Synthesis, Physicochemical Characterization, and Density Functional Theory Investigation of ZIL2 (3-(1,3-diethyl-1H-imidazol-3-ium-4-yl) propanoate)

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Figure 1. Synthesis of 3-(1,3-diethyl-1H-imidazol-3-ium-4-yl)propanoate (ZIL2). R = Et: ethyl, R-I: ethyl iodide, IRN78 resin: Amberlite IRN78 (OH^- form)

2.1.2 Thermal measurements (thermogravimetric analysis/differential thermal analysis)

TGA and DTA were performed to investigate the thermal stability, decomposition behavior, and phase transitions of the synthesized ZIL2. TGA measurements were carried out from room temperature to 650 ℃, whereas DTA measurements were conducted over the temperature range of 50-400 ℃. Performed to monitor phase transitions of this IL between 50 and 400 ℃. Thermal analyses were performed using an initial sample mass of 21.8 mg, at a heating rate of 10 ℃ min^-1 under a nitrogen atmosphere.

2.1.3 Ultraviolet-visible measurements

The UV-Vis absorption spectrum of ZIL2 was recorded using a SHIMADZU UV-1650 spectrophotometer. For the measurements, ZIL2 was dissolved in distilled water to obtain a homogeneous solution. The absorption spectra were recorded over the wavelength range of 200-800 nm at room temperature. The obtained data were used to determine the optical band gap of the IL.

2.1.4 Fourier transform infrared measurements

The ATR-FTIR spectrum of the synthesized compound was recorded using a Thermo Scientific Nicolet iN10 MX FT-IR microscope equipped with a ZnSe crystal. The spectrum was acquired over the spectral range of 3500-600 cm^-1 with a resolution of 4 cm^-1 by averaging 64 scans at room temperature. The obtained spectrum was used to identify and assign the characteristic vibrational bands of the investigated compound.

4.1 Thermal measurement

TGA was used to evaluate the thermal stability and decomposition behavior of ZIL2. As shown in Figure 2(a), the TGA thermogram indicates that ZIL2 is thermally stable up to 231 ℃. An initial mass loss of 0.53 mg (2.43% of initial mass) was observed at 61 ℃, which can be attributed to the evaporation of adsorbed moisture. An additional mass loss of 1.38 mg (6.33%) was observed at 107.75 ℃, which may be attributed to the removal of residual volatile species or to the early stages of thermal degradation. The cumulative mass loss before the main decomposition stage was 8.76%. A significant thermal degradation process began at approximately 225 ℃, as indicated by the onset of sharp mass loss in the TGA curve. Between 225 and 400 ℃, a rapid mass loss was recorded, corresponding to the principal decomposition stage of ZIL2.

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Figure 2. Thermal analysis of ZIL2: (a) Thermogravimetric analysis (TGA) and (b) differential thermal analysis (DTA) curves of ZIL2 as a function of temperature

The DTA thermogram in Figure 2(b) exhibits two endothermic peaks. The first endothermic peak at 98.66 ℃ corresponds to the melting point of ZIL2. The second endothermic peak at 281.10 ℃ is assigned to its thermal decomposition. This thermal event is associated with the breakdown of the IL structure and agrees well with the major mass loss stage observed in the TGA curve, confirming the thermal decomposition of ZIL2. The high thermal stability of ZIL2 up to approximately 231 ℃ indicates that it can withstand moderate operating temperatures without significant thermal degradation. Furthermore, the well-defined melting transition at 98.66 ℃ indicates a clear phase behavior and is consistent with the good thermal characteristics of ZIL2.

4.2 Ultraviolet-visible analysis

Electronic absorption corresponds to the transition of electrons from the ground state to one or more excited states, depending on the electronic structure of the molecule and the available energy levels [31]. Previous studies have shown that imidazolium-based ILs exhibit strong absorption in the UV region [32], which is consistent with the UV-Vis spectrum obtained for ZIL2. The absorbance spectrum presented in Figure 3 shows that ZIL2 absorbs below 250 nm, with a maximum absorbance of 1.97 at 218.92 nm.

The broad absorption band observed in the UV region can be attributed to π-π* electronic transition associated with the conjugated C=C group of ZIL2.

To better understand the electronic transitions of ZIL2, the theoretical UV-Vis spectrum was calculated using the TD-DFT method implemented in Gaussian 09. The calculated maximum absorption wavelength was 217.49 nm, which is in good agreement with the experimental value of 218.92 nm, corresponding to an oscillator strength of 9 10⁹ a.u.

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Figure 3. Experimental and theoretical ultraviolet-visible (UV-Vis) absorption spectra of ZIL2

The optical band gap of ZIL2 was estimated from the experimental and theoretical UV-Vis spectra using the corresponding energy relation (Eq. (1))

E=hν                (1)

where, E is the photon energy (J), h is Planck’s constant (6.62.10^-34 J.s), and ν is the frequency (s^-1).

To improve the reliability of the optical band gap determination, three different approaches, namely the first derivative method, the second derivative method, and the Tauc plot, were applied to both the experimental and theoretical spectra. The corresponding plots are presented in Figures A3-A8 (Appendix), while the obtained optical band gap values are summarized in Table 1.

Table 1. Experimental and theoretical optical gap values

Methods	Experimental	Theoretical
First derivative	Eg = 5.64 eV	Eg = 5.65 eV
Second derivative	Eg = 5.39 eV	Eg = 5.40 eV
Tauc plot	Eg = 5.27 eV	Eg = 5.31 eV

The experimental and theoretical optical band gap values of 5.64 eV and 5.65 eV, respectively, are in good agreement, indicating that the TD-DFT calculations successfully reproduce the optical behavior of ZIL2. This agreement confirms the reliability of the theoretical approach for investigating the optical properties of the synthesized IL.

4.3 Vibrational analysis

The vibrational analysis aimed to identify and assign the characteristic vibration modes of ZIL2. DFT frequency calculations were performed using Gaussian 09 at the same level of theory employed for geometry optimization. The optimized structure of ZIL2 contains 30 atoms, corresponding to 84 normal vibrational modes (3N-6). These modes were assigned using potential energy distribution (PED) analysis implemented in the VEDA program [33]. A scaling factor of 0.967 was applied to the calculated DFT/6-311G (d, p) [12] vibrational frequencies to improve their agreement with the experimental values. The good agreement between the calculated and experimental frequencies supports the reliability of the optimized structure of ZIL2. A comparison between the experimental and calculated vibrational spectra is presented in Figure 4, while the corresponding vibrational frequencies are provided in Table 2.

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Figure 4. Experimental and calculated IR spectrum of (3-(1,3-diethyl-1H-imidazol- 3-ium-4-yl)propanoate in spectral area 3500-600 cm^-1

C–H vibrations

According to the literature, the bands observed between 2800 and 3000 cm^-1 are assigned to the symmetric C-H stretching vibrations of the methyl and/or alkyl groups attached to the imidazolium ring [34, 35], whereas those between 3000 and 3200 cm^-1 correspond to C-H stretching vibrations of the imidazolium ring [36, 37]. In the present study, the calculated symmetric and asymmetric C-H stretching vibrations of ZIL2 occur in the range of 3393.50- 3072.93 cm^-1 with a PED contribution of 100% at the B3LYP/6–311G level of theory, while the corresponding experimental bands are observed at 3090 cm^-1. The calculated CH₂ bending vibrations are located between 1479.66 cm^-1 and 1272.49 cm^-1, in good agreement with the experimental bands observed between 1471 cm^-1 and 1276 cm^-1.

C–C and C=C vibrations

The C-C stretching vibrations are generally observed in the 1650-1400 cm^-1 region [37], whereas the C=C stretching vibrations occur between 1560 and 1670 cm⁻¹ [38]. For ZIL2, the calculated ν(C-C) and ν(C=C) vibrations occur at 1532.98 and 1653.43 cm^-1, respectively. Additional C-C stretching vibrations are predicted in the 935.17-1030.62 cm^-1 region, corresponding to the experimental band at 1017 cm^-1 in the FTIR spectrum.

C ̶ N and C=N vibrations

The C-N stretching vibrations are typically found in the 1511-1067 cm^-1 region. For ZIL2, experimental ν(C-N) vibrations are observed at 1062, 1120, 1229, 1370 and1452 cm^-1, whereas the corresponding calculated frequencies are at 1067.84, 1123.53, 1220.21, 1379.39 and1450.04 cm^-1, showing good agreement between the experimental and theoretical results. The C=N stretching vibration is commonly reported around 1555 cm^–1 [33]. Experimentally, this band is observed at 1557 cm^-1, whereas DFT calculations predict frequencies at 1548.22 cm^-1.

C–O and C=O vibrations

The C=O stretching vibration is generally found in the 1600–1800 cm^-1 region [39, 40]. Experimentally, the ν(C=O) band appears at 1597 cm^-1, whereas the corresponding calculated frequencies are at 1636.28 and 1598.02 cm^-1. The calculated ν(C-O) vibrations are found at 1316.50 and 1310.06 cm^-1, matching the experimental band at 1309 cm^-1.

Overall, the vibrational analysis of ZIL2 confirms the successful assignment of the main functional groups, including C–H, C–C/C=C, C–N/C=N, and C–O/C=O vibrations. The good agreement between the calculated and experimental FTIR frequencies supports the reliability of the DFT calculations and confirms the proposed molecular structure of ZIL2.

Table 2 summarizes the calculated harmonic vibrational frequencies, the experimental FTIR wavenumbers, and the corresponding vibrational assignments of ZIL2.

The vibrational assignments were performed using the PED. All calculated frequencies are scaled values expressed in cm^-1.

4.4 Structure description

A preliminary conformational analysis of 3-(1,3-diethyl-1H-imidazol-3-ium-4-yl)propanoate (ZIL2) was performed using DFT calculations at the B3LYP/6-311G(d, p) level of theory, which has been widely employed for the investigation of ILs [41]. The optimized molecular structure is presented in Figure 5. The most stable geometry corresponds to a minimum total energy of -650.59467090 a.u., -17696, 1750 eV. This conformation provides detailed information on the molecular geometry, including bond angles, torsional angles, and atomic charges, which form the basis for the subsequent investigation of the physicochemical properties of ZIL2.

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Figure 5. Density Functional Theory (DFT) optimized molecular structure of ZIL2

The geometric parameters of ZIL2, including bond lengths, bond angles, and dihedral angles, are summarized in Table 3. The imidazolium ring is characterized by the C₃=N₇ and C₃-N₄ bonds with bond lengths 1.3444 and 1.3472 Ǻ, respectively, while the C₁=C₂ bond measures 1.3680 Ǻ. The C-C bonds in the alkyl chains range from 1.49 to 1.56 Ǻ, whereas the C-H bond lengths are close to 1.09 Ǻ. Furthermore, the C₂₈=O₂₉ and C₂₈-O₃₀ bond lengths are 1.2878 and 1.2799 Ǻ, respectively. Overall, the calculated geometric parameters are in good agreement with previously reported data for similar ILs [42].

The bond angles of the imidazolium ring, namely α(C₃-C₁-C₂), α(N₇-C₃-C₁), and α(C₁-C₃-N₄), are 72.97°, 72.23°, and 36.47°, respectively. Among all the optimized bond angles, α(C₁-C₃-N₄) is the smallest (36.47°), whereas α(H₆-C₃-C₁) is the largest (161.67°). These values reflect the optimized molecular geometry of ZIL2 and provide additional structural information for the subsequent theoretical analysis.

The molecular planarity can be evaluated from the dihedral angles, which provide valuable information about the three-dimensional conformation of ZIL2. The calculated dihedral angles exhibit different values, indicating that the atoms are not coplanar and that the molecule adopts a non-planar geometry.

4.5 Atomic charges and molecular dipole moment

The analysis of atomic charge distribution is essential for understanding the electronic properties and chemical reactivity of ZIL2. In this study, the Mulliken population analysis method [31] was employed to calculate the atomic charges of the optimized structure, and the results are summarized in Table 4. The obtained charge distribution provides useful information for interpreting the electronic properties and molecular dipole moment of ZIL2 [36, 37].

The calculated atomic charges reveal the distribution of electron density within ZIL2, allowing the identification of electron-rich and electron-deficient regions that may influence its chemical reactivity and intramolecular charge transfer [30, 37].

Table 4. Mulliken atomic charges of the ZIL2 calculated at DFT/6-311G(d, p)

The Mulliken population analysis reveals a non-uniform charge distribution within the ZIL2 molecule. The most negative atomic charges are located on the nitrogen atoms N₇ (-0.653 a.u.) and N₄ (-0.587 a.u.), as well as on the oxygen atoms O₂₉ (-0.549 a.u.) and O₃₀ (-0.536 a.u.), reflecting their high electronegativity and electron-accepting character. In contrast, atom C₂₈ exhibits a relatively positive charge (+0.439 a.u.), which can be attributed to its bonding with the two oxygen atoms of the carboxylate group. Most hydrogen atoms carry positive charges, whereas several carbon atoms, including C₁₁, C₂₂, and C₂₅, are negatively charged due to electron density redistribution within the molecular framework. Overall, the calculated charge distribution indicates a pronounced electronic polarization, which may favor intermolecular electrostatic interactions and influence the chemical reactivity of ZIL2.

Figure 6 illustrates the dipole moment vector of ZIL2 together with its molecular charge distribution. The orientation of the dipole moment is consistent with the Mulliken charge analysis, showing the accumulation of negative charges on the oxygen and nitrogen atoms, whereas the hydrogen atoms carry positive charges. The calculated dipole moment of ZIL2 is 9.8718 D, indicating a highly polar molecule with pronounced separation of charge. Such a high dipole moment is expected to enhance intermolecular electrostatic interactions and may contribute to the physicochemical properties of ZIL2.

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Figure 6. Dipole moment vector and molecular charge distribution of ZIL2

4.6 Electronic properties

4.6.1 Frontier Molecular Orbitals analysis

FMO analysis provides valuable insight into the electronic structure and charge-transfer characteristics of molecules [43]. The highest occupied molecular orbital (HOMO) acts as an electron donor, whereas the lowest unoccupied molecular orbital (LUMO) acts as an electron acceptor [36]. The energy difference between these orbitals, known as the HOMO-LUMO energy gap Eg (ΔE = E_LUMO-E_HOMO), is an important parameter for evaluating molecular stability, chemical reactivity, optical properties, and the hardness and softness of a molecule [28, 37, 42].

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Figure 7. Frontier molecular orbitals (HOMO and LUMO) of ZIL2

The HOMO and LUMO energies, with the HOMO-LUMO energy gap of ZIL2, were calculated at the DFT/B3LYP/6-311G(d, p) level of theory. Figure 7 illustrates the three-dimensional distributions of the molecular frontier orbitals. The green and red colors represent the positive and negative phases of the molecular wave function, respectively. The HOMO is mainly located on the carboxylate group, whereas the LUMO is predominantly distributed over the imidazolium ring. This spatial separation suggests an intermolecular charge transfer from the carboxylate moiety toward the imidazolium ring, which may contribute to the electronic properties and chemical reactivity of ZIL2.

The calculated HOMO-LUMO energy gap of ZIL2 is 3.6842 eV, suggesting moderate semiconducting characteristics and moderate chemical reactivity. This behavior results from the relatively easy excitation of electrons from the HOMO to the LUMO under external perturbations, thereby facilitating interactions with other chemical species. Such an energy gap generally reflects a balance between molecular stability and chemical reactivity [34, 44], indicating that ZIL2 possesses adequate electronic stability while maintaining an appreciable reactive character. Furthermore, the spatial separation between the HOMO and LUMO orbitals supports the occurrence of intramolecular charge transfer, which contributes to the electronic properties of ZIL2.

4.6.2 Density of states

The density of states (DOS) is widely used to describe the electronic structure of molecular systems [45]. To support the frontier molecular orbital, the total DOS spectrum of ZIL2 was calculated at the DFT/B3LYP/6-311 G (d, p) level of theory using the GaussSum program [46]. Figure 8 illustrates the DOS spectrum, which confirms the HOMO-LUMO energy gap of 3.6842 eV obtained from the FMO analysis. The clear separation between the occupied and unoccupied electronic states indicates the electronic stability of ZIL2. Furthermore, the DOS profile is consistent with the HOMO and LUMO distributions, supporting the occurrence of intramolecular charge transfer within the molecule.

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Figure 8. Total density of states (DOS) of ZIL2

4.6.3 Global reactivity descriptors

Global reactivity descriptors provide valuable insight into the stability and electronic behavior of molecules [47]. These descriptors include chemical hardness (η), chemical softness (S), the electrophilicity index (ω), chemical potential (µ), and electronegativity (χ) [48]. Both chemical hardness and softness are used to describe how easily the electronic density of a molecule can deform or polarize when interacting with another molecule. The electrophilicity index is used to measure the electrophilic character of molecules. The chemical potential reflects the tendency of electrons to escape from a system and is associated with its electron-donating ability [37]. Electronegativity describes the ability of an atom to attract shared electrons in a covalent bond [25]. These descriptors provide useful information about the overall stability and chemical properties of a molecule.

Using Koopman’s theorem, these descriptors were calculated at the B3LYP/6-311G(d, p) level of theory [49]. The corresponding equations and calculated values are presented in Table 5.

The relatively low softness and high hardness values suggest that ZIL2 possesses good chemical stability.

Ionization potential: I_P = - E_HOMO                      (2)

Electron affinity: E_A = - E_LUMO                     (3)

Electronegativity: χ = -1/2 (E_HOMO + E_LUMO)                      (4)

Chemical potential: μ = 1/2 (E_LUMO + E_HOMO)                  (5)

Global hardness: η = 1/2 (E_LUMO – E_HOMO)                     (6)

Overall softness: S = 1 /2η                      (7)

Electrophilicity: ω = μ² /2η                     (8)

Table 5. Global reactivity descriptors of ZIL2

As summarized in Table 5, the calculated global reactivity descriptors confirm the electronic characteristics of ZIL2. The ionization potential (4.88884 eV) indicates that the molecule requires a moderate amount of energy to remove an electron, whereas the electron affinity (1.20464 eV) reflects its ability to accept electrons. The electronegativity (3.04674 eV) and chemical potential (-3.04674 eV) suggest a balanced electron-donating and electron-accepting capability. Furthermore, the relatively high hardness (1.8421 eV) and low softness (0.27592 eV) indicate good chemical stability. The calculated electrophilicity index (2.561289 eV) also suggests that ZIL2 possesses a moderate electrophilic character.

4.6.4 Molecular electrostatic potential

The MEP is a valuable tool for predicting the reactive sites of a molecule. It provides information about the distribution of electrostatic potential over the molecular surface, allowing the identification of electrophilic and nucleophilic regions as well as potential hydrogen-bonding sites [49]. The three-dimensional (3D) MEP map offers a visual representation of the charge distribution, facilitating the interpretation of intramolecular interactions and molecular reactivity [30, 31]. In the MEP map, the blue region corresponds to the most positive electrostatic potential, indicating electron-deficient sites that are susceptible to nucleophilic attack, whereas the green region represents areas of nearly neutral electrostatic potential [39, 50].

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Figure 9. MEP of ZIL2 (2D)

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Figure 10. MEP of ZIL2 (3D)

The MEP maps of ZIL2 are illustrated in Figures 9 and 10. The electrostatic potential ranges from -0.108 a.u. (deep red) to +0.108 a.u. (deep blue). Regions of negative electrostatic potential (red and yellow) correspond to electron-rich areas and represent the preferred sites for electrophilic attack. In contrast, regions of positive electrostatic potential (blue) correspond to electron-deficient areas and are susceptible to nucleophilic attack. According to the calculated MEP maps, the most negative potential is mainly localized around the oxygen atoms, whereas the nitrogen atoms display a lower negative electrostatic potential. The positive electrostatic potential is predominantly distributed around the hydrogen atoms and neighboring carbon atoms. This charge distribution highlights the reactive regions of ZIL2 that are likely to participate in intermolecular interactions, particularly hydrogen bonding and electrostatic interactions with surrounding species.

4.7 Thermodynamic properties

The thermodynamic parameters of ZIL2 were calculated at 298.15 K using the optimized structure obtained at the B3LYP/6-311G(d, p) level of theory. The calculated properties include the zero-point vibrational energy (ZPVE), rotational constants (A, B, C), rotational temperatures (θr), thermal energy (E), heat capacity at constant volume (Cv), entropy (S), and the thermal corrections to energy (E), enthalpy (H), and Gibbs free energy (G). The calculated values are summarized in Table 6.

The calculated thermodynamic parameters indicate that ZIL2 possesses good thermal stability under standard conditions. The relatively high heat capacity at constant volume (44.152 cal.mol^-1. K ^-1) reflects the ability of the molecule to store thermal energy, whereas the calculated entropy (106.237 cal.mol^-1. K ^-1) is consistent with the structural complexity of the molecule. In addition, the positive thermal corrections to energy, enthalpy, and Gibbs free energy confirm the thermodynamic consistency of the optimized molecular structure.

Table 6. Thermodynamic parameters of ZIL2 calculated at 298.15 K using the DFT/B3LYP/ 6-311G (d, p) level of theory