Spectroscopic investigations of (E)-5-(2-phenyldiazenyl)-2-hydroxybenzaldehyde using DFT method

. The Spectral Characterization of (E)-5-(2-phenyldiazenyl)-2-hydroxy benzaldehyde (PDHB) were carried out by using FT-IR, FT-Raman and UV-Vis spectroscopic techniques. All the recorded spectral results were compared with calculated results. The optimization has been performed on the title compound using B3LYP/6-311++G(d,p) level of theory. The optimized bond parameters of PDHB molecule was compared with X-ray diffraction data of related molecule. To study the intra-molecular charge transfers within the molecule the Lewis (bonding) and Non-Lewis (anti-bonding) structural calculation was performed. The Non-linear optical behavior of the title compound was measured using first order hyperpolarizability calculation. In addition of Mulliken atomic charges and thermodynamic properties were also calculated and analyzed.


INTRODUCTION
Azo compounds represent the largest and most important group of synthetic dyes. These compounds have a basic structure of Ar-N=N-Ar′ (where Ar and Ar′ =aromatic groups). The -N=Nazo group is the chromophore responsible for the color of these compounds [1]. The azo compounds are most widely used class of dyes due to their versatile application in various fields such as the dyeing of textile fibers, the coloring of different materials, colored plastics, biological and medical studies, and advanced application in organic synthesis [2][3][4]. Aromatic and heteroaromatic azo compounds constitute the largest and the most diverse group of synthetic dyes with application not only as textile colorants but in many other industrial fields for coloring different substrates, biological-medical studies, in the field of non-linear optics and optical data storage [5][6][7].
Azobenzenes have versatile applicability ranging from textile dyeing [8], leather dyeing [9], coloring of plastics and polymer [10] to advanced applications such as liquid crystal displays [11], biological and medical studies [12]. In this study, (E)-5-(2-phenyldiazenyl)-2-hydroxybenzaldehyde (PDHB) azo dyes were synthesized and geometrical parameters, and vibrational spectra were calculated. The calculated vibrational wavenumbers and chemical shifts were compared with the experimental data of the molecules. Furthermore, we interpreted the calculated spectra on the basis of total energy distributions (TED).

Synthesis
For synthesis of polyhydroxy azo-azomethine derivatives 6, substituted anilines 1 were first diazotized using sodium nitrite in the presence of hydrochloric acid, which coupled with salicylaldehyde 3 to afford 5-phenyldiazenylsalicylaldehyde dyes 4 in good yields. 5-Phenyldiazenylsalicylaldehydes 4 were purified by recrystallization from suitable solvents. The reaction of 4 with tris(hydroxymethyl)aminomethane (5) then afforded polyhydroxy azoazomethine derivatives 6, which were purified by recrystallization from suitable solvents.
(1) Where I i is the Raman intensity, RA i is the Raman scattering activities, ν i is the wavenumber of the normal modes and ν 0 denotes the wavenumber of the excitation laser [17]. The same formula (Eqn. 1) has been used to convert the Raman activity into Raman intensity.

Geometrical Parameters
In this study, the optimized molecular structure of the title molecule is depicted in Fig. 1. The comparative optimized structural parameters such as bond lengths, bond angles and dihedral angles were performed using B3LYP/6-311++G (d,p) basis set are presented in Table. 1. These parameters are compared with X-ray data of the related molecule [18].

Vibrational Assignments
The title molecule belongs to C1 point group symmetry. It consist of 27 atoms and 75 normal modes of vibrations are distributed amongst the symmetry species as: Γ3n-6 = 51´ (in-plane) + 24´´ (Out-of-plane). All these modes are found to be are active both in the Raman and IR absorption. To know the exact vibrational behaviour of this molecule the TED analysis was performed. The calculated wavenumbers were compared with recorded vibrational frequencies of FT-IR and FT-Raman spectra. Some discrepancies could be identified in between harmonic and observed frequencies, which were scaled down by proper scale factor [19]. The observed and calculated spectral results and their probable assignments are given in Table. 2 and the combined recorded and theoretical spectra are also shown in Figs. 2 and 3.s

O-H Vibrations
The O-H stretching vibrations are more sensitive to hydrogen bonding. The non-hydrogen bonded or a free hydroxyl group absorb strongly in the 3550-3700 cm −1 region [20]. Intra-

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ILCPA Volume 60 molecular hydrogen bonding if present in six membered ring system would reduce the O-H stretching band to 3200-3550 cm −1 regions. In the present study, the OH stretching mode assigned at 3677 cm −1 while its corresponding vibrational bands are absent (mode no: 1). The in-plane bending vibrations calculated at 1130 cm -1 (mode no: 30). The OH out-of-plane deformation vibration in phenol lies in the region 517-710 cm −1 for associated OH [Varsanyi, 1974] [21]. In this study, the FTIR bands observed at 708 cm -1 and its corresponding harmonic value 692 cm -1 (mode no: 50) are assigned to ΓOH mode. This results show that the frequency increases with hydrogen bond strength because of the larger amount of energy required to twist the O-H bond out-of plane [22].

C=O Vibrations
The carbonyl stretching vibrations has been most extensively studied by IR spectroscopy. This multiply bonded group is highly polar and therefore gives rise to an intense IR absorption band. Because of the different electro negativities are distributed between the two atoms [23]. The lone pair of electrons on oxygen also determines the nature of the carbonyl group. In the present study, the very strong band observed at 1667 weak band at 1660 cm −1 in FTIR and FT-Raman spectra are assigned to C=O stretching vibration and its corresponding harmonic value is 1692 cm -1 (mode no: 11). This assignment is further supported by TED 88%. The βHCO /ΓHCO vibrations are ascribed to harmonic values 1402/966 cm -1 (mode nos: 20/37) with TED 74/64 % respectively.The νC 21 -O 23 mode is ascribed to FT-Raman band at 1201 cm -1 , this is supported by harmonic frequency 1196 cm -1 (mode no: 27) with TED 20%.

C-H Vibrations
In case of CH stretching modes they are expected to occur in the range 2900-3200 cm −1 [24,25]. In this study, these vibrations are observed at 3064, 3041 cm -1 in FTIR spectrum and 3051 cm -1 in Raman spectrum. The harmonic frequencies in the range 3087-3041cm -1 (mode nos: 2 to 9) are belong to the same mode. In which mode nos: 8 to 9 are in excellent agreement with experimental value and also find support from TED > 86%.

Azo chromophoric group (-C-N=N-C-) vibrations
Azo chromophoric group contains two bands viz C-N and N=N. Each has its own well known characteristic vibrational wavenumber. The N=N stretching vibration has a very characteristic and intense band in the Raman and its difficult to observe in IR spectrum, which is due to its symmetry [26]. This molecule has two unequivalent C-N parts causes a change in dipole moment and hence the N=N stretching is IR active and is predicted to be medium strong. Literature survey reveals that the bands occurring between 1380 and 1450 cm -1 corresponds to the N=N stretching of an azo compound [27]. The νC-N modes of azo compounds are expected to appear in the region 1200-1300 cm -1 [28,29]. These bands shift in wavenumber and intensity in a complex fashion, depending on the neighbouring groups. Conjucation effects, H-bonding and molecular tautomerism [29]. Moreover, substituents on benzene ring 2 influence both the peak wavenumbers and intensity. The strong band at 1477 (FT-IR) and at 1482 cm -1 (FT-Raman) originate from the stretching vibration of (N=N) group fused in between the two rings. The observed FTIR: 1096/FT-Raman: 1106 cm -1 bands are assigned to νC14-N13 mode. These two bands have contribution from βCH vibrations of their respective benzene rings. These assignments are in agreement with harmonic values 1485, 1170 and 1104 cm -(mode nos: 16, 28 and 32), respectively.

NLO property
The first-order hyper polarizability β is a third rank tensor that can be described by 3x3x3 matrix. The 27 components of 3D matrix can be reduced to 10 components due to the Kleinman symmetry [Kleinman, 1962] [31]. The output from Gaussian 03W provides 10 components of this matrix as βxxx, βxxy, βxyy, βyyy, βxxz, βxyz, βyyz, βxzz, βyzz and βzzz respectively. The components of the first-order hyper polarizability can be calculated using the following equations.
Since the values of the polarizabilities α and the first-order hyper polarizabilities (β) of Gaussian 03W output [13] are reported in atomic units (a.u), the calculated values have been converted in to electrostatic units (esu). The calculated value of dipole moment and first hyper polarizability values are 0.78768 Debye and 9.2446 x 10 -30 esu, respectively and are listed in Table.

NBO analysis
NBO analysis has been performed to elucidate the intra-molecular interaction, rehybridization and delocalization. The second order perturbation analysis of Fock matrix of PDBH is summarized in Table. 4. The strong intra-molecular interactions are formed by the orbital overlap between σ(C-C), σ*(C-C) and π(C-C), π*(C-C) bond orbitals in the aromatic rings, which leads intra-molecular charge transfer (ICT) causing stabilization of the system.

Homo-Lumo analysis
The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are called frontier molecular orbitals as they lie at the outermost boundaries of the electrons of the molecules. The HOMO and LUMO are the main orbitals responsible for chemical stability. The HOMO-LUMO orbitals of the PDHB are as shown in Fig. 4. The positive and negative phases are represented in green and red colors, respectively. The calculated values of the HOMO and LUMO energy energies and band gap are given in Table. 5. HOMO represents the associated with the electron-donating ability of a molecule, whereas LUMO indicates its ability to accept electrons. The frontier orbital gap helps to characterize the chemical reactivity and kinetic stability of the molecule. A molecule with a small frontier orbital gap is generally associated with high chemical reactivity and low kinetic stability. In the present study, the HOMO/LUMO energies have been calculated using B3LYP/6-311++G(d,p) level are -6.514133/-2.695831 eV. In the most common case, ionization potential (I) and electron affinity (A) are related to HOMO and LUMO, respectively. The obtained values of I and A (Table. 5) were considered for the calculation of electronegativity (v), global hardness (g) and softness (r). These quantum chemical parameters were evaluated using Eqs. (5) to (7) as below; χ = (I+A/2); (5) η = (I-A/2); (6) σ = 1/ η (7)

UV-Visible analysis
The properties of azo benzene (-N=N-group), have been mostly investigated by UV-Vis spectroscopy in various theoretical and experimental results have been reported [32]. The (λ max ) of the title molecule were calculated by the TD-DFT/B3LYP/6-311++G(d,p) basis set. The observed and calculated UV-Visible absorption maxima of the molecule are given in Table. 6 and shown in Fig 5. The λ max is a function of the electron availability. This band was due to electronic transition of the azo group. . This excitation corresponds to (π-π*) transition.

Molecular electrostatic potential
The molecular electrostatic potential (MEP) is related to the electronic density and is a very useful descriptor in understanding sites for electrophilic attack and nucleophilic reactions as well as hydrogen bonding interactions [33,34]. To predict reactive sites for electrophilic and nucleophilic attack of PDHB, MEP's were calculated at the B3LYP/6-311++G (d,p). The color scheme for the MEP surface is red, electron rich, partially negative charge; blue, electron deficient, partially positive charge; light blue, slightly electron deficient region; yellow, slightly electron rich region; green, neutral; respectively. The color code of the maps in the range -6.919 a.u. (deepest red) to 6.919 a.u. (deepest blue) in title molecule, where blue color indicates the strongest attractions and red indicates the strongest repulsion. The predominance of light green region in the MEP surfaces corresponds to a potential halfway between the two extremes red and dark blue color. It is obvious from Fig. 7 that the region around the oxygen atom of the OH group for represents the more negative potential regions (red) while there is a more positive charge around the hydrogen atoms of the PDHB.

Mulliken Charges
The Mulliken population analysis [35] provides a partitioning of either a total charge density or an orbital density. The Mulliken atomic charges were calculated at B3LYP/6-311++G(d,p) level are given in Table. 7. The mulliken charge plot is shown in Fig. 6. The charge distribution on the molecule has an important influence on the vibrational spectra. The carbon: C14 and C21 have the highest negative charge. Therefore, this might have been given C-C bond a greater ionic character. The C14 and C21 atoms bonded with N13 and O23 atoms. The C17 has highest positive charge, which is due to the attachment of CHO group withC17 atom. It is evident from the Table. 7 that the bond length between (C17-C21 1.4156 Å) and (C21-C19 1.398 Å) are differ by 0.0176 Å. This is due to C21/C17 atoms posses highest negative/positive changes.

Thermodynamic analysis
On the basis of vibrational analysis at B3LYP/6-311++G(d,p) level, the standard thermodynamic functions: heat capacity (C 0 p,m ), entropy (S 0 m ) and enthalpy changes (H 0 m ) for the PDHB were obtained from the theoretical harmonic frequencies and are listed in Table. 8. Therefore, it can be observed that these thermodynamic functions are increasing with temperature ranging from 100 to 1000 K due to the molecular vibrational intensities increase with temperature [36]. The correlation equations among heat capacity, entropy and enthalpy changes due to the temperature were fitted by quadratic, linear and quadratic formula and the corresponding fitting factors (R 2 ) for these thermodynamic properties are 0.99957, 0.99997 and 0.99971. The corresponding fitting equations are as follows and the correlation graphs are shown in Fig. 8. C 0 p,m = -1.83066 + 0.1642T -6.4227x10 -5 T 2 (R 2 = 0.99957) S 0 m = 3.16547 + 0.14747T -1.84592x10 -5 T 2 (R 2 = 0.99997) ΔH 0 m = 107.84214 + 0.00567T + 1.89583x10 -5 T 2 (R 2 = 0.99971) All the thermodynamic data are supportive information for further study of the PDHB. They can be used to compute other thermodynamic energies according to the relationship of thermodynamic functions and estimate of chemical reactions according to the second law of thermodynamics in thermo chemical field [37].

CONCLUSION
A complete vibrational analysis has been performed for the first time for PDHB molecule. The detailed interpretations of the vibrational spectra have been carried out. The vibrational assignments are further justified with help of the TED. The optimized geometrical parameters were calculated and compared with the related XRD values. The angle between C3-N12=N13 and C14-N12=N13 have same value 115°, which shows that steric repulsion is absent in the present molecule. The νN12=N13 mode appeared at higher wavenumber with strong intensity which is due to the neighbouring groups and conjucation effects. The β0 value of PDHB is twenty five times greater than that of urea and hence the molecule (PDHB) has good NLO activity. In this study, the π-π* interaction can make larger intra-molecular interaction and hence the polarizability of the molecule increases. The HOMO-LUMO energy gap is 3.8183 eV, which explain the eventual charge transfer occur within the molecule. MEP gives the visual representation of the chemically active sites and comparative reactivity of atoms. The Mulliken atomic charges of PDHB has been calculated and also plotted. The recorded UV-Visible spectral value agree well with calculated value. The absorption max (336 nm) is assigned to π-π* type. Thermodynamic parameters have been calculated at different temperatures of PDHB molecule. The correlation between the statistical thermodynamics and temperature are also obtained.
International Letters of Chemistry, Physics and Astronomy Vol. 60