Spectrophotometric Investigation of the Complexation Mechanism of 3– (Trifluoromethyl) Acetophenone

Received: 13/May/2019, Accepted: 10/Jun/2019, Online: 30/Jun/2019 AbstractThe experimental and theoretical study on the structures and vibrations of 3-TRIFLUOROMETHYL ACETOPHENONE (abbreviated as 3TFMAP) are presented. The FT-IR and FT-Raman spectra of the title compound have been recorded in the region 4000−400 cm -1 and 3500−100 cm -1 respectively. The molecular structures, vibrational wavenumbers, infrared intensities and Raman activities were calculated using DFT (B3LYP) method with 6-31+G (d) basis set. The most stable conformer of 3TFMAP is identified from the computational results. HOMO and LUMO energies were determined by time-dependent TD-DFT approach. Molecular electrostatic potential map also analysis by this present compound, respectively.


I. INTRODUCTION
Acetophenone is one of the most aromatic carbonyl, which shows interesting photochemical properties [1,2]. Acetophenone are compound that exhibit interesting physicochemical and biological properties. They are found in nature [3,4] and they can also be obtained by means of diverse synthesis procedures [5,6]. Antibacterial activity can be mentioned among its biological properties. A recent study has linked the antibacterial activity of 20 acetophenone with their structural characteristics by using electronic and topological indices [7]. On the other hand, substituted acetophenone are employed as synthesis reagents of several organic reactions. A well known one is the condensation reaction in alkaline medium of acetophenones and benzaldehydes in the synthesis of flavonoids (chalcones, flavones and flavanones) [8,9]. This work was based on combined solid phase infrared and Raman vibrational spectroscopy and gas phase theoretical calculation with the Density Functional Theory (DFT) method. DFT approaches, especially those using hybrid functional, have evolved to a powerful and very reliable tool and been routinely used for the determination of various molecular properties. B3LYP functional has been previously shown to provide an excellent compromise between accuracy and computational efficiency of vibrational spectra for large and medium size molecule [10−11]. Hence in this work, a systematic study on the optimized structure of the most stable conformer of the molecule, a detailed vibrational analysis based on FT-IR, FT-Raman inter and intra molecular charge transfer, Mulliken population analysis, and biological activity of the molecule based on its electrophilicity index have been carried out. The optimized geometry and vibrational wavenumbers are calculated only for the most stable conformer at DFT/B3LYP level of theory using 6-31+G (d) basis set. The conformers IV and V form are more stable than others. The vibrational wavenumber of the conformer IV of 3TFMAP have also been calculated. These calculations are valuable for providing an insight into the vibrational spectrum and molecular parameters. The stable position of 3TFMAP with respect to phenyl ring is obtained by performing the potential energy surface (PES) scan with B3LYP/6-31+G (d) level of theory.

II. EXPERIMENTAL DETAILS
The pure sample of 3TFMAP in the light yellow liquid form was purchased from the Lancaster Chemical Company (UK), with a stated purity of greater than 98% and it was used as such without further purification. The FT-IR spectrum of this compound was recorded in the region 4000−400 cm -1 on IFS 66V spectrometer equipped with an MCT detector using KBr beam splitter and globar source. The FT-Raman spectrum of title molecule has been recorded using the 1064 nm line of a Nd:YAG laser as excitation wavelength in the region 3500−100 cm -1 on a BRUKER model RFS 66V spectrometer. The reported wavenumbers are expected to the accurate within ±1 cm -1 resolution with 250 mW of power at the sample in both the techniques.

III. COMPUTATIONAL DETAILS
The gas phase geometry of the compound in the ground state was optimized by using DFT/B3LYP and LSDA levels with the standard basis set 6-31+G (d) on personal computer using GAUSSIAN 09W [12] program package, invoking gradient geometry optimization [13]. The global minimum energy conformer is used in the vibrational wavenumber calculation at the B3LYP and LSDA with using 6-31+G (d) level. Subsequently, the vibrational in association with the molecule were derived along with their IR intensity and Raman activity. In order to fit the theoretical wavenumbers to the experimental, the scaling factors have been introduced by using a least square optimization method. The HOMO−LUMO analysis has been carried out to explain the charge transfer within the molecule. The global hardness (η), global softness (), electronegativity (χ) and chemical potential (μ) have been calculated using the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). After the ground state geometry optimization, the excitation spectrum of the compound has been computed with at TD-DFT/6-31+G (d) level. Since solvent effects play an important role in absorption spectrum of the compound, in this paper, the time dependent self consistent field (TD-SCF) dealing with solvent effect was chosen in excitation energy calculations. The natural bonding orbital's (NBO) calculations [14] were performed using NBO 3.1 program as implemented in GAUSSIAN 09W package at DFT level in order to understand various second order interactions between the another subsystem, which is a measure of the intra molecular delocalization or hyper-conjugation. To check whether the chosen set of symmetric coordinates contribute maximum to the potential energy associated with the molecule, the PED has been carried out. The transformation of force field, subsequent normal coordinate analysis and calculation of the PED were done on a PC with the MOLVIB program (Version 7.0-G77) written by Sundius [15]. The complete information's regarding population of electrons in sub-shells of atomic orbital's and electron densities of an atom in the title molecule.

Prediction of Raman intensity
The Raman activities (S i ) calculated with the GAUSSIAN 09W Program were converted to Raman intensities (I i ) using the following relationship derived from the intensity theory of Raman scattering [16,17].
where υ 0 is the laser exciting frequency in cm -1 (in this work, we have used the excitation wavenumber υ 0 =9398.5 cm -1 , which corresponds to the wavelength of 1064 nm of a Nd:YAG laser), υ i is the vibrational wavenumber of the i th normal mode (in cm -1 ) and S i is the Raman scattering activity of the normal mode υ i , f (is the constant equal to 10 -12 ) is a suitably chosen common normalization factor for all peak intensities. h, k, c, and T are Planck constant, Boltzmann constant, speed of light, and temperature in Kelvin, respectively.

Vibrational spectral analysis
The goal of the vibrational analysis is to find vibrational modes connected with specific molecular structure of calculated compound. Vibrational frequencies were calculated by using B3LYP method with 6-31+G (d) for the table molecule having 20 atoms with 54 normal modes. In this study, the full set of 69 standard internal coordinates (containing 15 redundancies) for 3TFMAP were given in Table 1. From these, a non-redundant set of local symmetry coordinates were constructed by suitable linear combinations of internal coordinates are summarized in Table 1. This method is also useful for determining the mixing of other modes. But mostly the maximum contribution is accepted as the significant mode. The observed FT-IR and FT-Raman bands with their relative intensities, calculated wavenumbers and assignments are given in Table 5. The experimental FT-IR and FT-Raman spectra with corresponding theoretically simulated FT-IR and FT-Raman spectra for 3TFMAP are shown in Figs 2 and 3, respectively, where the calculated infrared intensities and Raman intensities are plotted against the vibrational frequencies. In the spectra, the theoretically simulated spectra are more regular than the experimental ones because many vibrations presenting in condensed phase lead to strong perturbation of infrared and Raman intensities of many other modes. The RMS error of the observed and calculated frequencies (unscaled) of 3TFMAP is quite obvious since the frequencies are calculated on the basis of quantum mechanical force fields usually differ appreciably from observed frequencies. This is partly due to the neglect of anharmonicity and partly due to the approximate nature of the quantum mechanical methods. In order to reproduce the calculated frequencies, the scale factors were refined and optimized via a least squares refinement algorithm. To calculate optimal scaling factors (λ), we employed as a least-square procedure using the below equation

Phenyl ring modes:
The benzene ring possesses six stretching vibrations of which the four with the highest frequencies occurring near 1600, 1580, 1490 and 1440 cm -1 [18]. Generally, the C−C stretching vibrations in aromatic compounds give the bands in the region of 1650−1430 cm -1 [19].  Table 2 . From the results, it is clear that the C 1 atom of 3TFMAP which is attached to the functional group (C=O) experience a higher positive magnitude which is obviously due to the presence of electronegative atom which withdrawn electrons from C atoms of the ring. In present case, C 7 atom is more acidic due to more positive`e charge in carbonyl group.

Frontier molecular orbital's (FMOs)
The most important orbital's in molecule is the frontier molecular orbital's, called highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). These orbitals determine the way of molecule interacts with other species. The frontier molecular energy gap helps to characterize the chemical reactivity and kinetic stability of the molecule. A molecule with a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity, low kinetic stability and is also termed as soft molecule [35]. The low values of frontier orbital gap in IPDP it more chemical reactive and less kinetic stable. The frontier molecular orbital's plays an important role in the electric and optical properties [36]. The conjugated molecules are characterized by a small highest occupied molecular orbitallowest unoccupied molecular orbital (HOMO -LUMO) separation, which is the result of a significant degree of intramolecular charge transfer from the endcapping electron acceptor groups through π -conjugated path [37]. The 3D plot of the frontier orbital's HOMO and LUMO of DCPI molecule is shown in Fig. 4. The positive phase is red and negative phase one is green (For interpretation of the reference to color in the text, the reader is referred to the web version of the article). Many organic molecules, conjugated π electrons are characterized by large values of molecular first hyperpolarizabilities, were analyzed by means of vibrational spectroscopy [38,39]. In most cases, even in the absence of inversion symmetry, the strongest band in the FT-Raman spectrum is weak in the FT-IR spectrum vice versa. But the intramolecular charge transfer from the donor-acceptor group in a singledouble bond conjugated path can induce large variations of both the dipole moment and the polarizability, making FT-IR and FT-Raman activity strong at the same time. The analysis of wave function indicates that the electron absorption corresponds to the transition from the ground state to the excited state and is mainly described by one. An electron excitatio from the high occupied molecular orbital to the lowest unoccupied molecular orbital (HOMO -LUMO) Generally, the energy gap between the HOMO and LUMO decreases, it is easier for the electrons of the HOMO to be excited. The higher energy of HOMO, the easier it is for HOMO to donate electrons whereas it is easier for LUMO to accept electrons when the energy of LUMO is low.
The energy values of HOMO and LUMO levels are computed to be -0.20937 a.u. and -0.06573 a.u., respectively, and energy difference is -12,1888 a.u. in B3LYP/6-31+G (d). The energy values of HOMO and LUMO levels in B3LYP/6-311++G (d, p) is to be -0.20796 a.u. and -0.06420 a.u. and energy difference is 3.9119 a.u., respectively 2.764 eV HOMO LUMO Fig. 4 The molecular orbitals and energies gap for the HOMO and LU of 3-trifluoromethyl acetophenone.

Molecular electrostatic potential (MEP)
Molecular electrostatic potential map sites for electrophilic attack and nucleophilic reactions as well as hydrogen bonding interactions [36−38]. A predict reactive sites for electrophilic attack for the title compound. MEP was calculated at the B3LYP/6-31+G (d) optimized geometry. For the systems studied the molecular electrostatic potential values were calculated as described using the equation: where the summation runs over all the nuclei A in the compound and polarization and reorganization effects are neglected. Z A is the charge of the nucleus A, located at R A and ρ(r') is the electron density function of the molecule. The molecular electrostatic potential contour maps for positive and negative sites of the title molecule 4HAP are shown in Fig. 5. The MEP is a plot of electrostatic potential mapped onto the constant electron density surface. Different values of the electrostatic potential are represented by different colors. Red represents the regions of the most negative electrostatic potential and blue represent the regions of the most positive electrostatic potential. Potential increases in the order red < orange < yellow < green < blue. The color grading of resulting surface simultaneously displays molecular size, shape and electrostatic potential value which are very useful in research of molecular structure with its physiochemical property relationship [39]. As easily can be seen Fig.5, while region having the negative potential are over the electronegative atom (oxygen atom), the regions having the most positive potential are over the hydrogen atoms. As can be seen from the MEP of the title molecule, more reactive sites are near C=O group, the region having the most negative potential over the oxygen atom O 8 , respectively.

CONCLUSION
In the present work, the optimized molecular structure of the stable conformer, vibrational and electronic properties of the title compound have been calculated by DFT method (B3LYP) using 6-31+G (d) basis set. The optimized geometric parameters (bond lengths and bond angles) are theoretically determined and compared with the experimental results. Spectroscopic properties of the present molecule were examined by FT-IR and FT-Raman techniques. The complete vibrational assignments of wavenumbers are made on the basis of potential energy distribution (PED). The electronic properties are also calculated in different solvent with UV-Vis spectrum. The energies of MOs and the λ max of the compound are also evaluated TD-DFT/B3LYP method with 6-31+G (d) basis set. The calculated HOMO, LUMO energy and frontier orbital energy gap values along with study for better understanding of charge transfer occur within the molecule.