FT-IR, FT-Raman, Homo-Lumo and UV-Visible Spectral Analysis of E-(N′-(1H-INDOL-3YL) Methylene Isonicotinohydrazide)

1H-indole-3-carbaldehyde (1.45 g, 0.01 mol) and isonicotinic acid hydrazide (1.37 g, 0.01 mol) were added to ethanol (10 ml) and stirred for an hour in the presence of hydrochloric acid to form a white precipitate. The precipitate was washed with sodium bicarbonate solution and filtered and again washed with petroleum ether (40–60%) and dried in air. The compound was recrystallized from absolute ethanol.

The optimized structural parameters of E-^N¢ are calculated using B3LYP/6-311++G(d,p) basis set and listed in Table 1. The optimized structure is shown in Figure 1. The title molecule consists of pyridine and indole ring fused by hydrazone linkage. In ICINH, the hydrazone linkage plays an important role. For the carbonyl (C21=O29) bond length in hydrazone link is calculated about 1.223Å using DFT and corresponding X-ray data value is 1.236Å. The bond length of N18-N19 is acted as bridge between the phenyl and pyridine ring and its bond length is calculated using DFT calculation at 1.368Å, while the corresponding X-rays value is 1.392Å. Similarly, the C21-N19/C16=N18 bond lengths are calculated as: 1.384 : DFT; 1.330Å : X-rays data/1.288 : DFT; 1.278Å : X-rays data, respectively. In the present study, the average value of C-C bond lengths in indole ring is 1.403Å (DFT). Bikas et al., ¹⁶, observed the bond angle of O-C-C at 120.6˚, which was in consistent with calculated value 121.49˚ (O29-C21-C22) and its corresponding DFT value is positively deviated (~ 1˚) from the calculated value. The dihedral angles are also calculated and listed in Table 1.

Figure 1.The optimized molecular structure of ICINH

The harmonic vibrational frequencies were calculated using B3LYP/6-311++G(d,p) basis set. The title molecule belongs to C1 point group symmetry and it had 90 vibrational normal modes of same symmetry species are listed in Table 2.1. The internal and symmetry coordinates of ICINH are listed in Table 2.1 and Table 2.2, respectively. The percentage of PED obtained from MOLVIB program package was used for assigning vibrational peaks. The exclusion of anharmonicity factor and the level of basis set used, certain theoretical frequencies are not matched with that of the experimental values. Hence linear scaling procedure is adopted to scale down the frequency values. In this study, we have followed scaling factor of 0.968 for DFT ¹⁷. The observed FT-IR, FT-Raman and simulated spectra of ICINH are shown in Figure 2 and Figure 3, respectively.

Figure 2.The combined theoretical and experimental FT-IR spectra of ICINH

Figure 3.The combined theoretical and experimental FT-Raman spectra of ICINH

The ring stretching vibrations are very prominent in the spectrum of pyridine and its derivatives and are highly characteristics of aromatic ring itself ¹⁸. The C−C stretching vibrations of pyridine derivatives usually appear in the region between 1650–1400 cm−1 and 1100−1000 cm−1 ¹⁹. In the present study, the peaks identified at 1006 cm−1 in FT-IR and 1009 cm−1 in FT-Raman are assigned to C−C stretching vibrations. Due to the absence of peaks, the theoretically scaled values at 1553, 1318 and 1025 cm−1 are assigned as C−C stretching vibrations of the rest of other modes in pyridine ring .The presence of nitrogen in the ring of the pyridine structure gives rise to two C−N stretching vibrations. Identifying these vibrations is rather a difficult task as their vibrational frequency lies within the C−C stretching region. As expected, the peaks for C−N stretching vibrations are found at 1244 cm−1 in FT-IR and 1254 cm−1 in FT-Raman spectrum. The theoretically scaled values of this mode are found to be in good agreement with experimental values.

According to PED results, the C−C stretching vibrations of six and five members are assigned. The C−C stretching vibrations are observed at 1608, 1501 and 1336 cm−1 in FT-IR and 1606, 1563, 1345 and 1284 cm−1 in FT-Raman spectrum. The theoretically predicted scaled values are in excellent correlation with that of the experimental values.

Four C−H bonds in the pyridine ring of the title molecule give rise to four C−H stretching vibrations. The hetero-aromatic structure shows the presence of C−H stretching vibrations in the region 3000−3100 cm−1 ²⁰, which is the characteristic region for ready identification of this structure. In this region, the bands are not affected appreciably by the nature of the substituents. In the present study, the peak at 3107 cm−1 in FT-IR and at 3109 cm−1 in FT-Raman are assigned to C−H stretching vibrations of the pyridine ring. The percentage of PED results show that these modes are very pure modes. Similarly, the peaks appeared at 3159 and 3078 cm−1 in FT-IR are ascribed to C−H stretching vibrations of the benzene ring of the title molecule.

The characteristic IR absorption wavenumber of C=O is normally strong in intensity and found in the region 1600–1800 cm−1 ^{21, 22}. In the present study, the strong peak at 1677 cm−1 in FT-IR is assigned to C=O stretching. The calculated value of C=O stretching mode at B3LYP/6-311++G(d,p) shows good agreement with the experimental value. The C=O in-plane bending vibration is identified at 830 cm−1 in FT-IR. A peak for C=O out-of-plane bending vibration is not active in both IR and Raman spectra. Hence the theoretically predicted value (128 cm-1/mode no: 6) is assigned to this mode.

In primary amines, usually the N–H stretching vibrations occur in the region 3600–3300 cm−1 ²³. In the present study, the N–H bond in the five member ring produce a well defined peak at 3531 cm−1 in FT-IR. A peak for the N19−H20 bond present at the out-of-the ring is not active in both IR and Raman. Hence, the theoretical scaled value of 3540 cm−1 is assigned to N15−H13 stretching mode. All the vibrations in this group are in excellent agreement with experimental results as well as with literature ²⁴.

The identification of C=N stretching vibrations are difficult task since these are usually coupled with ring stretching and C-H in-plane bending vibrations. A bond C21=N19 at out of the ring possesses three vibrational normal modes. Since all these vibrations are inactive in both the spectra, the theoretically predicted wavenumbers 1604, 849 and 535 cm−1 are ascribed to C=N stretching, in-plane bending and out-of-plane bending, respectively. Similarly, the N−N stretching and bending vibrations are not present in both IR and Raman. Hence, the theoretically scaled values of 1074, 647 and 166 cm−1 are attributed to N−N stretching, in-plane bending and out-of-plane bending vibrations, respectively.

Non linear effect arise from the interactions of electromagnetic fields in various media to produce new fields altered in phase, frequency, amplitude or other propagation characteristics from incident fields ²⁵. The first hyperpolarizability (β0), dipole moment μ and polarizability α are calculated using DFT/6-311++G(d,p) basis set. The computed total static dipole moment (μ), the mean polarizability (α0) the mean first hyperpolarizability (β0), for the molecule under study are presented in Table 3 shows that the first order hyperpolarizability value play an important role in determining the NLO activity of the molecule. The first order hyper polarizability (β0) of the present molecule is 8.6424x10-30 esu while that of urea is 0.3728x10-30 esu. The (βo) of ICINH is 23 times greater than that of urea, hence, the molecule can be said to be highly NLO activity; which is naturally due to the contribution of oxygen atom which makes one part of molecule highly negative and other part as equally positive.

The NBO analysis is performed on ICINH using B3LYP/6-311++G(d,p) basis set and are listed in Table 4. In the study, the π bonds have higher ED than the σ bonds. Due to this reason, the σ-σ* transitions have minimum delocalization energy than the π-π* transitions. It is evident from the Table 4. The ED of π(C30-N32) bond transfer energy 116.4, 52.38 kJ/mol to the acceptor orbitals: C22-C24 and C23-C25, respectively. There occurs a strong intra-molecular hyperconjugative interaction of π electron from C23-C25 bond to the π*C22-C24→C30-N32 bonds, which increases the EDs: 0.3281 and 0.3727 kJ/mol, respectively. The lone pair electrons are readily available for the interaction with excited electrons of antibonding orbital. During n-π* transition, more energy delocalization takes place: N19→C16-N18; C21→O29 and N15→C1-C2; C7-C8. The corresponding excitation energy values are, 102.42, 170.41 and 140.71, 160.41, respectively. In which, these (N19→C21-O29 & N15→C7-C8) two transitions give the strongest stabilization to the system.

The HOMO–LUMO plot of ICINH molecule is shown in Figure 4 In HOMO diagram, the colored portions indicate the prominent donor levels which contribute in the electronic transitions and similarly the LUMO diagram indicates the prominent acceptors level through colored shades which involve in the electronic transitions. Homo localized in the indole and carbonyl group. The LUMO is located over the pyridine and hydrazone linkage. The molecule ICINH has lower energy gap and hence the probability of π-π* proton transition is highly possible in between HOMO and LUMO orbitals. The HOMO/LUMO energies are calculated using B3LYP/6-311++G(d,p) level.

Figure 4.The frontier molecular orbitals of ICINH

By using HOMO and LUMO energy value ICINH, the global chemical reactivity descriptors such as hardness (η), chemical potential (μ), softness (S), electronegativity (χ) and electrophilicity index (ω) have been calculated and are listed in Table 5. It can be expressed through HOMO and LUMO orbital energies as I=-EHOMO and A=-ELOMO, the electron affinity I and Ionization potential A of title molecule ICINH are also calculated by using B3LYP/6-311++G(d,p) basis set. The calculated values of the softness, hardness, chemical potential electronegativity and electrophilicity index, Homo, Lumo and energy gap of the molecule are: 4.341 eV, 3.954 eV, 3.954 eV and 1.801 eV, -6.125 eV, -1.784 eV and 4.341 eV, respectively. The soft molecule has small energy gap and hard molecule has large energy gap. In addition, the frontier molecular orbital energies are also calculated using the same basis set and are listed in Table 6.

The UV absorption spectrum for ICINH is recorded in the range 200-800 nm. All the structures allow strong π-π* and σ-σ* transition in the UV-Vis region with high extinction coefficients. The calculated results involving in the vertical excitation energies, oscillator strength (f) and wavelength are carried out and compare with measured experimental wavelength. Typically, according to Frank-Condon principle, the maximum absorption peaks (λmax) in a UV-Vis spectrum correspond to vertical excitation. The λmax of ICINH molecule are calculated using TD-DFT/6-311++G(d,p) basis set. It is evident from the Table 7, the possible π-π* transitions, with absorption maximum at 413.14, 389.2, 357.32 nm, belong to gas phase and the oscillator strength for respective transitions are 0.0009, 0.0136 and 0.0296, respectively. The calculated absorption maxima has been found to be 357.32 nm which is moderately coincides with the experimental value 342.42 nm. The combined theoretical and experimental UV-Vis absorption spectra are shown in Figure 5.

Figure 5.The combined theoretical and experimental UV-Visible spectra of ICINH

It is well known that the atomic charges are very much dependent on how the atoms are defined. It also plays an important role in the application of quantum chemical calculation to molecular system because of atomic charges affect the dipole moment, molecular polarizability, electronic structure and a lot of properties of molecular systems. The mulliken charges calculated at B3LYP/6-311++G(d,p) basis set for the molecule under study are given in Table 8. The mulliken atomic charges plot for ICINH is shown in Figure 7

The C2 atom has high positive charge which is due to the indole group. Similarly the C16 atom has high negative charge due to the attachment of C16=N18 group. All the hydrogen atoms have positive charge.

Figure 7.The Mulliken atomic charges of ICINH

On the basis of vibrational analysis, the standard statistical thermodynamic functions: heat capacity (C), entropy (S) and enthalpy changes (H) for the title molecule are obtained from the theoretical harmonic frequencies and are listed in Table 9. From Table 10, it can be observed that these thermodynamic functions are increasing with temperature ranging from 100 to 1000 K. The obvious reason for this is almost linear increase, and this is due to the increase in internal energy of the molecule in accordance with kinetic theory of gases due to the fact that the molecular vibrational intensities increase with temperature ³⁰. The correlation equations between heat capacities, entropies, enthalpy, changes and temperatures are fitted by quadratic formulas and the corresponding fitting factors (R2) for these thermodynamic properties are 0.99905, 0.9994 and 0.99998, respectively and the correlation graphics are shown in Figure 8.

Figure 8.The thermodynamic properties of ICINH at different temperatures

They can be used to compute the other thermodynamic energies according to relationships of thermodynamic functions and estimate directions of chemical reactions according to the second law of thermodynamics in thermo chemical field ³¹. In this study all thermodynamic calculations are done in gas phase and they could not be used in solution.

C0p,m = 6.76926 + 0.02858T + 2.52495x10-5 T2 (R2 = 0.99905)

S0m = 1.40875 + 0.00595T + 5.25468x10-5 T2 (R2 = 0.99998)

ΔH0m = 4.06057 + 0.01714T + 1.51461x10-5 T2 (R2 = 0.9994)