Experimental and Theoretical (ab initio and DFT) Analysis of UV-Vis Spectra, Thermodynamic Functions and Non-linear Optical Properties of 2-Chloro-3,4-Dimethoxybenzaldehyde

2122 | P a g e A p r i l 0 8 , 2 0 1 5 Experimental and Theoretical (ab initio and DFT) Analysis of UV-Vis Spectra, Thermodynamic Functions and Non-linear Optical Properties of 2-Chloro-3,4-Dimethoxybenzaldehyde Rajesh Kumar, Sarvendra Kumar, Jayant Teotia, M. K. Yadav a Department of Physics, Government Degree College, Budaun (U.P.) rajesh_phy007@yahoo.co.in b Amity Institute of Applied Sciences, Amity University, Noida, U.P, INDIA, sarvendraricky@rediffmail.com c Department of Physics, D.N.College, Meerut (U.P.) 250002 jayant.phy@gmail.com d Department of Physics, D.N.College, Meerut (U.P.) 250002 ABSTRACT


INTRODUCTION
The benzaldehyde and its derivatives have received attention because of their chemical and biological importance [1][2][3][4][5]. Benzaldehyde is the simplest representative of the aromatic aldehydes. It is found in cherries, almonds, apricot and peach pits. Benzaldehyde is best known as being artificial essential oil of almond and it has many other uses such as; the manufacturing of dyes, perfumes, flavorings, cinnamic and mandelic acids, and it is also used as a solvent. Some more recent developments in the use of benzaldehyde are for the health and agriculture industries. Benzaldehyde is being used as a pesticide and also as an anticancer agent. It is used as a bee repellant in the harvesting of honey. These uses are still in development but would be very helpful in solving some of these problems of today. The benzaldehyde and substituted benzaldehydes have been subjected to various spectroscopic studies [6][7][8][9][10][11][12][13][14]. Mono-, halo-, methoxy and ethoxy-substituted benzaldehydes, among others, have attracted the attention of the spectroscopists. Various spectroscopic studies of halogen and methyl substituted compounds have been reported in the literature from time to time. N.Sundaraganesan et al., [11] have studied the vibrational spectra of 3,4-dimethylbenzaldehyde. V.Krishnakumar and V.Balachandran have reported vibrational spectra of 2-hydroxy-3-methoxy-5-nitrobenzaldehyde [12]. H Ünver , et.al, [13 ] have studied the UV-vis spectra and NLO properties of 2-hydroxy-3-methoxy-N-(2-chloro-benzyl)-benzaldehyde-imine. V. Venkataramanan, et.al, [14] have studied NLO properties of 3-methoxy-4-hydorxy benzaldehyde. Likewise, several UV-Vis spectroscopic studies, NLO properties, thermodynamic functions on di-and tri-substituted benzaldehydes have been reported [15 -17].
It is observed that the electronic data, non-linear optical parameters and thermodynamic parameters at various temperatures, which are vital to elucidate the relations among energetic, structural and reactivity of the title molecule, are insufficient in the literature. Thus, an attempt has been made in the present work to compute and compare the ground state optimized molecular geometrical parameters, thermodynamic and electronic properties using the Hartree-Fock (HF) and Becke's three parameter exact exchange functional (B3) combined with gradient corrected correlation functional of Lee, Yang and Parr (LYP) methods with 6-31+G(d,p) and 6-311++G(d,p) basis sets.
The UV spectrum was measured in different solvents (Acetone, Diethyl Ether and Carbon Tetra Chloride). The excitation energy, wavelength corresponds to absorption maxima ( ) and oscillator strength are calculated by Time-Dependent Density Functional Theory (TD-DFT) using B3LYP/6-31+G(d,p) and B3LYP/6-311++G(d,p) as basis sets. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. The non-linear optical parameters (the electric dipole moment, polarizability and the first hyperpolarizability values) of the investigated compound were also calculated using HF (Hartree-Fock) and DFT methods with same basis sets. The experimental and theoretical values of thermodynamic parameters at different temperatures are also tabulated.

Molecular Geometry
The optimized structure parameters of 2,3,4-CDMB calculated by HF and DFT-B3LYP levels with the basis sets 6-31+G(d,p) and 6-311++G(d,p) are listed in Table 1, in accordance with the atom numbering scheme given in Fig.1. Since the crystal structure of the title compound is not available till now, the optimized structure can be only be compared with other similar systems for which the crystal structures have been solved [21,22]. For example, the optimized bond lengths of C-C in phenyl ring fall in the range from 1.375 to 1.399 Å for HF and 1.38 to 1.48 Å. The bond angles calculated using various methods show the same trends as significant variation in bond lengths. The C-C-C angle in the ring at the position of chlorine substitution is distinctly larger than the others. The remaining C-C-C angles are close to 120 0 . The HF and DFT calculations also give shortening of the angle C3-C2-Cl12 for 2,3,4-CHMB and increase of the angle C1-C2-C3 for 2,3,4-CDMB from normal 120 0 at the C2 position and similarly for 3,4,5-CDMB shortening of the angle C4-C3-Cl12 and increase of the angle C2-C3-C4 for from normal 120 0 at the C3 position, and this asymmetry of exocyclic angles reveals the repulsion between the Cl group and the phenyl ring. Based on above comparison, although there are some differences A p r i l 0 8 , 2 0 1 5 between our values and the literature data, the optimized structural parameters can well reproduce the literature ones and they are the bases for thereafter discussion. The C-Cl bond length is almost equal in all B3LYP basis sets. The C-Cl bond length indicates a considerable increase when substituted in place of C-H. This has been observed even in benzene derivatives [22].

UV-Vis Absorption Spectra
On the basis of fully optimized ground-state structure, DFT-B3LYP levels with the basis sets 6-31+G(d,p) and 6-311++G(d,p) calculations have been performed to determine the low-lying excited states of 2,3,4-CDMB. The calculated result involving the vertical excitation energies, oscillation strength (f) and wavelength ( max) are carried out and compared with measured experimental wavelength listed in Table 2.1. The experimental and calculated UV-Vis spectra in different solvents are shown in Fig.2. Typically, according to Frank-Condon principle, the maximum absorption peak max in UV-vis spectrum corresponds to vertical excitation [16]. transitions are obtained at wavelengths 328, 299, 285 nm in acetone solvent, at 331, 297, 283nm in diethyl ether solvent and at 335, 296 and 282nm in carbon tetra chloride solvent by using B3LYP/6-311++G(d,p). For the same compound at different level B3LYP/6-31+G(d,p) these transitions are obtained at wavelengths 330, 301, 286 nm in acetone solvent, at 332, 299 and 284 nm in diethyl ether solvent and at 336, 297 and 283 nm in carbon tetra chloride solvent. There are three values of wavelengths for each solvent. For each solvent the first wavelength corresponding to n → * and another two corresponding to → * transitions [23,24].
For compounds 2,3,4-CDMB, there are shifts in wavelength of transition on changing the solvents. Among these three solvents Acetone is most polar and CCl4 is least polar. Solvent effects: On increasing the polarity of solvent the n→ * transition shifts towards lower wavelength (blue shift) and → * shifts towards higher wavelength (red shift). On comparing the results of three solvents (Tables 2.1-2.3), the wavelength corresponding to first transition is lowest for acetone and highest for CCl4, (i.e. shift towards lower wavelength on increasing the polarity of solvent) it is clear that first transitions represents n→ *, another two represent → * [25]. We cannot say solvent effect completely according to experimental values as well as theoretical data. The absorption maxima values are fairly in good agreement with theoretical values. The HOMO (Highest Occupied Molecular Orbital) energy characterizes the ability of electron giving, the LUMO (Least Unoccupied Molecular Orbital) characterizes the ability of electron accepting, and the gap between HOMO and LUMO characterizes the molecular chemical stability. The energy gap between the HOMOs and LUMOs is a critical parameter in determining molecular electrical transport properties because it is a measure of electron conductivity. Both HOMO and LUMO are the main orbitals that take part in chemical stability [16,17]. The energy values of LUMO and HOMO and their energy gap reflect determines the kinetic stability, chemical reactivity and, optical polarizability and chemical hardness-softness of a molecule [23]. The molecule has a large energy gap is known as hard and having a small energy gap is known as soft molecule. The hard molecule is not more polarizable than the soft ones because they need big energy to excitation [23,24]. The decrease in the HOMO and LUMO energy explains the Intramolecular charge transfer (ICT) interaction taking place within the molecule which is responsible for the activity of themolecule. The HOMO-LUMO energy separation has served as a simple measure of kinetic stability. A molecule with a small or no HOMO-LUMO gap is a chemically reactive [25,26]. Pearson showed that the HOMO-LUMO gap represents the chemical hardness of the molecule [27,28].

Non-Linear Optical (NLO) properties
Nonlinear optical (NLO) effects arise from the interactions of electromagnetic fields in various media to produce new fields altered in phase, frequency, amplitude or other propagation characteristics from the incident fields [29,16]. NLO is at the forefront of current research because of its importance in providing the key functions of frequency shifting, optical modulation, optical switching, optical logic, and optical memory for emerging technologies in the areas such as telecommunications, signal processing, and optical interconnections [30][31][32][33][17][18][19][20]. Urea is one of the prototypical molecules used in the study of the NLO properties of molecular systems. Therefore it is used frequently as a threshold value for comparative purposes.
The dipole moment μ, polarizability and second-order polarizability or the first hyperpolarizability , are calculated using HF and DFT with 6-31+G(d,p) and 6-311++G (d,p) basis sets on the basis of the finite-field approach. The complete equations for calculating the magnitude of total static dipole moment μ, the mean polarizability , the anisotropy of the polarizability and the mean first hyperpolarizability , using the x, y, z components from Gaussian 09W output is as follows: The dipole moment = 2 + 2 + 2 (1) The mean polarizability and anisotropy of polarizability respectively are defined by the mean first-order hyperpolarizability is defined as  Table 3.
The polarizabilities and first hyperpolarizability are reported in atomic units (a.u), the calculated values have been converted in to electrostatic units (esu) (α: 1 a.u =0.1482x10 -12 esu, β: 1 a.u = 8.6393x10 -33 esu). [47,48] The mean polarizabilities and anisotropy of polarizability α are also included in Table 3 (molecules are in the xy plane). It is observed that the B3LYP/6-311++G(d,p) level of theory leads to higher in comparison to other applied methods. Moreover, addition of halogen atom or methyl group to the original molecules increases the mean polarizability; this is because the molecular charge distribution in the case of derivatives can be distorted more easily by an external electric field in comparison with the original molecules. The first static hyperpolarizability calculated values are presented in Table 3. The values of is 6.01 x 10 -30 esu with (B3LYP/6-311++G(d,p) for 2,3,4-CDMB respectively. The values of first static hyperpolarizability with all components are shown in Table 3. Urea is the prototypical molecule used in the study of the NLO properties of the molecular systems. Therefore urea was used frequently as a threshold value for comparative purposes. In this study, the calculated values are nearly 5-55 times that of urea (0.1947 × 10 −30 esu). The large value of calculated by the B3LYP and HF methods show that the studied compound is a good NLO material. The theoretical calculation of components is very useful as this clearly indicates the direction of charge delocalization. The smallest value indicates charge delocalization is along the bond axis and the involvement of orbitals in intra-molecular charge transfer process [36].

Thermodynamic Functions
The values of some thermodynamic parameters (such as zero-point vibrational energy, thermal energy, heat capacity, rotational constants, and entropy) of 2,3,4-CDMB molecule computed by using HF and B3LYP methods with basis sets 6-31+G(d,p) and 6-311++G(d, p) at 298K in ground state are listed in Table 4.1. The standard statistical thermodynamic functions: heat capacity, entropy, enthalpy and entropy for the title compound were obtained from the vibrational harmonic frequencies in temperature range 200-1500K and listed in Table 4.2. From Table 4.2, it can be observed that these thermodynamic functions are increasing with temperature ranging from 200 to 1500 K, which is attributed to the enhancement of molecular vibration while the temperature increase [37,38]. These values of thermodynamic functions are plotted as a function of temperature. The variation of enthalpy function and heat capacity with absolute temperature 2,3,4-CDMB is shown in Figure 3a and 3b. These figures show that at very high temperature the influence of anharmonicity will make itself felt and will no longer give an accurate representation. The variation of free energy function and entropy function with temperature for 2,3,4-CDMB is shown in Figure 3c and 3d. The process of vaporization is accompanied by the increase in the disorder in the system [16,39]. So as the temperature increases entropy increases, this is shown in Table 4.2 for the molecule under study. Results of the study reveal that free energy, enthalpy and entropy increases continuously with temperature while heat capacity initially increases with temperature and remain almost constant beyond a particular temperature [40][41][42][43]. At low temperatures only translational parts of motion gives its contribution but as temperature increases rotational and vibrational motions are excited [43][44][45][46]. Beyond a certain value of temperature (nearly at 1000 K) there is no further increase in molecular motion and therefore heat capacity becomes almost constant.  The variation of free energy function and entropy function for 2,3,4-CDMB have been shown in figure 3. These Figures show that in case of free energy function at temperature as high as 1000K, the effect is quite small. While in case of entropy at very high temperature the vibrational contribution is very small, compared to other contribution thus; statistical calculation of the entropy is much less dependent upon vibrational data than the other thermodynamic functions. This trend of variation of thermodynamic functions with temperature is similar to those reported by earlier researchers [47][48][49][50][51] for similar molecules. All the thermodynamic data supply helpful information for the further study on the substituted benzaldehydes. They can be used to compute the other thermodynamic energies according to relationships of thermodynamic functions and to determine directions of chemical reactions according to the second law of thermodynamics in thermo-chemical field [36].

Conclusion
In the present work, an attempt has been made on the experimental and theoretical UV-vis spectral studies, NLO properties and thermodynamic properties at various temperatures of industrially important 2,3,4-CDMB. Equilibrium geometries, electronic parameters and thermodynamic parameters of 2,3,4-CDMB have been analyzed at HF and DFT/B3LYP using 6-31+G(d,p) and 6-311G++(d,p) basis sets. The correlations between the thermodynamic parameters and temperature show the increase in the heat capacities, entropies and enthalpies with increasing temperature owing to the rise in intensities of the molecular vibrations. Overall, the TD-DFT calculations on the molecule provided deep insight into their electronic structures and properties. In addition, the calculated UV-Vis results and thermodynamic functions are all in good agreement with the experimental data. The lowering of HOMO-LUMO band gap supports bioactive property of the molecule. The predicted NLO properties show that the title compound is a good candidate as nonlinear optical material. The calculated first hyperpolarizability is comparable with the reported values of similar derivatives and is an attractive object for future studies of nonlinear optics.