FT-IR, FT-Raman and UV-Visible Spectral Analysis on (E)-N′-(thiophen-2-ylmethylene) Nicotinohydrazide

Generally, Pyridine ring is a heterocyclic organic compound with the chemical formula C5H5N. Pyridine is structurally related to benzene, with one methine group (=CH-) replaced by a nitrogen atom. It occurs in many important compounds, including azines and the vitamins niacin and pyridoxal. The precursor of pyridine is used to agrochemicals, pharmaceuticals and is also an important solvent and reagent. Mostly, it is used in the in vitro synthesis of DNA, sulfa pyridine (a drug against bacterial and viral infections), antihistaminic drugs tripelennamine and mepyramine, as well as water repellents, bactericides, and herbicides. Some chemical compounds, although not synthesized from pyridine, contain its ring structure. They include B vitamins niacin and pyridoxal, an anti-tuberculosis drug isoniazide, nicotine and other nitrogencontaining plant products [1].


Introduction
Generally, Pyridine ring is a heterocyclic organic compound with the chemical formula C 5 H 5 N. Pyridine is structurally related to benzene, with one methine group (=CH-) replaced by a nitrogen atom. It occurs in many important compounds, including azines and the vitamins niacin and pyridoxal. The precursor of pyridine is used to agrochemicals, pharmaceuticals and is also an important solvent and reagent. Mostly, it is used in the in vitro synthesis of DNA, sulfa pyridine (a drug against bacterial and viral infections), antihistaminic drugs tripelennamine and mepyramine, as well as water repellents, bactericides, and herbicides. Some chemical compounds, although not synthesized from pyridine, contain its ring structure. They include B vitamins niacin and pyridoxal, an anti-tuberculosis drug isoniazide, nicotine and other nitrogencontaining plant products [1].
The ring of Thiophene and its derivatives have been reported to possess broad spectrum of biological properties including anti-inflammatory, analgesic, antidepressant, antimicrobial and anticonvulsant activities [2][3][4]. Antiepileptic drugs (AEDs) like tiagabine, etizolam, brotizolam are containing thiophene moiety

FT-IR, FT-Raman and UV-Visible Spectral Analysis on (E)-N′-(thiophen-2-ylmethylene) Nicotinohydrazide
in their structures as active pharmacophore [5,6]. In addition, thiophene and its derivatives functionalized with the formyl group are versatile building blocks for the synthesis of donoracceptor substituted p-conjugated systems for several optical applications.
The hydrazone group in the organic compound brings out several physical and chemical properties. The hydrazones are bearing the >C=N-N< which leads the molecule towards nucleophilic and electrophilic in nature. In the hydrazone moiety, the nitrogen atom behaves as nucleophilic and carbon atom behaves as nucleophilic as well as electrophilic in nature [7][8][9]. The benzohydrazide derivatives shows wide spectrum of

Vibrational Assignments
The T2CNH molecule belongs to C 1 point group symmetry. It consists of 25 atoms which undergoes 69 normal modes of vibrations. In which 47 modes of vibrations are in-plane and 22 are out-of-plane bending vibrations and all of them are IR and Raman active [25]. The calculated and observed vibrational wavenumbers using DFT/B3LYP/6-311++G(d,p) basis set, along with their relative intensities are given in Table 2. The total energy distribution (TED) was calculated using SQM program [26]. The biological activities such as antibacterial [10], antifungal [11] and antitubercular [12] activities.
Subashchandrabose et al. [13] recorded the FT-IR, FT-Raman and UV-Vis spectra for the molecule N1-N2-bis((pyridine-4-l) methylene)benzene-1,2-diamine. The observed FT-IR and FT-Raman spectral values were compared with the calculated wave numbers. For the prediction of accurate vibrational assignments TED analysis was performed using SQM method. The bond lengths and bond angles of stable conformer were correlated well with the experimental values. The hyperconjugative interaction and charge delocalization around the bonds were studied using NBO analysis. Band gap energy was also determined.
Quantum chemical calculations of energies, geometrical structure and vibrational wavenumber of 1,2-bis(3-methoxy-4hydroxybenzylidene)hydrazine were carried out by Subramanian et al. [14] using DFT method with 6-31G(d) as basis set. The optimized geometrical parameters obtained by DFT calculations are in good agreement with single crystal XRD data. The vibrational spectral data were obtained from FT-IR and FT-Raman spectra are assigned based on the results of the theoretical calculations in solid phase.
From Literature survey reveals that the vibrational analysis of (E)-N′-(thiophen-2-ylmethylene) nicotinohydrazide (T2CNH) has not yet been reported. The T2CNH molecule was synthesized and its structural characterization was calculated by B3LYP/6-311++G(d,p) basis set. The spectral investigation such as FT-IR, FT-Raman and UV-Vis spectra were recorded. The observed spectral results were compared with the computed wavenumber. The vibrational assignments of the title molecule were carried out with the help of TED. The First order hyperpolarizability, Homo-Lumo energy gap was calculated and furthermore, the MEP and thermodynamic properties were also calculated.

N-H vibrations
The N-H stretching vibration appears in the region of 3300-3500 cm -1 [27]. In accordance with the above literature the νN-H vibration has been observed at 3401 cm -1 in FTIR spectrum, whereas the harmonic wavenumber assigned at 3365 cm -1 (mode no: 1) and its TED value is 100% it should be noted here that, a small deviation between theoretical and experimental value is only due to intra-molecular charge transfer between amino and carbonyl group in hydrazone linkage. The harmonic wavenumbers (1508/ mode no: 14 and 488 cm -1 /mode no: 53) of βN-H and ΓN-H modes were presented in Table 2 are found to be in good agreement with literature [28] data 1476 and 535 cm -1 as well as with the structurally related molecule. Mode no: 14 is in agreement with observed FT-Raman band at 1519 cm -1 and these assignments also find support from TED values (22% and 61%).

C=C, C-C vibrations
In the pyridine ring, the ν(C-C) stretching vibrations are usually occur in the ranges of 1590-1640, 1560-1580 and 1470-1510 cm -1 [33]. The computed wavenumber for ν(C-C) modes are lies at 1563, 1542, 1240 and 1173 cm -1 (mode nos: 12, 13, 22 and 25) with TED values. In the present study it has been established well and the FTIR band at 1549 cm -1 and FT-Raman bands at 1241, 1195 cm -1 are designated as νC-C vibrations. These assignments are find support from the literature [34] and also from TED values.

C=N, C-N vibrations
The identifications of C=N and C-N vibrations is a difficult task, since the mixing of vibrations is possible in this region [36]. However, with the help of Gauss View (3.0) software and TED results, those vibrations are described and assigned in this study. The C=N and C-N stretching vibrations appear in the ranges of 1670-1600 cm -1 and 1382-1266 cm -1 respectively [36]. In hydrozone linkage, the νC9=N11 and νC14-N12 stretching vibrations are assigned respectively at 1592 cm -1 (FTIR) /1589 cm -1 (FT-Raman) and at 1096 cm -1 (FT-Raman). The TED results show that these vibrations are mixed with βHCN and νC-C modes and their corresponding harmonic frequencies are 1591 (mode no: 11) and 1097 cm -1 (mode no: 27) well correlated with experimental observations. These vibrational assignments are also supported by literature [28] in addition to TED output (75% and 40%).
In pyridine ring, the harmonic/observed bands at 1563 (mode no: 12), 1240 (mode no: 22)/1241 cm -1 in FT-Raman spectrum can be assigned to C-N stretching vibration and TED also predict that these vibrations are mixed with νC-C modes. These assignments are in good agreement with our earlier study [28] and also find support from TED values (30%, 43%). The βC 21 -C 23 -N 19 , βC 16

C-S vibrations
In T2CNH the scaled wavenumbers 791 and 718 cm -1 (mode nos: 43 and 44) are assigned to νC-S modes of thiophen ring moiety. This assignment is in agreement with the assignments proposed by various authors [20,21]. This mode is well known to mix with neighboring modes (νCC, βCCN/mode no: 43 and βCCC/mode no: 44) as reported in the literature [37]. The harmonic frequencies of βCSC and ΓCSC vibrations are ascribed to wavenumbers: 586 cm -1 (mode no: 50) and 125 cm -1 (mode no: 64) with 62% and 37% of TED values, respectively. Further the wavenumbers 827 cm -1 (mode no: 40) and 205 cm -1 (mode no: 62) are assigned to βC 4 C 5 S 1 and βC 9 C 2 S 1 modes, respectively, which are in line with the observed bands (846 cm -1 : FTIR and 191 cm -1 : FT-Raman) in addition to support the TED values [80% and 30%].

C=O vibrations
The C=O is formed by Pπ-Pπ bonding between carbon and oxygen atoms. Carbonyl (C=O) group stretching vibration is expected to appear in the region of 1680-1715 cm -1 [38]. In this study, the carbonyl group stretching vibration appear at 1670 cm -1 as strong band in FT-IR and at 1673 cm -1 as weak band in FT-Raman spectra. The values of νC=O band is calculated at 1688 cm -1 (mode no: 10) with a TED of 85%. The βC=O and ΓC=O vibrations are assigned respectively to mode nos: 39 and 45, in which the predicted wavenumber related to ΓC=O mode is found to be in moderate agreement with the observed FTIR band at 700 cm -1 .

NLO Property
Analysis of organic compounds having conjugated π-electron systems and large hyperpolarizability using IR and Raman spectroscopy has evolved as a subject of scientific research. The application of the title molecule in the field of non-linear optics demands the investigation of its structural and bonding features

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contributing to the hyperpolarizability enhancement by analyzing the vibrational modes using IR and Raman spectroscopy. The DFT/ B3LYP/6-311++G(d,p) basis set has been used for the prediction of first hyperpolarizability.
The calculated first hyperpolarizability and the total molecular dipole moment of T2CNH is 1.5861 × 10 −30 esu, 0.9906 Debye, respectively obtained by B3LYP/6-311++G(d,p) level of theory. The total dipole moment of the title molecule is moderately equal and β0 value of T2CNH is 4 times greater than that of urea, hence the molecule T2CNH has considerable non-linear optical (NLO) activity and the hyperpolarizabilities of T2CNH are given in Table 3.

NBO Analysis
This method gives information about the intra-and inter-molecular interactions among bonds. Furthermore, it provides a convenient basis for investigating the interactions in both filled and virtual orbital spaces along with charge transfer and conjugative interactions in molecular system [39]. The natural bonding orbital (NBO) analysis was performed for T2CNH using B3LYP/6-311++G(d,p) basis set in order to elucidate the intra-molecular, hybridization and delocalization of ED within T2CNH. The strong intra-molecular hyperconjugative interaction of the σ and π electrons of C−C to the anti C−C bond of the pyridine ring leads to stabilization of some part of the pyridine ring. The NBO analysis has been carried out by B3LYP/6-311++G(d,p) basis set and the E(2) values and types of the transition are shown in Table 4.  The larger E(2) value shows the intensive interaction between electron donors and electron acceptors. The strong intramolecular hyper conjugative interactions of the σ and π electrons of the C=C, C=N to the anti C=C, C=N bond of the ring as well as C=O group leads to stabilization of some part of the ring system in T 2 CNH. In the present study, the π-character of the bond plays an important role on comparing with σ bond character. The hyper conjugative interactions π(C 2 -C 3 )→π*(C 9 -N 11 ), π(C 4 -C 5 )→π*(C 2 -C 3 ), π(C 16 -C 17 )→π*(C 18 -C 21 ), π(C 18 -C 21 )→π*(N 19 -C 23

HOMO-LUMO Analysis
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) analysis is very important parameters for quantum chemistry. The energy values of HOMO (π-donor) and LUMO (π-acceptor) and its energy gap which reflects the chemical activity of the molecule. The HOMO and LUMO energy was calculated by B3LYP/6-311++G(d,p) level of theory. The frontier molecular orbitals (FMOs) of T2CNH are listed in Table 5. The atomic compositions of FMOs are shown in Figure 4. The HOMO is located over the thiophene and hydrozone moieties. The LUMO is located over pyridine ring. The LUMO transition implies that an ED transfer to pyridine ring via hydrazone linkage. The HOMO and LUMO energies are predicted as -6.248 eV and -2.223 eV, respectively. The calculated HOMO-LUMO energy gap is 4.025 eV, which explains the eventual charge transfer taking place within the present molecule. The physicochemical properties are also listed in Table 6.

UV-Vis Spectral Analysis
The UV-Visible absorption spectrum of T2CNH was recorded in the range of 250-350 nm is shown in Figure 5. All the structures allow strong π-π* (or) σ-σ* transition in the UV-Vis region with high extinction coefficients. On the basis of fully optimized ground state structure at TD-DFT/B3LYP/6-311++G(d,p) calculation has been used to determine the low-lying excited states of T 2 CNH. The calculated results involving the vertical excitation energies, oscillator strength (f) and wavelength are carried out and compared with measured experimental wavelength. Typically, according to Franck-Condon principle, the maximum absorption peaks (λmax) in a UV-Vis spectrum corresponds to vertical excitation. It is evident from the  Table 5 The frontier molecular orbital of T 2 CNH.
286 nm which correlates well with the experimental values 340 and 290 nm. The more intense band at 340 nm has maximum oscillator strength (f=0.6909), corresponds to Homo-Lumo transition and is mostly characterized as n-π* type. This type of transition is attributed to the presence of large no of free lone pairs of electrons available on sulphar (S 1 ), Nitrogen (N 11 , N 12 ) and oxygen (O 15 ) atoms. The experimental and theoretical UV-Vis absorption spectrum is shown in Figure 5. The density of states spectrum of T2CNH is shown in Figure 6. It was used to calculate group contributions to the molecular orbitals (HOMO and LUMO). DOS plot shows population analysis per orbital and demonstrates a simple view of the character of the molecular orbitals (MOs) in a certain energy range.

MEP Analysis
The molecular electrostatic potential (MEP) is widely used as a reactivity map displaying most probable regions for the electrophilic attack of charged part on organic molecule. MEP plot provides a simple way in predicting the interaction of different geometries. In order to predict the reactive sites for electrophilic and nucleophilic attacks of the T 2 CNH, MEP was calculated with DFT/B3LYP/6-311++G(d,p) level of theory. The negative (red color) and positive (blue color) regions of MEP are related to electrophilic and nucleophilic reactivity respectively is shown in Figure 7. The negative region is located over the carbonyl group and the positive region is located over Hydrogen atom in the hydrazone linkage.

Mulliken Charges Analysis
The Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation to molecular system, because the atomic charges should affect dipole moment, polarizability, electronic structure and more a lot of properties of molecular systems. The total atomic charges of T2CNH are calculated by Mulliken population analysis with B3LYP/6-311++G (d,p) basis set and its values are listed in Table 8. The Mulliken atomic charge plot for T2CNH is shown in Figure 8.

Figure 6
The DOS spectrum of T 2 CNH. Figure 7 Molecular electrostatic potential map of T 2 CNH.  Table 7 The electronic transition of T2CNH. due to the fact that the molecular vibrational intensities increase with temperature. The correlation equations between these thermodynamic parameters and temperature were fitted by parabolic formula and the regression coefficient is also given in the parabolic equation. The correlation relation between the thermodynamic parameters and temperature are as follows: The correlation graphs between various thermodynamic functions and temperature are graphically presented in Figure 9.

Conclusion
The complete vibrational analysis has been performed to using quantum chemical calculation at DFT method for the first time to the title molecule T 2 CNH. The calculated bond parameters are good agreement with the related single crystal X-ray diffraction (XRD) data. In T 2 CNH, the thiophene and hydrazone moieties are co-planar, which shows a good conjuction between p-orbitals of all atoms of thiophene and hydrazone moieties. The vibrational atoms have the highest negative/positive charges, respectively among the other atoms in T2CNH due to the resonance.