Experimental and Theoretical Investigation of Molecular Structure and Vibrational Frequencies of 3-Cyano-7-hydroxycoumarin by Density Functional Theory

In this research, geometrical structure and vibrational spectra of 3-cyano-7-hydroxycoumarin (3C7HC) were studied experimentally and theoretically. Experimentally, FT-IR (infrared) and FT-R (Raman) spectra of 3C7HC molecule were respectively obtained in the range of 4000-400 cm -1 and 3500-100 cm -1 . Theoretically, the optimized geometric structures, vibrational spectra and corresponding assignments according to two possible configurations of the 3C7HC molecule were investigated using the density functional method (B3LYP) with 6-311++G (d,p) basis set. When analyzes were performed, C S symmetry property of 3C7HC molecule was used. It was seen that the experimental and calculated data of the 3C7HC molecule were highly compatible with each other.


Introduction
3-Cyano-7-hydroxycoumarine is known as 3-Cyanoumbelliferone, 7-hydroxy-2-oxo-1-benzopyran-3-carbonitrile and 7-hydroxy-2-oxochromene-3carbonitrile, respectively.It is the derivative molecule of Coumarine, having a chemical formula of C10H5NO3.Coumarine derivatives exhibit various biological activities and therapeutic properties.As a result of their presence in the chemical structures, they have been extensively studied in biomedical and industrial areas [1][2][3][4][5][6][7][8][9][10][11][12][13][14].In recent years, the vibrational spectra and geometrical structures of Coumarine molecules have been experimentally and theoretically studied by many researchers.DFT method has been widely used in investigation of electronic molecular system.Density functional theory is based on the assumption that the electron density of an electronic system's energy can be defined using probability terms .In the literature, no study was found on experimental and theoretical investigation of 3C7HC molecule.
In this study, experimental and theoretical conformations of 3C7HC molecule were carried out.The vibrational frequencies and geometrical structure of 3C7HC molecule were calculated with DFT/B3LYP method using 6-311++G(d,p) basis set.The calculated geometrical parameters and vibrational frequencies were compared with experimental results and previous studies.
Company USA and used in pure form without further purification.The 3C7HC sample is found as powder at room temperature.3C7HC was pelleted with KBr at room temperature using approximately 10 MPa pressure.The FT-IR spectrum of the 3C7HC was obtained within 4000-100 cm -1 range with the Perkin Elmer Spectrometer.FTR spectrum of 3C7HC was obtained within the range of 3200-100 cm −1 with Jasco NRS-3100 Raman Spectrometer.

Computational method
Optimized geometric structures, vibrational frequencies, zero-point energies and dipole moments according to two different conformation of 3C7HC were calculated with B3LYP 6-311++G(d,p) basis parameters of DFT method.The computations were made with Gaussian 09 [25] and Gauss-View software [26].While the vibrational frequencies of 3C7HC were calculated theoretically, the 0.9614 scale was used [27].

Molecular geometry
3C7HC molecule has a planar geometric structure represented by the Cs symmetry which consists of identity E and a mirror plane σ elements.In this study, two probable conformers of optimized geometrical structures of the 3C7HC molecule with hydroxyl group (O-H) oriented in line with C-O bonding were investigated.When O-H is farther away from the oxygen atom of the pyrone ring of 3C7HC, the conformer is called as rotamer (Rot1).When O-H is closer to the oxygen atom, the conformer is called as rotamer2 (Rot2).The optimized geometrical structures of the Rot1 and Rot2 conformers of 3C7HC molecule are shown in Fig. 1.Orientation angles of O-H for conformers of 3C7HC are 110.6 0 for Rot1 and 249.6 0 for Rot2.
In the case of Rot1 and Rot2 of the 3C7HC molecule the zero-point, relative energy and dipole moments are given in Table 1.
The relative energy between two conformers is considerably low and it has a value of ~0.32 kcal/mol.As indicated by zero-point energies and dipole moments of the molecule, Rot1 conformer is more stable.Experimental [28,29] and computed geometric parameters of conformers of 3C7HC molecule were compared in Table 2.The largest differences in computed and experimental parameters were obtained as Rot1 (0.04 Å), Rot2 (0.042 Å) for the bond lengths, and Rot1 (1.7 0 ), Rot2 (1.8 0 ) for the bond angles.

Γ
(1) In the event of translation ( ) and ) from Cs point group table, vibrational modes are expressed as; In infrared (FT-R) and Raman (FT-R) all vibrational values are active.In the Cs symmetry, the 2N-3 of the vibrations is in plane and the N-3 of the vibrations is out of plane [30].As a result, 35  FT-IR and FT-R spectra of 3C7HC molecule are given in Fig. 2 and Fig. 3. Observed and computed vibrational frequencies, Raman activities, infrared intensities, in plane -out of plane assignments are given in Table 3. Correlation values of vibrational frequencies for both conformers are given in the Table .The obtained results, normal coordinate analysis and the vibrational frequencies are significantly consistent with the results of previous studies.

C-H vibrations
CH stretching vibrations in the aromatic molecular structures widely occur in the form of weak multiple bands within 3100-3000 cm -1 interval.This region generally represents C-H stretching vibrations [31].
In this region the characteristics of substituent have no significant effect on the bands and four C-H stretching vibrations occur in 3C7HC molecule.In this study, aromatic C-H stretching vibrations of 3C7HC were obtained as 3090, 3043 cm −1 in FT-IR and

O-H vibrations
O-H bending and stretching vibrations can be identified with stretching vibration of the frequency band that is related to the hydrogen bonding extent.
Because the oxygen atom is lighter than the hydrogen atom, the stagnation of the molecular structure had no effect on the O-H stretching vibration.O-H stretching vibrations are demonstrated by strong absorption bands in 3700-3584 cm -1 region [16].In this study, the O-H stretching vibration was obtained as 3690 cm -1 in FT-IR.[16,[38][39][40].

CN vibrations
There is a limited number of absorbing groups in the area where C≡N stretching is, so C≡N stretching restraint is quite easy.The bands showing the absorption vibration of cyano groups are quite dense within the range 2240-2220 cm -1 and the stretching vibrations in this region show a characteristic feature of C≡N.The amplitude of C≡N stretching vibration is demonstrated as a significantly strong band 2223 cm -1 in FT-IR and 2224 cm -1 in FT-R for 4-methy-2cyanobiphneyl [15,39] and 2229 cm -1 at FT-IR and 2221 cm -1 at FT-R for 3-cyano-4-methylcoumarin [15].

A
In this research, the stretching vibration was observed as 2235 cm -1 at FT-IR and 2239 cm -1 at FT-R.C≡N in plane bending vibrations are demonstrated in 617-437 cm -1 region [15].In plane C≡N bending vibrations were obtained as 652, 554 cm -1 at FT-IR and 650, 553, 427 cm -1 at FT-R, whereas out of plane C≡N bending vibration was obtained as 526 cm -1 at FT-IR and 516 cm -1 at FT-R.C≡N rocking vibration was obtained as 115 cm -1 at FT-R.

Discussion and Conclusion
In this research, FT-IR and FT-R spectra were obtained and vibrational assignments are shown in detail for the first time for 3C7HC.The differences in observed and scaled vibrational frequencies were found to be significantly low.FT-IR and FT-R spectra of 3C7HC were studied by using B3LYP 6-311++G(d,p) parameter of density functional theory.Vibrational frequencies of optimized molecular structures, geometry parameters, relative energy and dipole moment, corresponding vibrational assignments, FT-IR intensities and FT-R activities of the two different conformers of 3C7HC were obtained using DFT method.The calculated geometrical parameters and vibrational frequencies of these conformers were found to be consistent with the experimental data.

Table 1 .
Zero-point, relative energy and dipole moment of 3C7HC

Table 3 .
Experimental and computed vibrational frequencies of Rot1 and Rot2 conformers of 3C7HC