Molecular structure simulation of (E)-2-(butan-2-ylidene) hydrazinecarbothioamide using the DFT approach, and antioxidant potential assessment of its complexes

3.1 Synthesis of 2-butanone thiosemicarbazone

The ligand was synthesized taking reference from a earlier published study with some alterations (Supplementary Fig. 1) (Khan et al., 2017; Kumar and Kumar, 2013).

4.1 Spectral characterization of the ligand

The ¹H NMR spectrum of the ligand had signals due to CH₃, CH₂, NH and NH₂ groups. Significant shifting in values confirmed the formation of complexes. In the proton spectrum, a triplet at δ 1.1, singlet at δ 1.9, quartet at δ 2.3 were obtained corresponding to CH₃, CH₃ and CH₂ groups respectively. Two small singlets at δ 4.9 and 7.69 were obtained for NH and NH₂ groups. The ¹³C spectrum of the ligand revealed signals at δ 10.70, 18.72, 31.62, 155.5 and 178.72 corresponding to CH₃, CH₃, CH₂, C⚌N and C⚌S groups. NH and NH₂ protons were most downfield. Mass spectrum of the ligand furnished a M⁺¹ peak at 146 corresponding to its molecular weight of 145. The IR spectrum of the ligand showed a strong band at 1619 cm⁻¹ related to the azomethine group, ν(C⚌N) (Tada et al., 2011). The strong bands at 1077 cm⁻¹ & 772 cm⁻¹ were assigned to ν(C⚌S) (Bindu and Kurup, 1997). λ_max of the ligand (267 nm) corresponded to π-π* transition (Misner et al., 2003).

5.1 Frontier molecular orbital analysis

The energy gap between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) mainly governs the molecular electrical transport properties (Silverstein et al., 1981). HOMO denotes the nucleophilicity, whereas the LUMO represents the electrophilicity. They are also referred to as frontier molecular orbitals (FMOs). The HOMO and LUMO of the ligand is presented in Fig. 1. The negative value of the energy gap indicated molecular stability. Lower HOMO energy value indicated that the molecule has low electron donating ability (Barakat et al., 2015). The energy gap also predicts the intramolecular charge transfer (Padmaja et al., 2009).

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Fig. 1

HOMO-LUMO plot of ligand in isolated (gaseous) phase.

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HOMO-LUMO energy gap:

HOMO = −5.3015 eV

LUMO = −0.6642 eV

HOMO-LUMO energy difference = −4.6373 eV

5.2 Optimized geometry parameters

The main aim of geometry optimization is to find a new low energy conformation of the molecule. The geometry optimization was done by B3LYP/6–31++G(d,p) method. The N4-C5 bond length was 1.287 Å indicating double bond character (Table 1) (Mary et al., 2008). Bond lengths of S1-C2 aand C2-C9 were 1.663 Å and 1.378 Å respectively indicating partial double bond character. Though the bond lengths were higher than that obtained for a pure double bond length (1.22 Å) suggesting multiple bond character (Politzer and Truhlar, 1981). The optimized structure of the ligand is presented as Supplementary Fig. 2.

5.3 Molecular electrostatic potential (MEP)

The electrophilic and nucleophilic sites on a molecule are predicted through MEP (Luque et al., 2000). The sites of negative potential are susceptible to nucleophilic attack whereas sites having positive potential are electrophilic. The positive electrostatic potential is shown by blue colour and depicts the area of low electron density. This is the state of repulsion of the proton by the nucleus. The negative electrostatic potential depicted by red colour shows the attraction felt by the proton towards the electron density. Potential increases in the order red < orange < yellow < green < blue. MEP diagram of the ligand (Fig. 2) showed the presence of positive region around hydrogen atoms, whereas the negative region was found around sulphur atom associated with its’ lone pair.

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Fig. 2

The MEP of Ligand, (a) 3D structure, (b) embedded 2D structure.

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The color range, in kcal/mol, is: red greater than 4.5, 4.5 > yellow > −11.0, −11.0 > green > −26.6 and blue < −26.6.

5.4 NMR spectral analysis (¹H NMR and FT-IR)

Experimental NMR spectrum of the ligand was obtained in DMSO. The calculated and experimental chemical shifts are given in Tables 2-3. The structure was optimized in Gaussian 09. The experimental values were obtained between 1.1 and 7.69 ppm whereas the calculated values were between 1.03 and 8.03 ppm. The highest deviation was obtained for H14, 15 viz. 0.3 ppm. The ¹³C chemical shifts were obtained in the range 10.70–178.72 ppm experimentally. The theoretical values were between 8.70 and 180.98 ppm. The highest variation was for C6 viz. 8.5 ppm at the B3LYP level. A larger deviation is obtained for ¹³C chemical shifts than for ¹H. ¹H and ¹³Ccorrelation coefficients (R²) were 0.9964 and 0.9974 respectively (Supplementary Fig. 3). A higher degree of correlation was obtained between ¹³CNMR experimental and calculated chemical shifts (Asadi et al., 2016).

5.5 Vibrational spectral analysis

The experimental and calculated (scaled as well as non-scaled) wave numbers were observed between 400 and 4000 cm⁻¹ and calculated PED are given in Table 3. The calculated and experimental IR spectra are given in Fig. 3. The correlation coefficient (R²) was 0.9984 showing good agreement between the calculated and experimental values (Supplementary Fig. 4). The experimental and theoretical peak of C⚌ was found at 1619 and 1639 cm⁻¹ respectively. The experimental and theoretical C⚌S peaks were obtained at 772 and 765 cm⁻¹ respectively (Roeges, 1994). The experimental and theoretical asymmetric ν_as NH vibrations were obtained at 3448 and 3546 cm⁻¹ respectively. The experimental and theoretical symmetric ν_sym vibrations were obtained at 3371 and 3410 cm⁻¹ respectively. The difference may be due to intermolecular interactions as the experimental spectrum was obtained in solid state. The stretching N-N vibration was at 1107 cm⁻¹ and the calculated value was 1133 cm⁻¹ (Singh et al., 2013; Gambino et al., 2007). The C–H in plane deforming vibration was observed at 1435 cm⁻¹ experimentally and coincided with the observed value. The difference between the experimental and calculated harmonic frequencies may be due to non competent treatment of electronic correlation (James et al., 2011). A scaling factor of 0.9613 was used to scale down the calculated wave number to remove any possible harmonicity (Pople et al., 1981).

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Fig. 3

Calculated (a) and Experimental (b) IR spectrum of ligand.

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5.6 UV–vis spectral analysis

The UV–visible spectrum of 2-butanone thiosemicarbazone was calculated using the time dependent density functional theory (TD-DFT). The spectrum was obtained both in gaseous phase as well as in solvent phase in DMSO (Fig. 4). The experimental spectrum was recorded in DMSO with the λ_max obtained at 267 nm and a shoulder band (Khan et al., 2017). In gaseous phase three transitions at 303, 269 and 251 nm were obtained corresponding to π → π* charge transfer transition. The π → π* transition is mainly governed by HOMO → LUMO excitation. In solvent phase (DMSO) transitions at 305, 261 and 248 nm were obtained. The electronic transitions are presented in Table 4. Transitions involved with oscillator strength viz. f = 0.1455 and 0.1022 were not obtained experimentally. The electronic transitions were accompanied by a slight red shift in DMSO which may be attributed to electrostatic and dispersion contributions and H-bonding (Krawczyk, 2015). Solvatochromic changes also occur when polarity of the medium is changed (Zakerhamidi et al., 2012).

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Fig. 4

TD-DFT predicted UV–vis spectra in gaseous and solvent (DMSO) states.

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5.7 Global reactivity descriptors

Hardness (η), global softness (S), electronegativity (χ), chemical potential (µ) and electrophilicity index (ω) are the global reactivity descriptors generally used to predict the stability and reactivity (Geerlings et al., 2003). The electrophilicity index describes the tendency of a molecule to accept electron. A nucleophile has a lower value of ω, whereas an electrophile has high value of ω (Babu and Jayaprakash, 2015). The value of chemical potential (μ) describes the reactivity of the compound. The greater the electronic potential, more reactive would be the compound. According to Koopman’s theorem, global reactivity descriptors are calculated using HOMO and LUMO of the FMOs (Parr and Yang, 1989). The low value of global electrophilicity index (1.9185) indicated ligand’s nucleophilic nature. The ionization energy (I) and electron affinity (A) can be predicted through HOMO and LUMO orbital energies as I = -E_H and A = E_L. ΔN_max defines the electrophilic charge transfer (ECT) between the two molecules. If ECT is greater than 0 then charge flows from second molecule to first. ECT is calculated from the equation- ECT = (ΔN_max)₁-(ΔN_max)₂, here (ΔNmax)₁ = −(µ/η)₁ and (ΔNmax)₂ = −(µ/η)₂. The ΔN_max for the reacting molecules is 1.2868 indicating that the charge flows from molecule 2 to 1 (Singh and Singh, 2015). Calculated global descriptors are given in Supplementary Table 2.

5.8 Natural bond orbital analysis (NBO)

NBO also helps in understanding the hybridization, covalent bonding, hydrogen bonding and van der Waals interaction (Glendening et al., 2001). Second order perturbation theory analysis of the Fock matrix in the NBO basis for intermolecular interactions was calculated by DFT/B3LYP/6–311++G(d,p) method is presented in Supplementary Table 3. Stable donor to acceptor interactions involve delocalization of electron density between occupied Lewis-type (bonding or lone pair) NBO’s and unoccupied (antibonding) non-Lewis NBO’s (Glendening et al., 2001). The most important interaction was related to electron donation from n(2)S1 atom to the σ*C2-N3 resulting in the maximum stabilization energy 19.08 kcal/mol.

5.9 Thermodynamic calculations

Thermodynamic energies were calculated using vibrational frequency calculations for the ligand at 298.15 K. The values of enthalpy (ΔH_Reaction), Gibbs free energy (ΔG_Reaction) and entropy change of reaction (ΔS_Reaction) are given in Supplementary Table 4. The negative values of ΔH and ΔG indicated that the ligand formation is exothermic and spontaneous at room temperature. The free energy of the molecule was calculated including the zero-point vibrational energy.

6.1 Free radical scavenging activity

The free radical scavenging activity of all the synthesized metal complexes was evaluated by DPPH Assay (Scherer and Godoy, 2009). The antioxidant activity was tested at 10, 50, 80 and 100 µM concentration. The absorbance was measured at 517 nm. Lower the absorbance, higher would be the free radical scavenging activity. The scavenging the DPPH radical was calculated by using the following formula: $$\text{DPPH scavenging effect}\left(\%\text{inhibition}\right)=\left[\left({\text{A}}_{\text{c}}--{\text{A}}_{\text{s}}\right)/{\text{A}}_{\text{c}}\right)\times 100]$$ where, A_c = absorbance of the control reaction (DPPH solution) and

 A_s = absorbance of the complexes

Uncontrolled generation of ROS causes redox imbalance and oxidative stress harmful for many diseases including cancer and neurological disorders. The antioxidant activity is summarized in Table 5. [Fe(C₅H₁₁N₃S)₂(SO₄)] showed maximum antioxidant activity viz. 81.70% followed by [Co(C₅H₁₁N₃S)₂Cl₂]⁺ (77.15%).