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Keywords

DFT Study; FT-IR; FT-Raman; PED; NMR; NLO

Introduction

Schiff base compounds have been derived from aromatic amines and aromatic aldehydes. Schiff-base compounds exhibit a wide range of biological activities [1], anti-HIV [2], anti-tumor activities [3] and the transition metal complexes [4,5]. Very often it undergoes a variety of chemical reactions, being as a key precursor for new compounds exhibiting diverse molecular structures and properties [6,7]. It is worth mentioning that the salicylaldehyde moiety appears in many compounds exhibiting various biological activity, including reactants used in the design of compounds exhibiting anti-viral activity [8], as well as in reactions resulting in new compounds with anti-cancer [9,10] or anti-microbial activity [11]. It is also present during the synthesis of new products called “aspirin-like molecules” exhibiting antiinflammatory activity [12].

The computational techniques are recently very useful in exploring nonlinear properties theoretically in useful manner. The simultaneous researches in the field of nonlinear materials have influenced the development of efficiencies of organic molecules as NLO material, since organic NLO materials often employ materials and device structures that have direct application to light multiplication.

In our present investigation we have focused on a simple organic molecule (E)-1-(4-methylbenzylidene) urea. We planned to investigate the nonlinear property of that molecule theoretically using first hyperpolarizability calculation. In addition to that, the vibrational spectral studies, NLO activity, NBO analysis, MEP surface and energy gap of the molecule are studied and discussed in detail.

Experimental Details

Synthesis of (E)-1-(4-Methylbenzylidene)urea

Equimolar amount of 4-methylbenzaldehyde and urea were dissolved in 30 ml of absolute ethanol. The mixture was shaken to make homogenous solution. Few drops of catalyst acetic acid were added to increase the rate of reaction. The content was refluxed at 90°C for 3 hours. The completion of the reaction was monitored by thin layer chromatography. After the reaction was completed, the content was cooled the mixture was poured into water. The solid product obtained was filtered and purified using absolute ethanol

Spectral measurements

FT-IR, FT-Raman and NMR Spectrum: The FT-IR spectrum of the synthesized EMBU molecule was measured in the Mid- IR (4000–400 cm^-1) region at the spectral resolution of 4 cm^-1 using on SHIMADZU FT-IR affinity Spectrophotometer (KBr pellet technique) in Faculty of Marine Biology, Annamalai University, Parangipettai. The FT-Raman spectrum was recorded on BRUKER: RFS27 spectrometer operating at laser 100mW in the spectral range of 4000–50 cm^-1. FT-Raman spectral measurements were carried out from SAIF, IIT, Chennai. NMR spectral studies were carried out using Bruker 400 MHz spectrometer, using TMS as an internal standard and DMSO-d6 as solvent and recorded at Annamalai University, Annamalainagar, Chidambaram. The 1H and 13C-NMR spectrum were shown in Figure 1.

Figure 1: The ¹H and ¹³C-NMR spectra of EMBU.

Computational Details

The quantum chemical calculations were performed at DFT method with B3LYP/6-31G(d,p) basis set using Gaussian 03W [13] program package, invoking gradient geometry optimization [13,14]. The optimized structural parameters were used in the vibrational frequency calculations at the DFT level to characterize all the stationary points as minima. The vibrational modes were assigned on the basis of PED analysis using VEDA4 program package [15]. NBO analysis was done by using NBO 3.1 program.

The Raman activity was calculated by using Gaussian 03W package and the activity was transformed into Raman intensity using Raint program [16] by the expression:

(1)

Where Ii is the Raman intensity, RAi is the Raman scattering activities, νi is the wavenumber of the normal modes and ν₀ denotes the wavenumber of the excitation laser [17].

Results and Discussion

Molecular geometry

The Optimized structure of EMBU molecule is shown in Figure 2 with atom numbering scheme. The bond parameters such as bond lengths, bond angles and dihedral angles values are calculated by DFT/B3LYP/6-31G(d,p) basis set and are listed in Table 1. In this case, the bond length of C-C and C-H are calculated around 1.40 and 1.08 Å, respectively. The bond lengths of related molecule was observed at C-C and C-H values are 1.38, 1.09 Å, respectively [18]. The carbonyl group bond C₁₃=O₁₈ is appeared as π bond character; its bond length value is calculated about 1.2182 Å. Similarly, the π bond character of C=O is observed at 1.212 Å [19], which nearly coincides with calculated value. The σ and π bond characters of N₁₂-C₁₃ and C₁₁=N₁₂ are calculated as 1.4341 and 1.281Å are positively deviated with related molecule data (1.348 Å and 1.273 Å) data [19].

Figure 2: The optimized molecular structure of (E)-1-(4- methylbenzylidene)urea (EMBU).

Table 1: The optimized bond parameters of EMBU.

The bond angles of N₁₂-C₁₃-O₁₈ and N₁₄-C₁₃=O₁₈ is calculated about 126.06 and 123.77°, respectively. Similarly, the reported molecule calculated the bond angle value of N-C=O is 123.36°, which nearly matches with calculated values [18]. These results fairly explore that, during the course of rotation of the bond to the entire molecule gets disturbed and the properties of the molecule also changes. From the literature [20] and our theoretical investigation, the optimized structure is more stable.

Vibrational assignments

The EMBU molecule belongs to C₁ point group symmetry. It consists of 22 atoms and 60 normal modes of vibrations are distributed among the symmetry species as;

Γ3N-6 = 41′ (in-plane) + 19′′ (Out-of-plane)

All these modes are found to be are active both in the IR and Raman absorption. The harmonic wavenumbers are calculated using B3LYP/6-31G(d,p) basis set and compared with recorded vibrational frequencies of FT-IR and FT-Raman spectra, respectively. Some discrepancies could be identified in between harmonic and observed frequencies, which are scaled down by proper scale factor [21,22]. The vibrational assignments are given in Table 2. The combined recorded and theoretical spectra are shown in Figures 3 and 4.

ν: Stretching, β: in-plane-bending, Γ: out-of-plane bending, τ: Torsion,
^aScaling factor: 0.9620,
^bRelative IR absorption intensities normalized with highest peak absorption equal to 100,
^cRelative Raman intensities normalized to 100,
^dPotential energy distribution calculated at B3LYP/6-31G(d,p) level.

Table 2: The fundamental vibrational assignments of EMBU.

Figure 3: The combined Theoretical and Experimental IR spectra of EMBU.

Figure 4: The combined Theoretical and Experimental FTRaman spectra of EMBU.

C=O Vibrations: The C=O stretching vibrations has been most extensively studied by IR and Raman 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 oxygen atom also determines the nature of the carbonyl group. In this study, the weak bands observed at 1672 and 1648 cm^-1 in FTIR and FT-Raman spectra are assigned to C=O stretching vibration and its corresponding harmonic value lies at 1692 cm^-1 (mode no: 11). This vibrational assignment is further supported by PED 88%.

C-C Vibrations: Generally, the phenyl ring carbon-carbon (C-C) stretching vibrations are observed in the region of 1625-1590, 1590-1575, 1540-1470, 1465-1430 and 1380-1280 cm^-1 by Varsanyi (1974) [24]. In the present study, the FT-Raman bands observed at 1539 and FTIR band observed at 1598 cm^-1 and are assigned to C-C stretching vibrations and their corresponding harmonic values appeared in the range of 1583 and 1541 cm^-1 (mode nos: 13 and 14). These assignments are good agreement with literature [25].

The C-C in-plane bending vibrations observed at 1174 cm^-1 in Raman and 613 cm^-1 in IR spectrum and its corresponding calculated value appeared at 1182 and 623 cm^-1 (mode no: 26 and 44). The ?C₄C₃C₅H₉ mode predicted at 962 cm^-1 (mode no: 34) is in agreement with IR band observed at 950 cm^-1 with the help of PED 81%.

C-H Vibrations: In General, the CH stretching modes are expected to occur in the range 2900-3200 cm−1 [26,27]. In our study, C-H vibrations are observed at 3061, 2987 and 2929 cm^-1 in FTIR spectrum. The corresponding harmonic frequencies lies at 3056, 2989 and 2936 cm^-1 (mode nos: 4, 7 and 9) are belong to the same mode. These results were good agreement with experimental value with PED >95%. In benzene like molecule the C-H inplane bending vibrations are appeared in the region 1000-1300 cm−1 and are usually weak intense. In our study, the frequency of the βCH vibrations are calculated in the region of 1438-1290 cm^-1 (mode nos: 16 and 22) for this molecule. These modes are observed in the FTIR: 1298 cm^-1/FT-Raman: 1468 cm^-1 spectra with weak intensity. The harmonic frequencies in the range 962 cm^-1 (mode nos: 34) and FTIR band observed at 950 cm^-1 assigned to ΓCH mode. These assignments are find support from PED.

N-H Vibrations: In general, the N-H stretching vibrations observed in the region 3400-3200 cm^-1 [28]. In the present case, the N-H band assigned at 3460 cm^-1 (mode no: 2). This assignment is further justified on the basis of their calculated PED value (100 %). The calculated wavenumber for β_N-H (1534 cm^-1/ mode no: 15) and Γ_N-H (585 cm^-1/ mode no: 45) modes well reproduced the experimental ones in FT-IR (1506 cm^-1) and FT-Raman (546 cm^-1) spectra with PED values (83% and 93%), respectively.

C-N Vibrations: The assignment of C=N stretching frequency is a rather difficult task since there are problems in identifying these frequencies from other vibrations. The ring C–N vibrations are appeared in the region 1650–1550 cm−1 [29]. The bands obtained at 1598 cm^-1 in FT-IR and at 1539 cm^-1 in FT-Raman spectra have been assigned to C=N stretching vibrations of title molecule. The corresponding theoretical wavenumber lies at 1583 and 1541 cm^-1, which is comparable to experimental wavenumber with PED contribution ≥ 45%. Silverstein et al., [30] assigned C–N stretching vibrations occurred in the region 1382–1266 cm^-1 for the aromatic amines. The C-N stretching vibrations of title molecule were obtained at 1298 cm^-1 in FT-IR spectrum. The band calculated at 1290 cm^-1 from DFT is assigned to C-N stretching vibration of the present molecule.

NLO property

In this study, the electronic dipole moment, molecular polarizability, anisotropy of polarizability and molecular first hyperpolarizability of EMBU were calculated at DFT/B3LYP/6- 31G(d,p) basis set and are presented in Table 3. It is well-known that the higher values of dipole moment, molecular polarizability, and hyperpolarizability which enhances the NLO property. Urea is one of the prototypical molecule used in the study of the NLO property of molecular systems, and thus, it was used frequently as a threshold value for comparative purposes. The first hyperpolarizability value of EMBU was calculated as 10.595 × 10−30 esu. According to these result, the β₀ value of present molecule is twenty-eight times larger than the magnitude of urea, which implies that the title molecule might become a kind of good NLO material.

Standard value for urea (μ=1.3732 Debye, β₀=0.3728x10^-30esu)

Table 3: The NLO measurements of EMBU.

NBO analysis

The NBO analysis provides an efficient method for studying intraand inter-molecular bonding and interaction among bonds, and also provides a convenient basis for investigating charge transfer or conjugative interaction in molecular systems [31].

In this present study, the NBO analysis has been carried out with DFT/B3LYP/6-31G(d,p) level of basis set and which deals the intra-molecular charge transfer within the molecule. In any molecule, the π character of the bond plays an important role when compare with σ bond character. In such a way that this molecule delivers maximum delocalization energy during the transition between π and π* bond whereas the ED of the donor (Lewis) bond decreases with increasing of ED of acceptor (Non- Lewis) bonds. In our case, the conjugative π bonds in the phenyl ring shows maximum delocalization during the interaction with π* acceptor bonds. It is evident from our title compound that the energy transfer from πC₁-C₆ to π*C₂-C₃, πC₄-C₅ to π*C₁-C₆ and πC₂-C₃ to π*C₁₁-N₁₂ are reveals the hyperconjucative energy about 97.74, 92.51 and 86.69 KJ/mol, respectively. Similarly, the lone pair atoms such as oxygen and nitrogen also transfer more energy to donor and acceptor bonds. The LP(1)N₁₄ to C₁₃-O₁₈ and LP(2)O18 to N₁₂-C₁₃ bonds transfer the energy about 252.71 and 106.78 KJ/mol, respectively and are listed in Table 4. The maximum hyperconjugative E⁽²⁾ energy of lone pair atoms during the intra-molecular interaction, which leads the molecule towards medicinal and biological applications.

*E⁽²⁾ means energy of hyperconjucative interactions,
LP = Lone pair (non-bonding molecular orbital)

Table 4: The NBO analysis of EMBU.

HOMO-LUMO analysis

The HOMO and the 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 EMBU are as shown in Figure 5. The calculated values of the HOMO and LUMO energies and band gap energy are listed in Table 5. The positive and negative phases are represented in green and red color, respectively. HOMO represents the electrondonating 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. In the present study, the energy gap have been calculated using B3LYP/6-31G(d,p) level are 4.76415 eV. This small energy gap is associated with high chemical reactivity and low kinetic stability.

Figure 5: The frontier molecular orbitals of EMBU.

Table 5: The frontier molecular orbitals of EMBU.

MEP analysis

The MEP surface map was calculated at B3LYP/6-31G(d,p) method. MEP is related to the ED. It is very useful descriptor in understanding the sites for electrophilic attacks and nuleophilic reactions as well as hydrogen bonding interactions [32]. The importance of MEP lies in the fact that it simultaneously displays molecular size, shape as well as positive, negative and neutral electrostatic potential regions in terms of colour grading as shown in Figure 6. Potential increases in the order red < orange < yellow < green < blue. The colour code of these maps, where blue indicates the strongest interaction and red indicates the strongest repulsion. In this study, the negative region is located over the carbonyl group and the positive region is located over Hydrogen atom in the imine linkage.

Figure 6: The MEP surface of EMBU.

Mulliken charges

Mulliken populations can be used to characterize the electronic charge distribution in a molecule and the bonding, anti-bonding, or non-bonding nature of the MOs for pair of atoms [33]. Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation to molecular system. The total atomic charges of EMBU are calculated by Mulliken population analysis with DFT method B3LYP/6-31G(d,p) basis set are listed in Table 6. The Mulliken atomic charge plot for EMBU is plotted in Figure 7. The most positive charges calculated at C_3/0.636162 and C_6/0.651105 in the present molecule. The C₆ atom has high positive charge due to the attachment of methyl group. Similarly, the negative charges were calculated at C_1/-0.764424 and C_19/-0.612242, respectively. From the results, C₁ atom shows the higher negative charge due to the presence aromatic ring. The nitrogen and oxygen atoms also be a negative charge such as N_12/-0.118445, N_{14/- 0.393447} and O_18/-0.362573, respectively.

Table 6: The Mulliken atomic charges of EMBU.

Figure 7: The Mulliken atomic charges plot of EMBU.

Thermodynamic properties

The standard thermodynamic functions: heat capacity (C⁰ _p,m), entropy (S⁰ _m) and enthalpy changes (H⁰ _m) for the EMBU were calculated using DFT method at B3LYP/6-31G(d,p) basis set and are listed in Table 7. 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 [34]. The correlation equations among thermodynamic functions due to the temperature were fitted by quadratic, linear and quadratic formula and the corresponding fitting factors (R2) 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 Figure 8.

Table 7: Thermodynamic properties of EMBU at different temperatures.

Figure 8: The thermodynamic properties of EMBU at different temperatures.

C⁰_p,m = -1.73055 + 0.1542T – 6.3237x10^-5 T² (R² = 0.99957)

S⁰_m = 3.06457 + 0.13745T – 1.83581x10^-5 T² (R² = 0.99997)

ΔH⁰_m = 107.82213 + 0.00557T + 1.88572x10^-5 T² (R² = 0.99971)

All the thermodynamic data are supportive information for further study on the molecule.

Conclusion

The quantum chemical calculations were performed at DFT method for the first time to the EMBU molecule. The bond parameters (bond lengths, bond angles and dihedral angles) are agree well with the related molecule XRD data. The observed and calculated wavenumbers of EMBU are well supported by the literature values. The hyperpolarizability (β0) value of EMBU molecule is calculated about 10.595x10-30 esu, which is twentyeight times greater than that of standard urea. The NBO analysis reveals that, the charge transfer occur within the molecule and the maximum energy takes place during π-π* transition. The energy gap of EMBU molecule is calculated at 4.76415 eV, which leads the title molecule to become less stable and more reactive. The reactive sites of EMBU molecule is predicted by MEP surface. In addition, Mulliken atomic charges and thermodynamic properties are also calculated and analyzed.

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