Molecular, Electronic, Nonlinear Optical and Spectroscopic Analysis of Heterocyclic 3-Substituted-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones: Experiment and DFT Calculations

Geometric Optimization

The three-dimensional approximate geometry of the 3-substituted-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones (1) are plotted in Gauss View 5.0 program [28] (Figure 1). Using these geometric structures, Gaussian 09W was used to determine the most stable positions of each atom in space. The minimum energy space structure of the most optimized compounds was analyzed with the 6-311G (d, p) basis set over polarized functions with B3LYP/DFT and B3PW91/DFT methods [29, 30].

Figure 1

The optimized gas-phase molecules at DFT theoretical level using 6-311G(d,p) basis set. 1a: 3-p-methoxybenzyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one, 1b: 3-m-chlorobenzyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one, 1c: 3-phenyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one

The C-C bond lengths of the C₃ bonded aryl groups of the different analogues of 1 were compared according to the Ikizler, based on the optimized structures [31]. According to the Ikizler, C-C bond lengths in the benzene ring have been observed as 1.397 Å and C-H bonds as 1.084 Å [31]. The average of C-C bond lengths in the thiophene ring in the structure of type S compounds was found to be 1.392 Å and 1.390 Å, according to the B3LYP/6-311G(d,p) and B3PW91/6-311G(d,p) methods, respectively. When the theoretical values were compared with values given according to the Ikizler [31], value obtained with B3LYP method was observed to be closer to the literature [31]. The average of C-S bond lengths in the synthesized compounds were found to be 1.741 Å according to B3LYP method and 1.730 Å according to B3PW91 method (Table 1).

The average C-C bond length values of the C₁-linked benzene ring in the triazole ring in the structure of type 1 compounds were found to be 1.392 Å, according to B3LYP/6-311G(d,p) and B3PW91/6-311G(d,p) methods. It was observed that the B3LYP method was 1.084 Å, and the B3PW91method was 1.085 Å, when the C-H bond lengths in the benzene ring were examined (Table 2), According to B3LYP method, the obtained value was found to be the same as that of the literature and the obtained theoretical data were confirmed against the values according to the Ikizler [31].

According to the Ikizler, the experimental C-N length was 1.49 Å and C=N length was 1.27 Å [31]. The results obtained were observed to be 1.368 Å in the B3LYP/6-311G (d, p) method and 1.365 Å, according to the B3PW91/6-311G (d, p) method (Table 3). The average bond lengths observed with the B3LYP and B3PW91 methods were experimentally determined to be between the suggested C-N single bond and C=N double bond lengths. Therefore, it has been observed that the C-NH bond has a partial double bond property in the 1,2,4-triazole-5-on ring.

Electronic Properties

LUMO (π acceptor) and HOMO (π donor) are successively called to be the lowest unoccupied molecular orbital and the highest occupied molecular orbital. E_LUMO is the lowest energy of unmatched electrons and E_HOMO is the highest energy of matched electrons. HOMO and LUMO can offer an appropriate qualitative estimate of excitation properties and a molecule's electron carrying ability [13, 32]. HOMO and LUMO from frontier molecular orbitals play an important role in determining electrical and optical properties, which are the most important parameters of quantum chemistry. The transitions of selected frontier molecular orbitals in the gas phase are as shown in Figures 2 and 3, with positive and negative phases being indicated in red and green, respectively. The electronic properties of the synthesized titled compounds were obtained from the calculated HOMO and LUMO energies, by using B3LYP/6-311G(d,p) and B3PW91/6-311G(d,p) methods [18]. The results obtained are given in Table 4.

Figure 2

Frontier orbitals (HOMO–LUMO) views, corresponding energies and energy gap of titled compounds (1) according to B3LYP/6-311G(d,p) method

Figure 3

Frontier orbitals (HOMO–LUMO) views, corresponding energies and energy gap of titled compounds (1) according to B3PW91/6-311G(d,p) method

The electron distribution is quite variable and polarization is low, especially when the LUMO-HOMO gap is small. The electron distribution within the molecule is less variable and polarization is low when the energy gap is large. The molecules examined contain substrates such as p-methoxybenzyl (1a), m-chlorobenzyl (1b) and phenyl (1c) moieties bound to C₁ in the 1,2,4-triazole-5-on ring. When the donor and acceptor substituents examined the effect on structures, LUMO/HOMO energies differences of 1a, 1b and 1c molecules are calculated as 4.166/4.178, 4.182/4.195 and 4.199/4.129 eV according to DFT/B3LYP and DFT/B3PW91, respectively (Table 4). It was found that the greater the energy gap of the molecule, the higher the intramolecular charge density. The energy gap in the studied molecules is 1b > 1a > 1c in the B3PW91/6-311G(d,p) set. Therefore, the 1b molecule with an electron-donating substituent in the ring has the highest energy gap, within the substituents explored.

Nonlinear optical Features

Polarizability and hyperpolarizability provide useful information for frequency changing, optical modulation, optical switching and optical logic for technologies evolving in areas such as non-linear optical (NLO) activity, communication, signal processing, and optical interconnection [42]. Organic materials are expected to have relatively strong NLO properties, due to the delocalized electrons in the π->π^* orbitals [43].

The first hyperpolarizability (β₀) of the Schiff base molecular systems under consideration is calculated using the DFT method based on the finite-field approach. The first hyperpolarizability is a third-grade tensor that can be defined by a 3×3×3 matrix. 27 components of the 3D matrix can be reduced to 10 components due to Kleinman symmetry. The components of β are defined as coefficients in the expansion of energy in the external electric field in the Taylor series energy. When the electric field is weak and homogeneous, expansion occurs.

Where E₀ is the energy of the free molecule, Fⁱ is the area in origin; μ_i, μ_ij, β_ijk and γ_ijkl are components of the dipole moment, polarizability, first hyperpolarization and second hyperpolarizability.

Total static dipole moments (μ_tot), average polariz-ability (α₀), anisotropy (α) and average first hyperpolarizability values of polarizations (β) were determined according to Zhang et al., using the X, Y and Z components [44]. Total static dipole moment calculation equation;

Isotropic polarization calculation equation

The average calculation equation availability hyperpolarized

The NLO properties of the molecules were calculated with the above equations using the basis sets B3LYP/6-311G (d, p) and B3PW91/6-311G (d, p). Total static dipole moment, polarizability and first order hyperpolarizability are given in Table 5. The data obtained were compared with the reported values of similar derivatives reported by Binil et al. [45]. The related compounds were compared to urea, referenced as a NLO material (urea: 0.3728 × 10⁻³⁰ esu), according to Adant et al. [46]. The calculated hyperpolarizability of 1 analogues appears to be approximately 10 times higher than the urea value, a noted significant increase.

Molecular Electrostatic Potential Analysis

Molecular electrostatic potential (MEP), which is related to electron density proves to be useful in understanding the regions of electrophilic and nucleophilic reactions [47]. Electrostatic potential is also well suited to analyzing processes based on the “recognition” of one molecule by another, such as drug-receptor and enzyme-substrate interactions [48]. Molecular electrostatic potentials were calculated in optimized geometry with the B3LYP and B3PW91 methods and the basis set of 6-311G (d,p) to estimate the reactive regions of electrophilic and nucleophilic attacks for the studied molecules. Different values of the electrostatic potential on the surface are indicated by different colors. Potential increases are listed as red < orange < yellow < green < blue. On the molecular electrostatic potential, negative regions (red and yellow) are associated with electrophilic reactivity, and positive regions (blue) are associated with nucleophilic reactivity (Figure 4) [49]. It appears that the negative charge covers the carbonyl group and the positive region is above the remaining groups. The highest electronegativity is located in the carbonyl group, the most reactive parts of the molecules are therefore elsewhere (1).

Figure 4

Molecular electrostatic potentials of 1 type compounds according to B3PW91 method

UV-vis Spectral Analysis

UV-vis absorption spectra of analogues of 1 were obtained in ethanol (Figure 5). Calculations were obtained with TD-DFT/B3LYP and TD-DFT/B3PW91 methods and 6-311G(d,p) polarized set based on optimized structure. The calculated absorption wavelengths (λ), oscillator power (f) and excitation energies are shown in Table 6 in the ethanol solvent phase. The stronger the donor character of the substitution in the molecules, the more electrons pushed into the molecule and the greater the λ_max. These values may change slightly due to the effect of a given solvent. The role of the substrate and solvent effect acts on the UV spectrum too.

Figure 5

Theoretically generated (DFT/B3LYP, DFT/B3PW91 and experimental) UV-vis spectra graphics of 1 type compounds, respectively

Three absorption bands were seen in the theoretically obtained electronic spectrum of the synthesized compounds (1a–c) in ethanol (Figure 5). The calculated absorption wavelengths were determined to be close to experimental values (Table 6).

The absorption bands below 300 nm belong to the π->π* transitions in the benzene ring and azomethine group. Absorption bands between 300–400 nm are due to n->π* transitions of the imine group [50]. Gauss-Sum3.0 program was used to determine the stimulation contributions in UV-visible transitions (Table 7) [51]. According to B3LYP/B3PW91 for TD-DFT calculations, for 1a, the main transition contribution from HOMO to LUMO (H->L) (96%/96%) was determined as n->π* transitions at 324.82/323.58 nm and the main transition contribution from HOMO-1 to LUMO (H-1->L) (99%/99%) was determined as n->π* transitions at 318.39/316.43 nm. In addition, the main transition contribution from HOMO-2 to LUMO (H-2->L) (77%/80%) was determined at 281.96/280.58 nm and are observed in the benzene ring and in the azomethine group π>π* transitions. Secondly for 1b, the main transition contribution from H->L (96%/96%) was determined as n->π* transitions at 324.21/322.80 nm and In addition, the main transition contribution from H-1->L (77%/80%) was determined at 281.47/279.99 nm. The main transition contribution from H-4->L (58%/64%) and H-2->L (39%/34%) were determined as π->π* transitions at 318.39/316.43 nm. It is observed that these π>π* transitions are in the benzene ring and in the azomethine group. Finally, the 1c molecule, the main transition contribution from H->L (94%/94%) was determined as n->π* transitions at 331.14/330.09 nm and the main transition contribution from H-1->L (91%/90%) was determined as n->π* transitions at 303.43/301.66 nm. In addition, the main transition contribution from H->L+1 and (89%/40%) was determined at 280.90/279.88 nm are observed in the benzene ring and in the azomethine group π>π* transitions.

Infrared Spectral Analysis

Derivatives of 1 were calculated by B3LYP and B3PW91 methods, and 6-311G(d,p) polarized set of vibration frequencies in gas phase to generate infra-red spectral information. There are 3N–6 free vibrational motions, therefore the synthesized compounds are of planar and nonlinear structure. 1a–c consist of 39, 35 and 32 atoms respectively, and have 111, 99 and 90 normal modes of fundamental vibrations, respectively. The calculated FT-IR spectra were obtained from B3LYP and B3PW91 levels with 6-311G(d,p) set (Figure 6). Negative frequency was not found in the data obtained from the optimized structure. The vibrational frequencies obtained by Gaussian 09W are multiplied by 0.9516 for the B3LYP/6-311G(d,p) method and 0.9905 for the B3PW9/6-311G(d,p) method [29]. Veda4f program was used to determine the vibrational types obtained by both methods [52]. The experimental IR spectral values were compared with the theoretical IR spectral values and some functional group regions were analyzed experimentally and theoretically. The obtained data were made compatible with experimental data.

Figure 6

Theoretically generated (B3LYP and B3PW91) IR spectrums of 1 type compounds, respectively

The corresponding heterocyclic 1,2,4-triazole compounds have signals corresponding to N-H stretching vibrations. While NH stretching vibrations are observed in the range of 3160–3184 cm⁻¹ in the experimental data, theoretical signals were obtained using the B3LYP/6-311G(d,p) method in the range of 3641–3646 cm⁻¹ and for the B3PW91/6-311G(d,p) method in the range of 3515–3520 cm⁻¹. The carbonyl peaks in the 1,2,4-triazol-5-one ring were observed the range of 1695–1702 cm⁻¹ in the experimental data, whereas the theoretical ranges of 1782–1785 cm⁻¹ for B3LYP/6-311G(d,p) method and 1733–1735 cm⁻¹ for B3PW91/6-311G(d,p) method were obtained. As shown in Table 7, the peaks of the imine group in the Schiff base ring are observed in the experimental in the range of 1575–1590 cm⁻¹ [27], whereas the calculated values in the range of 1621–1632 cm⁻¹ for the B3LYP/6-311G(d,p) method and 1574–1584 cm⁻¹ for the B3PW91/6-311G(d,p) method were obtained. Experimental data [27] were found to be more compatible with the data obtained from B3PW91 when comparing vibrational frequencies obtained by both methods.

NMR Spectral Analysis

The isotropic chemical shift analysis allowed us to identify relative ionic species, and to calculate reliable magnetic properties in nuclear magnetic resonance (NMR) spectroscopy, providing accurate predictions of molecular geometries [53,54,55]. In the study, ¹³C-NMR and ¹H-NMR chemical shift values of 1a–c were obtained from optimized structures with minimum energy. Chemical shift values of 1a–c were obtained by using optimized structures, obtained from of B3LYP and B3PW91 methods, by using Gauge-Independent Atomic Orbital (GIAO) NMR using 6-311G(d,p) basis set in a DMSO solvent phase (Tables 8–10) [56].

¹³C-NMR and ¹H-NMR chemical shift values were calculated by regression analysis via analysing experimental data using the least squares method. The obtained R² values were found to be nearly 1, especially for ¹³C-NMR data (Figure 7).

Figure 7

Comparison of experimental data with theoretical ¹³C-NMR and ¹H-NMR chemical shift values obtained by B3LYP (DMSO) and B3PW91 (DMSO) methods of 1 type compounds, respectively

It is well known that aromatic carbon atoms give NMR signals in the range of 100–150 ppm. However, in coordination with electronegative atoms, these NMR signals resulting from aromatic carbon atoms shift to higher values [57, 58]. Experimentally and theoretically generated ¹³C-NMR and ¹H-NMR isotropic shift values were compared and a linear correlation was observed (Figure 6). Theoretical chemical shifts of 3-substituted-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one compounds were optimized to the most stable structure using the B3LYP and B3PW91 methods using the 6-311G(d,p) basis set. It has been found out that ¹³C-NMR chemical shift values are highly compatible between the GIAO-NMR approach and experimental data [27]. In the ¹H-NMR chemical shift values, it was determined that the R² value was lower than expected, since the N-H proton in the 1,2,4-triazole-5-on ring has an acidic value [59].

Experimental Method

In the study, 3-p-methoxybenzyl-4-(3-methyl-2-thieny lmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one (1a), 3-m-chlorobenzyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one (1b) and 3-phenyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one (1c) were obtained from reaction between 3-methylthiophene-2-carbaldehyde and three different 4-amino-3-p-methoxybenzyl/m-chlorobenzyl/phenyl-4,5-dihydro-1H-1,2,4-triazole-5-ones [27] (Scheme 1).

Scheme 1

Synthesis route of 1 type compounds

General procedure for the synthesis of 1 type compounds

3-methylthiophene-2-carboxialdehyde A (0.01 mol) was dissolved in acetic acid (15 mL) and reacted with the corresponding compounds T (0.01 mol) to 3-p-methoxybenzyl/3-m-chlorobenzyl/3-phenyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-ones (1a–c) and was refluxed for 1.5 hour. Then, the solution evaporated at 50–55 °C in vacuo. The residue was crystallized several times in ethanol and pure 1a–c compounds were obtained as white crystals.

3-p-Methoxybenzyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one (1a) Yield (white solid) 94%; IR (υ, cm⁻¹): 3161 (NH), 1702 (C=O), 1590 (C=N), 850 (1,4-disubstituted benzenoid ring); ¹H-NMR (400 MHz, DMSO-d₆): δ 11.93 (s, 1H, NH), 9.84 (s, 1H, N=CH), 7.71 (d, 1H, ArH; J=5.20 Hz), 7.24 (d, 2H, ArH; J=8.80 Hz), 7.02 (d, 1H, ArH; J=5.20 Hz), 6.85 (d, 2H, ArH; J=8.80 Hz), 3.89 (s, 2H, CH₂Ph), 3.70 (s, 3H, PhOCH₃), 2.32 (s, 3H, CH₃); ¹³C-NMR (100 MHz, DMSO-d₆): δ 151.14 (triazole-C₂), 147.33 (N=CH), 146.18 (triazole-C₁), 158.11, 142.87, 131.77, 131.22, 129.98, 129.93 (2C), 127.44, 113.84 (2C) (Ar-C), 55.01 (OCH₃), 30.28 (CH₂Ph), 13.72 (CH₃); mp 207 °C (dec).

3-m-Chlorobenzyl-4-(3-methyl-ı2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one (1b) Yield (white solid) 74%; IR (cm⁻¹): 3180 (NH), 1700 (C=O), 1579 (C=N), 788 and 622 (1,3-disubstituted benzenoid ring); ¹H-NMR (400 MHz, DMSO-d₆): δ 12.01 (s, 1H, NH), 9.85 (s, 1H, N=CH), 7.72 (d, 1H, ArH; J=5.20 Hz), 7.41 (s, 1H, ArH), 7.26–7.34 (m, 3H, ArH), 7.02 (d, 1H, ArH; J=4.80 Hz), 4.00 (s, 2H, CH₂Ph), 2.32 (s, 3H, CH₃); ¹³C-NMR (100 MHz, DMSO-d₆): δ 151.14 (triazole-C₂), 147.51 (N=CH), 145.33 (triazole-C₁), 143.00, 138.02, 132.95, 131.65, 131.23, 130.21, 130.06, 128.93, 127.62, 126.78 (Ar-C), 30.78 (CH₂Ph), 13.73 (CH₃); mp 165 °C (dec).

3-Phenyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one (1c) Yield (white solid) 97%; IR (υ, cm⁻¹): 3160 (NH), 1695 (C=O), 1575 (C=N), 766 and 686 (monosubstituted benzenoid ring); ¹H-NMR (400 MHz, DMSO-d₆): δ 12.40 (s, 1H, NH), 9.82 (s, 1H, N=CH), 7.94-7.92 (m, 2H, Ar-H), 7.71 (d, 1H, ArH; J=5.20 Hz), 7.52 (t, 3H, ArH; J=6.40 Hz), 7.05 (d, 1H, ArH; J=4.80 Hz), 2.38 (s, 3H, CH₃); ¹³C-NMR (100 MHz, DMSO-d₆): δ 151.55 (triazole-C₂), 150.23 (N=CH), 144.11 (triazol-C₁), 143.33, 131.60, 131.28, 130.27, 130.01, 128.39 (2C), 127.74 (2C), 126.66 (Ar-C), 13.77 (CH₃); mp 202 °C (dec).

Calculation Methods

Approximate geometry of three dimensions in Dennington et al., the gas phase and basis state molecules were recorded and drawn in GaussView5.0 molecular imaging software (Figure 8) [28]. The initial geometries of the molecules were obtained in GaussView 5.0 package software and transferred to Gaussian 09W software as input data [29, 30]. Many parameters such as geometric, spectroscopic, electronic and thermodynamic properties of molecules to be examined from the optimized structure can be analysed. The basis or excited states of compounds or atoms can be used in theoretical calculation processes [29, 30, 60]. All calculations were made on computers located in Chemistry Department of Kafkas University Science Faculty.

Figure 8

The optimized molecular structure of 3-benzyl/p-methylbenzy/p-chlorobenzyl-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazol-5-one (1) with DFT/B3LYP/6-311G(d,p) level

The ab-initio method is based on the analysis of the Schrödinger wave equation without experimental values [61]. It seems that the solution of my Schrödinger wave equation is possible with a single electron hydrogen atom. However, mathematical approaches such as DFT (density function theory) are used as it has been challenging to analyse in multi-electron structures. In an attempt to determine the electronic properties of the structures better, the DFT method was used which takes into account the electron density and generates the desired data on this electron density. In addition, the B3LYP hybrid function in the Gaussian 09W software has been applied suitable for workstation capacity and polarized 6-311G (d,p) basis set [62].

In this study, we were optimized using DFT/B3LYP and DFT/B3PW91 methods In order to find the minimum energy and the most stable structure of the synthesized molecules, the bond lengths of the related compounds were determined from the optimized geometric structure with minimum energy found. Vibration frequencies were calculated from the optimized structure of the molecules. Veda4f program [44] was used to determine the vibration types of the calculated IR frequencies by using computer-aided Gaussian 09W package program. The theoretically calculated vibration frequency values are multiplied by appropriate scale factors, and they are compared with the experimental values [27]. Theoretical IR spectra were drawn according to DFT/B3LYP and DFT/B3PW91 methods. Chemical shift values of ¹H-NMR and ¹³C-NMR were calculated according to GIAO method using optimized structure. The theoretically obtained chemical shift values were compared with the experimental values and it was observed that they were compatible. In addition, the 3-substituted-4-(3-methyl-2-thienylmethyleneamino)-4,5-dihydro-1H-1,2,4-triazole-5-ones calculated HOMO-LUMO energies, energy differences and Electronic parameters such as I; Ionization potential, A; electron affinity, ΔE; Energy Gap, χ; electronegativity, S; molecular softness, ω; Electrophilic Index, IP; Nucleophilic Index Pi, Chemical Potential derived from HOMO-LUMO energies and Total Energy.