and FT-Raman spectral investigations of 4-Methylesculetin supported by DFT calculation to understand the structural and bonding features, electron delocalization, and the intramolecular charge transfer interactions. The natural bond orbital (NBO) analysis and the distribution of electric charges on atoms of free molecule of 4-Meth-ylesculetin compounds also were investigated by using the DFT computation.
2. EXPERIMENTAL
The FT-IR spectrum of this molecule is recorded in the region 4000—400 cm—1 on IFS 66V spectrophotometer using KBr pellet technique is shown in Fig. 1.
Transmittance 1.00
0.75
0.50
0.25
4000 3000 2000
Fig. 1. Experimental FT-IR spectra of 4-Methylesculetin molecule.
1000
Wavenumber, cm-1
The FT-Raman spectrum of 4MESC has been recorded using 1064 nm line of Nd : YAG laser as excitation wavelength in the region 50—3500 cm-1 on Bruker FRA 106/S are shown in Fig. 2. The 1H and 13C NMR spectra are taken in chloroform solutions and all signals are referenced to TMS on a Bruker Superconducting FT-NMR Spectrometer. All NMR spectra are measured at room temperature.
3. COMPUTATIONAL DETAILS
The calculations were performed at DFT levels by using Gaussian 09 [29] program package; invoking
Raman intensity, a.u. 1.00
0.75
0.50 -
0.25 -
3000
gradient geometry optimization [30]. In order to establish the stable possible conformations, the conformational space of title molecule was scanned with molecular mechanic simulations. This calculation was performed with the Spartan 10 program [31]. For meeting the requirements of both accuracy and computing economy, theoretical methods and basis sets should be considered. Density functional theory (DFT) has been proved to be extremely useful in treating electronic structure of molecules. The basis set cc-pVDZ was used for the conformational analysis. The optimized structural parameters were used in the vibrational frequency calculations at the DFT level to characterize all stationary points as minima.
Then, vibrationally averaged nuclear positions of 4MESC were used for harmonic vibrational frequency calculations resulting in IR and Raman frequencies together with intensities and Raman depolarization ratios. In the present work, the DFT method B3LYP with cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets were used for the computation of molecular structure, vibrational frequencies and energies of optimized structures. The vibrational modes were assigned on the basis of TED analysis for higher basis set (B3LYP/cc-pVDZ), using SQM program [32].
It should be noted that Gaussian 03W package is able to calculate the Raman activity. The Raman activities were transformed into Raman intensities using Raint program [33] by the expression:
2000 1000
Wavenumber, cm-1
I = 10-12(vo -viRAi, v i
(1)
Fig. 2. Experimental FT-Raman spectra of 4-Methylesculetin molecule.
where I is the Raman intensity, RAt is the Raman scattering activities, vt is the wavenumber of the normal
Fig. 3. All conformer and atomic numbering of the 4-Metyhylesculetin molecule.
modes and v0 denotes the wavenumber of the excitation laser [34].
The XH and 13C NMR chemical shifts calculations of the conformer 1 of the 4MESC molecule were made by using B3LYP functional with cc-pVDZ, cc-pVQZ, and cc-pVTZ basis sets. The GIAO (Gauge Including Atomic Orbital) method is one of the most common approaches for calculating isotropic nuclear magnetic shielding tensors [35, 36]. For the same basis set size GIAO method is often more accurate than those calculated with other approaches [37, 38]. The NMR spectra calculations were performed by Gaussian 03 program package. The calculations reported were performed in chloroform solution using IEF-PCM model as well as gas phase in agreement with experimental chemical shifts obtained in chloroform solution.
4. RESULTS AND DISCUSSION
4.1. Molecular Geometry
The numbering scheme for 4MESC is shown in
Fig. 3. Optimized bond parameters were calculated by using B3LYP with cc-pVDZ basis set. To find stable
conformers, a meticulous conformational analysis was
carried out for the title molecule. Rotating each 10 degree intervals around the free rotation bonds, confor-
mational space of the title molecule was scanned by
molecular mechanic simulations and then full geome-
try optimizations of these structures were performed by B3LYP/cc-pVDZ method. Results of geometry optimizations indicated that the title molecule is rather flexible molecule and, in theory, may have at least four conformers as shown in Fig. 3. Ground state energies, zero point corrected energies (^eiect + ZPE), relative
energies and dipole moments of conformers were pre-
sented in Table 1. Zero point corrections have not caused any significant changes in the stability order.
The optimized molecular geometry (Fig. 3) represents an isolated molecule under ideal conditions with a stationary point at the potential energy surface; the convergence was confirmed by observing no imaginary vibrational wavenumbers. Table 2 shows the selected optimized parameters of 4MESC molecular unit in solid phase using B3LYP (cc-PVDZ, cc-PVTZ, and cc-pVQZ) level along with experimental value [39]. The optimized geometry shows that the calculated bond lengths are slightly longer than the experimental values. This variation is due to the fact that the optimization was performed in an isolated condition. The changes in bond length of the C-H bond on substitution are due to the electron donation group within the benzene ring which reduces the electron density of the carbon atom. The agreement for bond angles is not as good as that for the bond distances. The optimized bond lengths of C-C in the ring vary in the range from 1.385 to 1.454 A for cc-PVDZ level, 1.377 to 1.447 A for cc-PVTZ and cc-pVQZ levels are in good agreement with 4MESC (1.370-1.445 A) [39]. The bond length of C-CH3 (C8-C19) the calculated value of all
Table 1. Energetic of the four conformers calculated at the B3LYP/cc-pVDZ level
Conformation E, Hartree AE, kcal/mol Dip. Mom., D
1 -686.83073582 0.000 4.345
2 -686.82438414 3.985 4.732
3 -686.83047963 0.160 7.327
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