Role of Multiple Intermolecular H-Bonding Interactions in Molecular Cluster of Hydroxyl-Functionalized Imidazolium Ionic Liquid: An Experimental, Topological, and Molecular Dynamics Study

Abstract

Multiple intermolecular H-bonding interactions play a pivotal role in determining the macroscopic state of ionic liquids (ILs). Hence, the relationship between the microscopic and the macroscopic properties is key for a rational design of new imidazolium ILs. In the present work, we investigated how the physicochemical property of hydroxyl-functionalized imidazolium chloride is connected to the molecular structure and intermolecular interactions. In the isolated ion pair, strong N-H···Cl H-bonding interactions are observed rather than H-bonding interactions at the acidic C₂-H site and alkyl-OH···Cl of the hydroxyl-functionalized imidazolium chloride. However, the N-H···Cl H-bonding interaction of the cation plays a significant role in ion-pair formations and polymeric clusters. For 3-(2-Hydroxy)-1H-imidazolium chloride (EtOHImCl), the oxygen atom (O) engages in two significant interactions within its homodimeric ion-pair cluster: N-H···O and alkyl OH···Cl. Vibrational spectroscopy and DFT calculations reveal that the chloride ion (Cl⁻) forms a hydrogen bond with the C2-H group via a C2-H···Cl interaction site. Natural Bond Orbital (NBO) analysis indicates that the O-H···Cl hydrogen-bonding interaction is crucial for the stability of the IL, with a second-order perturbation energy of approximately 133.8 kJ/mol. Additional computational studies using Atoms in Molecules (AIMs), non-covalent interaction (NCI) analysis, Electron Localization Function (ELF), and Localized Orbital Locator (LOL) provide significant insights into the properties and nature of non-covalent interactions in ILs. Ab initio molecular dynamics (AIMD) simulations of the IL demonstrate its stable states with relatively low energy values around −1680.6510 atomic units (a.u.) at both 100 fs and 400 fs due to O-H···Cl and C-H···Cl interactions.

1. Introduction

Ionic liquids (ILs) are fascinating ionic materials that have interesting physio-chemical properties, which are helpful to prepare not only potential electrochemical devices, such as fuel cells, dye sensitized solar cells, and capacitors, but also precursors for various chemical syntheses [1,2,3,4]. For example, the physical properties of ILs can be tailored by adding various substituents of the imidazolium ring [5]. Task-specific hydroxyl-functionalized imidazolium-based ILs have shown higher chemical and thermal stability, with well-established structural characterization, making them a unique material for the reversible capture of greenhouse gasses [6,7] as media for organic synthesis to enhance hydrophilicity and enzyme activity [8], to upgrade the solubility of inorganic salts [9], and to produce metal oxide powders with a specified size [10,11]. The polarity of task-specific hydroxyl-functionalized ILs was found to be anion-dependent and varied over a significantly wider rage due to significant differences in H-bonding strength [12]. The physical and chemical properties of ILs are mainly governed by a subtle balance of various non-covalent interactions such as Coulombic interactions, H-bonding, and dispersion forces [13,14,15,16]. The local and H-bonding interactions have an influence on determining the physical properties of ILs [17,18]. The combination of cations and anions is an important parameter for controlling the properties of ionic liquids [19,20]. The rational design of a new class of ILs depends on their macroscopic properties, which can be achieved by choosing the proper composition of actions and anions related to their structural parameters. However, the structure–property relationships for ILs have been systematically explored and reported by several groups [21,22]. These hydroxyl-functionalized imidazolium-based ILs are considered a special class of protic ionic liquids (PILs), with a unique proton-conducting ability used in water-free fuel cells [23]. PILs play an important role in designing next-generation proton exchange membrane material [24]. The proton conduction mechanism of PILs has been investigated for designing next-generation proton exchange membrane material [25,26]. According to the proton jump mechanism, PILs with multiple H-bonding interaction sites play an important role in enhancing ionic conductivity and creating more efficient proton exchange membrane materials [26].

Task-specific hydroxyl-functionalized imidazolium-based ILs have mainly demonstrated two types of H-bonding: (i) H-bonding interactions between a cation and an anion, and (ii) multiple-ion H-bonding interactions between a cation and an anion. The multiple-ion H-bonding interactions are significantly declining due to the presence of repulsive Coulomb forces, observed in neutral water and alcohol clusters in a solution [27,28,29]. Recently, hydroxyl-functionalized PILs were also studied via various spectroscopic techniques and theoretical studies focused on multiple interactions in these ILs [30,31,32]. A variety of cations with a hydroxyl group (e.g., ammonium-, imidazolium-, pyrrolidinium-, piperidinium-, and pyridinium) were reported [33,34]. In hydroxyl-functionalized PILs, cation–cation-type H-bonds might act as an important driving force for making the local structures of ILs as they compete against the repulsive Coulomb interactions between cations and with the Coulomb-enhanced cation–anion-type H-bonds between cations and anions [35,36]. Precisely, FT-IR measurements mainly showed two well-defined vibrational bands that are clearly assigned to cation–anion- and cation–cation-type H-bonded species [36]. It is interesting to note that the H-bonding interactions in the cation–cation case are stronger than the H-bonding interactions in the cation–anion case, showing the red shifts of their corresponding vibrational bands [36,37,38]. Despite this expectation, however, in the bulk liquid phase, structural motifs with H-bonded cationic clusters were mainly observed [38,39].

The present work aims to understand how the physicochemical property of hydroxyl-functionalized imidazolium chloride is connected to the molecular structure and intermolecular interactions between anions and cations. For this purpose, FT-IR and density functional theory were applied to 3-(2-hydroxyethyl)-1H-imidazolium chloride (EtOHImCl; Scheme 1), which has potential applications in chemistry. Furthermore, our goal is to explore the similarity in the theoretical results for isolated ion pairs and small gas-phase clusters to the structures in liquid form at room temperature.

2. Experimental Section

2.1. Chemicals

All starting materials were bought from Sigma-Aldrich. These materials are of analytical grade. D₂O and CDCl₃ were also procured from Sigma-Aldrich and used for NMR studies. Ultrapure water, dichloromethane, acetonitrile, ethanol, DMSO, and other solvents were obtained from Sigma Aldrich (Milwaukee, MI, USA).

Synthesis Procedure

EtOHImCl was synthesized from imidazole precursor (Scheme 2) according to reported procedures [40]. Firstly, a mixture of imidazole (Im: 0.01 mol, 0.735 g) and 2-chloroethanol (ClEtOH: 0.67 mL, 0.01 mol) was stirred at 120 °C for 24 h. At reduced pressure, the mixture was evaporated and the obtained product was further repeatedly washed using diethyl ether (5 × 20 mL). Then, the solvent was removed and the product was dried under vacuum for 6 h to obtain a product with high purity. A viscous yellowish liquid was obtained with a yield of 82%. The water content was measured from coulometric Karl Fischer titration and found to be below 360 ppm.

2.2. Synthesis and Characterization of EtOHImCl

Characterization of EtOHImCl IL: State: Liquid; Yield: ~82%. ¹H-NMR: (500 MHz, D₂O): δ (ppm) 8.91 (s, 1H, NH), 8.22 (s, 1H, C₂H), 7.63 (d, J = 4.8 Hz, 1H, C₄H), 7.56 (d, J = 4.8 Hz, 1H, C₅H), 4.16 (t, J = 4.0 Hz, 2H, -CH₂-), 3.85 (t, J = 4.8 Hz, 2H, -CH₂-), 1.99 (s, 1H, OH). ¹³C-NMR: (150.93 MHz, D₂O): δ (ppm) 135.2, 122.0, 121.3, 56.8, 53.4.

2.3. Infrared Spectroscopy

Attenuated total reflectance Fourier-transform infrared (FTIR) spectra were measured for the study of the vibrational properties. The spectra are obtained using a Vertex 70-RAM II Bruker spectrometer (Bruker Analytical, Madison, WI, USA) operating with a Golden Gate TM diamond ATR accessory (Specac Ltd., Slough, UK). The FT-IR spectra (400–4000 cm⁻¹) were collected using a nominal resolution of 1 cm⁻¹ by co-adding 64 scans for each spectrum.

2.4. Quantum Chemical Calculation

2.4.1. Structure Calculations

The geometries for EtOHImCl ILs were optimized using DFT calculations applying a Becke, 3-parameter, Lee–Yang–Parr (B3LYP) functional [41,42,43]. A further B3LYP functional with a 6-311++G(d,p) basis set was employed for predicting the most stable geometries for ILs and the interactions between cation and anion in EtOHImCl [44,45]. These geometrical optimizations were performed using the Gaussian 09 suite whereas the structures were visualized using the Gauss View 5.1 program [46].

2.4.2. Wavenumber Calculations

FT-IR as well as Raman vibrational wavenumbers of EtOHImCl IL were calculated using a B3LYP functional with a 6-311++G(d,p) basis set through the Gaussian 09 program [46]. The absence of negative (imaginary) wavenumbers was monitored using harmonic vibrational wavenumber calculation for each optimization of the structure of EtOHImCl to confirm the convergence to minima of the potential surface. The assignment of FT-IR bands was performed using the VEDA program [47].

2.4.3. Binding Energy Calculation

The binding energy (BE) of EtOHImCl IL was also separately calculated using the B3LYP functional with a 6-311++G(d,p) basis set through the Gaussian 09 program [46].

The binding energy for the ion-pair formation (ΔE) was measured using Equation (1), according to Turner et al. [48].

$$∆\mathrm{E}={\mathrm{E}}_{\mathrm{A}\mathrm{X}}-{\mathrm{E}}_{{\mathrm{A}}^{+}}^{\mathrm{M}\mathrm{i}\mathrm{n}}-{\mathrm{E}}_{{\mathrm{X}}^{-}}^{\mathrm{M}\mathrm{i}\mathrm{n}}+{\mathrm{E}}_{\mathrm{B}\mathrm{S}\mathrm{S}\mathrm{E}}+\Delta \mathrm{ZPE}$$

(1)

where ΔE is the binding energy for the ion-pair formation whereas E_A+, E_X⁻ and E_AX are the energy of the isolated cation, anion, and ionic pair, respectively.

Basis Set Superposition Error (BSSE) correction was calculated to correct the interaction energies [49,50]. The BSSE calculation was performed using the counterpoise procedure developed by Boys and Bernardi. Further, frequency calculation provided the zero-point energy (ZPE) correction to the interaction energy.

2.4.4. Natural Bond Orbital (NBO) Analysis

NBO analysis delivers the essential information of inter- and intra- molecular interactions [51,52,53]. The analysis provides fundamental bonding understanding by using localized Lewis-like chemical bonds [54,55]. The NBO search are formulated from the natural atomic orbital through the contribution of a natural Lewis structure [56,57,58]. The Lewis structure is mainly accepted when the occupancy is greater than the threshold.

2.4.5. Energy Decomposition Analysis (EDA)

EDA is a computational technique used to dissect the total interaction energy between molecules or fragments of a molecular system into distinct, interpretable components. This helps in understanding the nature and contributions of various forces involved in the stability and interactions within a molecular system.

The decomposition of energy was measured using quantum chemical calculations into various components with multiple chemical origins. These components were electrostatic, exchange, repulsion, polarization, and dispersion energies. The EDA calculation was performed using the GAMESS program package 2023, R2 [59].

2.4.6. Topology Analysis

Atom-In-Molecule (AIM) analysis, Electron Localized Function (ELF) and Localized Orbital Locator (LOL) surface map analysis were performed using Multiwfn Software Package [60]. AIM analysis is an intuitive tool to study electron delocalization using the calculation of electron density and its Laplacian. ELF and LOL surface maps determine the topological properties related to the probability of finding electrons OK in atomic and molecular systems. These obey the first-principle method for all the electrons present in the molecular system at full-potential local orbitals.

2.4.7. Non-Covalent Interaction (NCI) Analysis

Non-covalent interaction analysis provides information on the underlying chemistry involved in various types of bonds [61]. It delivers quantitative data regarding hydrogen bonds, van der Waals interactions, and steric repulsion. The calculation was performed using NCIPLOT software package 4.2 [61].

2.4.8. Ab Initio Molecular Dynamics Assay

Ab initio molecular dynamics (AIMD) study was performed for ILs using the Orca 5.0.2 program package [62]. This study used the timestep of 0.5 fs at a timecon of 10 fs for 500 fs at 350 K [62]. The “timecon” parameter sets the coupling strength of the thermostat using the NVT ensemble, where larger time constants correspond to weaker coupling.

3. Results and Discussion

Our aim is to establish the links between the molecular structure of the isolated ions, ion pairs, and small clusters and the properties of the condensed phase for EtOHImCl. In the current section, the results are reported based on our combined computational and experimental studies in several steps starting with the most simplistic case, i.e., isolated ion pairs, via the more realistic situation of small clusters in the gas phase, to, eventually, consider the real, condensed state.

3.1. Structure of Isolated Ion-Pairs and Small Clusters

To obtain the orientation patterns of the Cl⁻ anion and the EtOHIm⁺ cation and its potential molecular interactions, in an isolated ion-pair and a cluster comprising two ion pairs, the models were optimized at the B3LYP/6-311++g(d,p) level of theory. Each optimized structure was checked to be a local minimum through normal-mode frequency calculations through the inexistence of imaginary frequencies. Initially, the cation–anion interactions were initiated by positioning the Cl⁻ anion near C₂-H, N-H and the hydroxyl group (-OH) of the EtOHIm⁺ cation (Figure 1). The B3LYP method was successfully used to optimize the molecular structure (Figure 1). Interestingly, in the most stable geometry, the anion was not found at the C2-H position of the cation in EtImCl, where the anion is generally located in most imidazolium ILs [63], and at the hydroxyl group of the EtOHIm⁺ cation [35,64], but it was found at the N-H position of the cation in the ion pair of EtOHImCl IL. The most stable position of the Cl⁻ anion is at the N-H position and then the hydroxyl group (alkyl OH), whereas it is least probable at the C2-H position of the cation. The difference in binding energy (B.E.) with BSSE correction was 21.3 kJ/mol when the anion was positioned at the hydroxyl group (alkyl OH) site of the cation instead of the N-H site, while the difference was significantly larger, at 116.8 kJ/mol, when the anion was located at the C2-H position of the cation. These three H-bonding interaction sites make EtOHImCl a special class of PIL. Instead of an H-bonding interaction between the hydroxyl group and the C2-H position of the cation with the Cl⁻ anion, the N-H···Cl H-bonding was preferable and observed for the ion pair of EtOHImCl from the DFT calculation (Figure 1). The distance of C₂-H···Cl and alkyl OH···Cl was found to be 2.25 Å and 2.07 Å, respectively (Figure 1b). The N-H···Cl distance was observed at 1.36 Å and the anion was preferably located on the plane of the cation (Figure 1c).

As indicated previously, from the calculation of the ion pairs of EtImCl, it is challenging to attain resemblance with the real material. Therefore, the next step is to increase the level of complexity by performing a small molecular cluster comprising two ion pairs. The most stable optimized geometry of the ion-pair cluster is displayed in Figure 1d. The H-bonding between the hydroxyl group (-OH) and the chloride anion (Cl⁻) can be observed at 1.98 Å in the cluster (Figure 1d). Similarly, H-bonding between the hydroxyl group (-OH) and the N-H can be observed at 1.94 Å in terms of alky-O···H-N interaction in the cluster (Figure 1d). The C2-H···Cl H-bonding interaction is observed at 2.25 Å in the cluster. An interesting factor is that there are unique H-bonding sides available in the ion-pair cluster via multiple H-bonding center Cl anions in both ways (Figure 1d).

In other words, this intermolecular H-bonding interactions in the cluster becomes even stronger in the presence of another ion pair. Interestingly, these twin H-bonding interactions between the hydroxyl oxygen atom associated with the acidic hydrogen atom attached to the C₂ position of the other cation and the Cl anion play an important role in the stability of the ion-pair cluster and the 1D polymeric structure of EtOHImCl IL. It can be concluded that the interesting molecular interactions between the reported ion pairs are mainly dominated by cation–cation interactions and cation–anion interaction; on the other hand, there do exist further strong/directional interactions, featuring a cation with the other anion.

The described interactions and configuration of the hydroxyalkyl group mean that each hydroxyl group is involved in three H-bonds with another unit of the same molecule, similarly to the situation often found in the alkyl alcohol and water system. Such intermolecular H-bonding interactions are identified in a small cluster rather than in complex and bulky molecular systems. In the liquid state, this H-bond network exhibits dynamic behavior, resulting in different bonding states. This gives rise to distinguishable vibrational frequencies manifesting as broad -OH stretching bands [38]. Furthermore, the later section of this paper, focused on the discussion of the experimental vibrational spectra recorded in the solid state, will shed some light on this issue.

3.2. FT-IR Studies

In order to investigate the role of vibrational modes towards intermolecular molecular interactions, FTIR measurements were carried out in solid state. Figure 2 represents the C-H/O-H stretching regions for IM (Imidazole) and ClEtOH (2-Chloroethanol).

Table 1. Observed FTIR bands along with their assignment.

FTIR Band	Vibrational Band Assignment
651	ω(N-H)/CH3(N) CN Str
702	CH2(N)/CH3(N)CN bend
755	ω(N-H)
868	ring CC bend
944	ρas(CH2)
1069	νC-O
1166	Ring as Str CH2(N) and CH3(N)CN Str/CC
1255	Ring ip as str
1338	Ring, ν(CH2-N)
1386	δ(N-H) ip vibration
1428	δ(O-H) ip vibration
1455	δ(CH2)/CCH, HCH as bend
1475	Str CH2(N)/CH2(N)CN Str
1634	Ring vibration C=C
1636	Ring vibration C=N
2882	νs(CH2) as Str
2955	νas(CH2) as Str
3095	ν(N)CH2 as Str
3149	νC(4/5)-H str
3364	ν(N-H)/(O-H)

(ν = Str = stretching; δ = deformation; bend = bending deformation; ω = wagging; ρ = rocking; s = symmetric; as = antisymmetric; ip = in plane; op = out plane).

summarizes the assignment of the experimental vibrational bands, and the detailed rigorous analysis of FTIR bands is presented in the Supporting Information (see Table S3). The aliphatic C-H stretching modes ranged from 2700 to ~3000 cm⁻¹ (alkyl group), while the aromatic C-H groups vibrations were observed at a higher frequency range between 3000 and 3200 cm⁻¹ for IM (imidazole). Strong OH stretching bands were observed and located at 3366 cm⁻¹ and 3569 cm⁻¹ for ClEtOH (Figure 2). OH stretching peaks appeared at two distinct wavenumbers, which indicates that OH groups can exist in two distinct states: strongly H-bonded OH groups and free OH groups. The free OH stretching vibration of IL has shown similar spectral behavior to alcohols and water [38,65,66]. Interestingly, this supports the idea that these OH groups of IL are not contributing to the 1D arrangement via any intermolecular interaction. The band at 3417 cm⁻¹ of EtOHImCl is close to the asymmetrically H-bonded O-H of water molecules [66]. These experimental results strongly support our hypothesis that each OH group of EtOHImCl is involved in two different H-bonding interactions [64]. These H-bonding interactions are responsible for the stability of various molecular systems [52,58,67,68,69].

We systematically assigned the vibrational bands for our present system (EtOHImCl; Figure 3) with the help of our previous studies [30]. The stretching vibrations of the C₄/₅-H groups were observed at 3151 and 3132 cm⁻¹. These vibrational frequencies were found to be slightly different depending on the chemical environment and can be observed in Figure 3. The C₂-H stretching vibration is observed at 3106 cm⁻¹ due to the presence of an H-bonding interaction between C₂-H and the Cl anion. The above experimental results support that an H-bonding interaction takes place between the C₂-H groups of the cations and the Cl⁻ anion. Strong interactions between the cation C₂-H and the Cl⁻ anion do take place in EtOHmimCl, but the Cl anion also takes part in an H-bonding interaction with the -OH group [5]. The stretching vibrational modes of the aliphatic C-H groups appeared in the region between 2800 and 3000 cm⁻¹. Interestingly, two different vibrational modes of CH₂ group were observed due to the presence of the hydroxyethyl chain with a chemically different environment as one is neighboring an oxygen and the other one a nitrogen atom. The vibrational mode of EtOHImCl at the fingerprint region provides useful molecular information related to the cation–anion interaction of EtOHImCl PIL. In the experimental FTIR spectrum, the vibrational mode of C=C bond is found at 1565 cm⁻¹. Interestingly, the non-symmetric shape of the vibrational mode of the C=C bond consists of a weak shoulder at 1570 cm⁻¹.

The experimental results suggest that the C₂-H group of the cation forms an H-bond along with the Cl anion which is also connected to the hydrogen atom of the alkyl hydroxyl group of another cation. The other C-H groups (aliphatic and aromatic) seem not to be contributing to H-bonding interactions.

Further, temperature-dependent FT-IR spectra were also measured in the 25–80 °C temperature range (Figure 4). It was observed that no new vibrational band appeared in the 500–4000 cm⁻¹ range but the intensity of spectral band was observed. With an increase in temperature, only the absorbance decreases, but there is no change in the spectral pattern or peak position of each band. It is clear that there is no significant change in intermolecular interaction up to 80 °C as there is no change in the vibration modes involved.

3.3. Raman Studies

In the Raman spectra of EtOHImCl, very strong bands could be found in the region (1500–500 cm⁻¹). In the C-H stretch region, strong bands were also observed. The deconvoluted Raman spectra are also presented in Figure 5. Figure 5 was also fitted by using the nonlinear curve-fitting algorithm. The vibrational wavenumbers in the region 3200–2800 cm⁻¹ dictated the C-H stretching vibrational modes of the imidazolium ring along with the aliphatic C-H of the alkyl chain [70]. In the region 3200–3500 cm⁻¹, the vibrational modes of hydroxyl group and N-H stretching vibration were assigned from the experimental and theoretical study. Therefore, the Raman results provide crucial structural information on EtOHImCl PIL. The alkyl C-H region and imidazolium ring C-H region showed highly Raman and FT-IR active modes for EtOHImCl. Significantly strong signals were observed in the C-H stretching vibrational region due to the strong Raman scattering cross-section of the CH groups. It was found that the C₂-H bond of the cation was able to form an H-bond with the Cl anion in the form of a C₂-H···Cl interaction in a cluster from the theoretical calculation. Therefore, the C₂-H stretching vibrational mode in the imidazolium cation may provide an idea about the existence and strength of inter- and intramolecular H-bonds in EtOHImCl PIL.

However, the interpretation of the C-H region has been discussed in previous studies [71,72]. The C₂-H vibrational band appeared at ~3070 cm⁻¹ for the ion pair of EtOHImCl. The observed vibrational bands at 3110 cm⁻¹ and 3140 cm⁻¹ were attributed to the C₄H and C₅H stretching modes of EtOHImCl. The C₂-H stretching observed at 3070 cm⁻¹ is at a lower wavenumber than C₄/₅H, which was explained by the strong hydrogen bond formation of C₂-H for the ion-pair network. Initially, the Cl anion was located at the top of the EtOHIm cation in the monomeric ion pair but a strong C₂-H⋯Cl interaction was observed in the dimeric ion-pair cluster. Further, the alkyl-OH stretching mode was observed at 3020 cm⁻¹, which is also lower than the C₂-H stretching vibration. A redshift of the alkyl-OH stretching was observed due to the presence of a strong interaction between alkyl OH and the Cl anion in EtOHImCl. A strong stretching vibrational mode of the CH₂ group was observed at 2964 cm⁻¹. Further, aliphatic C-H vibrational modes appeared at 2830–2890 cm⁻¹.

The experimental vibrational mode of the C=C bond (

Table 2. Vibrational band assignment from experimental and theoretical studies.

Band Assignment	Experiment: Raman	Theory: Ion Pair	Dimeric Ion-Pair Cluster
${\nu }_{AS}\left({\mathrm{C}}_{\mathrm{4,5}}\text{-}\mathrm{H}\right)$	3140/3110	3140	3140
$v\left({\mathrm{C}}_2\text{-}\mathrm{H}\right)$	3070	3171	3018
$v\left(\mathrm{O}\text{-}\mathrm{H}\right)$	3020	3125	3178
${\nu }_{AS}\left({\mathrm{N}\text{-}\mathrm{C}\mathrm{H}}_2\right)$	2964	3021	3005
$v(\mathrm{C}=\mathrm{C})$	1567	1590	1579

For the C-H region 3300–2700 cm⁻¹, the C_4,5-H stretching at 3140 cm⁻¹ was selected as a reference peak for scaling the theoretical wavenumbers and the scaling factor was found to be 0.99 [73].

) appeared at 1567 cm⁻¹ and showed that its line shape is symmetric in nature. A new vibration at 1451 cm⁻¹ was assigned to the CH₂ scissor and can be correlated to the 1451 cm⁻¹ mode in the experimental Raman spectrum.

Two rotational isomers of the bmim cation can coexist, namely, one with a trans and the other with a gauche conformation in a n-butyl chain attached to the imidazolium ring [35,74,75]. From the experimental results, the gauche marker band was observed at 601 cm⁻¹, while the weaker trans marker band was observed at 625 cm⁻¹ (Figure 6). From the above experimental results, it was confirmed that the conformation in the EtOHmim cation with the Cl anion only existed as gauche, and this was also confirmed by the DFT calculation (Figure 1 and Figure 2). Figure 7 shows the experimental Raman spectra of EtOHImCl PIL in the region 500–3600 cm⁻¹ along with scaled theoretical Raman spectra at the B3LYP/6-311++G(d,p) level of theory for better understanding [73].

3.4. Natural Bond Orbital (NBO) Analysis

NBO analysis mainly depicts the molecular interaction analysis through second-order Fock matrix calculation. It is carried out for all feasible interactions between filled (donor, i) Lewis-type along with unfilled (acceptor, j) non-Lewis type NBOs [76]. The loss of occupancy occurs for the localized Lewis-type NBOs while the gain of occupancy occurs for non-Lewis orbitals. The second-order Fock matrix calculation (ΔE_ij) is described using the off-diagonal NBO Fock matrix element (F(i,j)), donor orbital occupancy (q_i), and diagonal elements (εj and εi). The selected NBO parameters for the optimized structures of the dimer of the EtOHImCl IL are depicted in

Table 3. Selected NBO parameters for optimized structures of EtOHImCl IL dimer [77]. Atom numbering is shown in Figure S2 in Supporting Information.

NAOs	Donor NBO (i)	Occupancy	NAOs	Acceptor NBO (j)	Occupancy	E(2) kJ/mol
27	n(1)O1	1.96153	106	σ*(1)N21-H34	0.03328	30.00
28	n(2)O1	1.94428	106	σ*(1)N21-H34	0.03328	5.10
30	n(1)O17	1.96158	87	σ*(1)N4-H19	0.03318	29.92
31	n(2)O17	1.94426	87	σ*(1)N4-H19	0.03318	4.94
35	n(3)Cl35	1.95316	93	σ*(1)C11-H12	0.05446	46.19
36	n(4)Cl35	1.87669	93	σ*(1)C11-H12	0.05446	15.90
33	n(1)Cl35	1.99900	98	σ*(1)O17-H18	0.10664	6.69
36	n(4)Cl35	1.87669	98	σ*(1)O17-H18	0.10664	134.31
37	n(1)Cl36	1.99900	79	σ*(1)O1-H2	0.10635	6.65
40	n(4)Cl36	1.87718	79	σ*(1)O1-H2	0.10635	134.01
39	n(3)Cl36	1.95307	112	σ*(1)C28-H29	0.05436	46.40
40	n(4)Cl36	1.87718	112	σ*(1)C28-H29	0.05436	15.61

and in Figure 8.

Interestingly, the NBO parameters nicely show the various inter- and intra-molecular hydrogen bonding interactions in the ion pair of EtOHImCl PIL.

3.5. Topology Analysis

3.5.1. AIM Analysis

AIM analysis provides useful information for a quantitative understanding of intra- and inter-molecular interactions. The analysis uses parameters such as electron density ρ_bcp(r) and the Laplacian of electron density ∇²ρ_bcp (r), and provides information regarding covalent and non-covalent bonding interactions [51,78]. Figure 9 shows the AIM representation of the dimer of EtOHImCl PIL calculated using the structure optimized at the B3LYP/6-311++G(d,p) level of theory whereas the related data are plotted in Figure 10. The IL was stabilized through important H-bonding interactions such as O1-H2···Cl36, C29-H29···Cl36, C11-H12···Cl35, O17-H18···Cl35, N4-H19···O17, and N21-H34···O1. Interestingly, bond critical points (BCPs) were observed between the connected atoms, showing the strength of the H-bonding interaction using ρ_bcp(r) and ∇²ρ_bcp (r). The ρ_bcp(r) values at the BCP between H2···Cl36, H29···Cl36, H12···Cl35, H18···Cl35, H19···O17, and H34···O1 were −0.0078, 0.00112, 0.00111, −0.0078, 0.00232, and 0.00231 a.u., respectively. The corresponding ∇²ρ_bcp (r) values of the dimer of EtOHImCl PIL were 0.07218, 0.06475, 0.06475, 0.07216, 0.09144, and 0.0916 a.u., respectively.

3.5.2. Electron Localized Function (ELF) and Localized Orbital Locator (LOL) Surface Map Analysis

The electron delocalization in ILs is measured using ELF and LOL surface map analysis. The localization of electrons at the atomic and molecular level is understood using ELF and LOL surface maps [79]. ELF determines the possibility of finding the electrons in the molecular system. It provides a visualization of both lone pairs and bond pairs, which are localized close to interacting atoms, and the degree of delocalization of electrons into σ and π components. These functions are an intuitive method to understand the chemical bonds. Figure 11a,b show the ELF and LOL plots for the compound EtOHImCl dimer calculated using the structure optimized at the B3LYP/6-311++G(d,p) level of theory. The related data are shown in Figure 12. Figure 11 shows the diagrams with separate color pallets from blue to red. This corresponds to a range from 0.000 to 1.000 for the ELF plot whereas the range reaches 0.800 for the LOL plot. The maxima of localization of electron pairs are found close to covalent bonds while the presence of non-covalent interactions is understood through the relevant colors. The delocalization of electrons is depicted using colors with a range below 0.5 while electron localization is highlighted using colors with a range above 0.5. Thus, the light blue observed in the plot shows the delocalization of electrons. Thus, the ELF and LOL plots provide a clear picture of the interactions present in the ion pairs of EtOHImCl PIL.

3.6. Non-Covalent Interaction (NCI)

NCI provides a two-dimensional representation of reduced density gradient (RDG) as well as electron density, which is shown in Figure 13a. Figure 13b represents the isosurface extraction of the NCI analysis for the non-covalent interaction present in the dimer of EtOHImCl PIL. The calculation was performed using the optimized geometry at the B3LYP/6-311++G** level of theory. The pallet in Figure 13a describes the color scheme for the plot with a λ₂ range from −0.06 (blue) to 0.06 (red). The blue color shows the most attractive interaction while the red color indicates the most repulsive forces [80]. The green color in the plot corresponds to a λ₂ equal to zero and represents van der Waals interactions. The blue and green colors in the middle of the plot are due to hydrogen bonding interactions. Each spike in the plot originates as a result of a specific interaction. The spike in the NCI plot near the λ₂ value close to 0.03 represents H-bonding interactions.

3.7. Energy Decomposition Analysis (EDA)

A quantitative interpretation of interactions in the dimer of EtOHImCl IL was performed using EDA at the m05-2x/cc-pVDZ level of theory. Interestingly, LMO-EDA calculated the total interaction energy as −0.441350 a.u. and the electrostatic, exchange, repulsion, polarization, and dispersion energies were measured at −1110.6428, −167.15, 576.05, −259.91, and −197.11 kJ/mol, respectively [59]. The electrostatic contribution was found to be nearly five times that of dispersion interaction, which dictates that the IL is mainly stabilized electrostatically. The dispersion component was measured at ~17% of the total interaction energy. The contributing interactions to keep the IL stable were mainly N-H···O, C-H···Cl, and O-H···Cl interactions.

3.8. Ab Initio Molecular Dynamics (AIMD) Assay

Molecular dynamics using the AIMD assay computation was performed at the finite temperature dynamics trajectories to evaluate microscopic mechanics [76,81]. The computation was carried out using forces computed “on the fly” through electronic structure quantum calculation. The AIMD simulations of the dimer of EtOHImCl PIL were performed at an initial temperature of 350 K using a time step of 0.5 fs by applying a Timecon of 10.0 fs. The AIMD study of 500 fs is shown in Figure 14. Figure 14a presents the total energy of IL during the AIMD simulation of the dimer of EtOHImCl PIL, where a maximum energy of −1680.6068 a.u. was observed close to 237 fs. A relative low energy of −1680.6510 a.u. during the AIMD calculation was observed close to 100 fs as well as 400 fs. Figure 14b depicts the drift change during the molecular dynamics calculations. The change in important H-bonding interactions such as O1-H2···Cl36, C29-H29···Cl36, C11-H12···Cl35, and O17-H18···Cl35 during the AIMD simulation is shown in Figure 14c. The distance of H2···Cl36 and H29···Cl36 H-bonding interactions varied between 1.72 and 2.11 Å, whereas the distance of H12···Cl35 and H18···Cl35 interactions varied between 1.95 and 2.56 Å.

4. Conclusions

In this paper, we investigated the structural properties of hydroxyl functionalized 1H-imidazolium Chloride, namely 3-(2-Hydroxyethyl)-1-H-imidazolium chloride (EtOHImCl), by experiments and theoretical results. Here, we are also reporting the effect of the theoretically predicted isolated ion pairs and ion-pair dimers of EtOHImCl on the physical properties of the IL. Moreover, the common C₂-H···anion H-bonding interactions are not observed in the ion pairs of EtOHImCl. In EtOHImCl, the hydroxyl group of the cation significantly contributes to a H-bond with the Cl anion in the ion pair and dimeric ion-pair of EtOHImCl. When two ion pairs interact with each other, they form dimeric ion pairs with highly interesting H-bonding patterns governed by cation–cation interactions. The hydrogen atom of the OH group interacts with the Cl anion via H-bonding interactions. Further, the oxygen atom of the hydroxyl group forms H-bonds along with N-H and the Cl anion which is also connected to the C₂-H protons via H-bonding interactions to establish a distinct dimer, and so on. These center H-bonding arrangements result in a 1D layered molecular structure and promote the stability of the ion-pair dimers. The systematic analytical approach of DFT calculations of isolated ion pairs and dimers has shown its potential for studying the physical chemistry of complex materials. From the second-order perturbation energy calculation, the strength of the O-H···Cl H-bonding interaction was measured at ~133.8 kJ/mol. The AIMD calculation showed that a relative low energy of −1680.6510 a.u. was found close to 100 fs as well as 400 fs.