Exploring the non-covalent interactions, vibrational and electronic properties of 2-methyl-4-hydro-1,3,4-triazol-thione-5 in different solutions

3.1 Molecular structure

Fig. 1a shows the optimal geometric structure of MHTT calculated at the B3LYP/6–311++G(d,p) level by the DFT method. Table S1 presents the geometric parameters (bond length, angles) corresponding to the optimal structure of MHTT. The 2D potential energy surface of MHTT (C4-N3-O12-H13) was calculated along the deviation of the dihedral angle in the range 0–360 Degrees (Fig. 2). Calculations showed that C4-N3-O12-H13 has the minimum potential energy at a value of 0 Degree dihedral angle.

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Fig. 1

The optimized molecular structures of of 2-methyl-4-hydro-1,3,4-triazole-thiol-5 (a) and its complexes of with acetonitrile (b), acetone (c), dioxane (d), DMF (e) molecules calculated B3LYP/6–311++G(d,p) level.

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Fig. 2

Potential energy surface scan for dihedral angle C⁴-N³-O¹²-H¹³ of MHTT.

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A scan of potential energy surface (PES) was used to determine the preferred position of the MHTT molecule. A search to identify low-energy structures was performed for the (C4-N3-O12-H13) dihedral angle of MHTT, which is also a relevant coordinate for intramolecular conformational flexibility. In calculations, all geometric parameters were simultaneously relaxed and and rotation angles ranged from 0 to 360 degrees. The minimum energy curve for this rotation is −755.067265 Hartrees, with the minimum energy at 0 Degree. Therefore, in this study, we concentrated on the most stable form of the MHTT molecule to clarify the assignment of the molecular structure and vibrational spectra.

The optimal geometries of complexes of MHTT with solvent molecules such as acetonitrile, acetone, dioxane, and DMF are depicted in Fig. 1b–e. MHTT molecules form complexes with solvent molecules using N-H…N≡C (with acetonitrile), N-H…O = C (with acetone), N-H…O (with dioxane), and N-H…O = C (with DMF) hydrogen bonds. The hydrogen bond lengths are 1.919 Å (MHTT + ACN), 1.796 Å (MHTT + AC), 1.789 Å (MHTT + DO), and 1.742 Å (MHTT + DMF), respectively. Intermolecular interaction energies in molecular complexes are 7.15 kcal/mol (MHTT + ACN), 5.55 kcal/mol (MHTT + AC), 5.13 kcal/mol (MHTT + dioxane), and 5.93 kcal/mol (MHTT + DMF), respectively. The intermolecular interaction energies in molecular complexes were determined based on ΔE = E_complex – E_MHTT – E_solvent where E_complex, E_MHTT, E_solvent are the total energies of the complex, MHTT and solvent molecules, respectively (Allangawi, 2024; Sajid, 2021).

3.2 Vibrational analysis

The title molecule contains 13 atoms, and the vibrational spectrum shows 33 fundamental vibrational modes. The Raman and IR spectra of MHTT calculated at the DFT: B3LYP/6–311++G(d,p) in the gas phase are shown in Fig. 3a,b. VEDA software (Jamroz, 2004) was used to analyse the potential energy distribution (PED) of vibrations connected to each group of atoms in the molecule. The calculated vibrational frequencies, scaled values, Raman activity, IR intensities and PED assignments are all listed in Table 1. PED can predict the vibrational mode for every vibrational frequency (Thamarai, 2020).

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Fig. 3

Simulated vibrational (Raman (a) and IR (b)) and UV–Vis (c) spectra of MHTT.

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It is known from the literature (Sagaama, 2022; Bellamy and Williams, 1957) that the N-H stretching vibration frequencies lie in the range of 3300–3500 cm⁻¹. For the MHTT molecule, the calculated N-H stretching vibration frequency in the gas phase is 3657 cm⁻¹ (Ghatfaoui et al., 2021). Fig. 5 shows the FTIR spectra of N-H stretching vibration of binary solutions of MHTT in different solvents. The N-H stretching vibration of MHTT in carbon tetrachloride (CCl₄) solution is 3396 cm⁻¹ (Fig. 4). The figure shows that the NH stretching band maximum of MHTT shifts to the low frequency region up to 3249 cm⁻¹ (acetonitrile), 3201 cm⁻¹ (acetone), 3168 cm⁻¹ (dioxane) and 3063 cm⁻¹ (DMF) in different solutions. Also, the half-width of the band and the integral intensity increase. The reason for this spectral change is due to the interaction between solvent–solute molecules in the solution.

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Fig. 4

Experimental FTIR spectra of N-H stretching of the 2-methyl-4-hydro-1,3,4-triazole-thiol-5 in CCl₄ (a), acetonitrile (b), acetone (c), dioxane (d), and DMF (e).

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Fig. 5

FMOs and MEP surfaces of MHTT and its complexes with different solvent molecules.

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It is known from the literature (Mani, 2023) that the frequency of O-H stretching vibration is fined in the range of 3450–3600 cm⁻¹. The calculated O-H stretching vibrational frequency for our title molecule is in the range of 3495 cm⁻¹. Also, spectral lines at 1540 cm⁻¹ belong to the combination of N-O-H bending and C-N stretching vibrations.

Analysis of the literature (Thamarai, 2020; Sagaama, 2022) showed that C-H stretching vibration frequencies are in the range of 2800–3300 cm⁻¹. For our title molecule, the frequency of CH₃ asymmetric stretching vibrations was found to be 3141 cm⁻¹, the frequency of CH₂ asymmetric stretching vibrations was 3086 cm⁻¹, and the frequency of CH₂ symmetric stretching vibrations was 3141 cm⁻¹. CH₃ bending vibration lines are located at 1420 cm⁻¹.

3.3 Frontier molecular orbital (FMO) analysis

The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are known as frontier molecular orbitals (FMO), and their values can be used to analyze intermolecular interactions and the electron donating and accepting abilities of molecules (Medimagh, 2023; Ullah, 2023). FMO also provides information on compound kinetic stability and chemical reactivity. The energy gap (E_g) of frontier orbital represents the energy difference between the HOMO and LUMO. The energy gap is a good predictor of a molecule's kinetic stability and chemical reactivity. A molecule with a low energy gap is polarizable, reactive, and soft. The high energy gap indicates the presence of a solid molecule. Table S2 presents the chemical reactivity parameters energy gap (E_g), hardness (η), chemical potential (μ), global electrophilic index (ω), ionisation potential (IP), and electron affinity (EA) calculated for the title molecule in the gas phase and its complexes with different solvent molecules (ACN, AC, dioxane, and DMF) using HOMO and LUMO energies. Fig. 5 indicates the HOMO and LUMO orbitals of the MHTT and its complexes with different solvent molecules. These values are determined using the following mathematical formulas based on Koopman's theorem (Koopmans, 1934):

3.4 Molecular electrostatic potential surface (MEPS) analysis

MEP surface of molecules and molecular systems is a helpful technique for studying hydrogen bonding interactions and the biological recognition process by identifying nucleophilic and electrophilic reaction sites (Koopmans, 1934; Fukui, 1982; Kazachenko, 2023). The MEPS has an essential role in characterizing the physicochemical properties of molecules, such as size, shape, electron density, delocalization, and chemical reactivity site (Ahmed, 2017).

Fig. 5 shows the MEP surfaces of the MHTT molecule and its complexes with various solvent molecules. Different colours on the MEPS show different levels of electrostatic potential. Blue > green > yellow > orange > red is in order of decreasing potential. The color code of the MEP map ranges from −7.615 a.u. (deepest red) to + 7.615 a.u. (deepest blue) for MHTT, respectively. It can be seen that in the MHTT molecule, a large electropositive potential is detected near the N-H group, and a small electropositive (blue) region is detected near the oxygen (O) atom. As a result of the formation of N-H…N and N-H…O hydrogen bonds during the formation of complexes, the electrostatic potential around the N-H group decreases and becomes neutral. In addition, the highly electronegative region is activated near the oxygen (O), sulphur (S), and nitrogen (N) atoms of the MHTT molecule. The surroundings of the CH₃ group have an almost neutral electrostatic potential and are therefore inactive.

3.5 UV–vis absorption studies

The UV–Vis spectrum of MHTT was simulated using TD-DFT theory with several solvents. Table S3 displays the maximum absorption wavelength, oscillator strength, and transition probabilities for MHTT in solvent phases. Because each solvent provides various maximum absorption peaks, UV–Vis spectra for the MHTT molecule and its various complexes are shown in Fig. 3c in the 100–400 nm range. For the MHTT molecule, there are three absorption peaks in the gas phase: the primary absorption at 274 nm, the second absorption at 260.21 nm, and the third absorption at 259.73 nm. In this case, the oscillator strength of the second absorption is the largest (f = 0.1222), and the main contribution is 67 % from HOMO to LUMO + 1. The absorption is different for the complexes formed with different solvents. In MHTT + ACN and MHTT + DMF complexes, S₀ → S₃ absorption peaks are 245.15 and 243.62 nm, respectively, and have the largest oscillator strength. In these, the main transition contributions from HOMO to LUMO are 64 and 63 %, respectively. The first absorption peak for the MHTT + AC complex is 287.60 nm (f = 0.0039), and the main contribution is 71 % from HOMO to LUMO. The second absorption peak for the MHTT + DO complex is 253.23 nm, and the main contribution from HOMO to LUMO + 1 is 68 %. These indicate charge transfers within the complexes.

3.6 Atoms in molecules (AIM) analysis

The Atoms in Molecules (AIM) analysis, which is based on Bader's theory (Bader, 1985), is frequently used to understand intra- and intermolecular interactions in molecular complexes. Geometrical and topological characteristics are also important tools for determining the strength of hydrogen bonding. The geometric conditions for the presence of hydrogen bonds are as follows (Khan, 2015):

1. The distance between hydrogen (H) and acceptor (A) atoms is smaller than the sum of their van der Waals radii;

2. Hydrogen bond angle α(D-H∙∙∙∙A) > 90⁰;

3. An increase in the D–H bond length is observed in the hydrogen bond.

The bond critical point (BCP) is a point on the bond path with the lowest electron density. Electron density (ρ), Laplacian of electron density (∇²ρ), energy density (H) and potential energy (V) in BCPs are important topological parameters describing the nature of hydrogen bonding (Asif, 2022). The following criteria are used to describe the presence of a hydrogen bond (Tang, 2006; Bader, 2010; Rozas, 2000):

1. BCP must be present between hydrogen (H) and acceptor (A) atoms;

2. Electron density and its Laplacian in BCPs should be in the interval of 0.002–0.040 a.u and 0.024–0.139 a.u, respectively.

The hydrogen bond energy is calculated using the relationship suggested by Espinosa et al. (Espinosa, 1998):

E_HB = -V(r)/2

Fig. 6 shows the AIM plot for these compounds, and some calculated topological parameters in BCPs are presented in Table 2.

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Fig. 6

AIM plot, RDG scatter and NCI coloring maps of complexes.

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It can be seen from Table 2 that there is an N5-H6…O14 H-bond between MHTT and acetone molecules, and the values of electron density and Laplacian of electron density are 0.0345 a.u. and 0.1164 a.u., respectively. The hydrogen bond energy is 9,036 kcal/mol. Between MHTT and acetonitrile molecules, there is an N5-H6…N14 H-bond, and the electron density and its Laplacian values are 0.0284 a.u. and 0.0906 a.u., respectively. The energy of this H-bond is 6,369 kcal/mol. There is an N5-H6…O19 H-bond between MHTT and dioxane molecules, and the the electron density and its Laplacian values are 0.0362 a.u. and 0.1194 a.u., respectively. The energy of this H-bond is 9,758 kcal/mol. Also, there is a N11-H12…O1 H-bond between MHTT and DMF molecules, and the electron density and its Laplacian values are 0.0375 a.u. and 0.1312 a.u., respectively. This H-bond energy is 10,573 kcal/mol. The results of the analysis showed that the MHTT molecule forms a stronger hydrogen bond complex with the DMF molecule compared to other solvent molecules. If the energy density at BCPs is negative (H < 0), the hydrogen bond is covalent, while if it is positive (H > 0), it is electrostatic. It can be seen that the H-bonds in the MHTT-acetone and MHTT-acetonitrile complexes have an electrostatic nature, and the H-bonds in the MHTT-dioxane and MHTT-DMF complexes have a covalent nature.

3.7 RDG-NCI interaction analysis

It is very important to study the nature of intermolecular interactions between solute–solvent molecules. Reduced density gradient and non-covalent interaction (RDG-NCI) analysis was used to characterize weak interactions in MHTT-acetone, MHTT-acetonitrile, MHTT-dioxane, and MHTT-DMF complexes. These analyses are widely used to distinguish non-covalent interactions such as van der Waals, hydrogen bonding, and steric repulsion in molecular systems. RDG is a dimensionless quantity determined by electron density and its first derivative, and it is expressed using the formula follows (Rasul, 2023): $$\mathrm{R}\mathrm{D}\mathrm{G}(\mathrm{r})=\frac{1}{2{(3{\pi }^2)}^{1/3}}\frac{\left|\mathrm{\nabla }\rho (r)\right|}{\rho {(r)}^{4/3}}$$ The RDG scatter plot is obtained by estimating the electron density of the RDG-sign(λ2)ρ peaks. This plot describes the strength and kind of interactions in a molecule. If sign(λ2)ρ > 0, it represents a repulsive effect. If sign(λ2)ρ < 0, it represents an attractive effect. If sign(λ2)ρ≈0, Van der Waals represents the effect(Daghar et al., 2021). Furthermore, blue represents hydrogen bonding, green represents Van der Waals interactions, and red represents strong steric effect (Zahid, 2023).

Fig. 6 shows RDG scatter graphs and NCI coloring maps for MHTT-acetone, MHTT-acetonitrile, MHTT-dioxane and MHTT-DMF complexes. The blue spines in the range of −0.04 < sign(λ2)ρ < -0.03 in the RDG scatter graphs and the blue discs in the NCI coloring maps confirm the presence of intermolecular hydrogen bonds in the studied complexes.

3.8 Electron localisation function (ELF) and localized orbital locator (LOL) studies

For a better understanding of the electronic structure and chemical bonding in molecular systems, we conducted topological analyses of the electron localization function (ELF) and the localised orbital locator (LOL). Fig. 7 depicts ELF and LOL colour shade maps for the examined molecular complexes. ELF values range from 0.0 to 1.0, indicating a region with comparably bonding and nonbonding localised electrons. Fig. 7 shows a high ELF zone (in red) around the hydrogen atoms and a blue region with the carbon atoms, indicating a delocalized electron cloud. The LOL colour map reveals white in the centre of some hydrogen atoms, suggesting that the electron density exceeds the colour scale limit of 0.80. The complexes' hydrogen atom surrounds, as well as the centres of some carbon and nitrogen atoms, are coloured red. This location has high ELF and LOL values, indicating strong electron localization. The blue region surrounding the carbon atoms represents the delocalized electron cloud. The circular blue area surrounding the nitrogen and oxygen atoms represents the electron decrease area between the inner and valence shells.

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Fig. 7

ELF and LOL diagrams of complexes of MHTT wit solvent molecules.

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