**3. Results and discussion**

#### **3.1 Optimized structures of the complex at the B3LYP/6-31G(d) level**

Optimized structures of estrone/ionic liquid complexes are depicted in **Figure 1**. Estrone was found closer to the anion species, especially just near the electronegative atoms having most negative partial charges in all the three cases. For instance, in the case of [BMIM]<sup>+</sup> [NTF2]<sup>−</sup> and estrone, apart from O▬H and F▬H interactions which are visible from the proximity of the groups at the optimized structure, it can be seen that there is a substantial difference in the partial atomic charges of S(1.125e) and C(−0.394e) at one end followed by partial atomic charges 0.6e and −0.49e on the carbon-bearing fluoride and hydrogen atoms on [NTF2]<sup>−</sup> and estrone, respectively. While in the other case such as [BMIM]<sup>+</sup> [BF4]<sup>−</sup> and estrone, the vicinity of the bromine and fluoride is not justified by oppositely polarized moieties in estrone, however, also not indicative of an open-shell or covalent interaction as well simply because all the outer shell electrons of the atoms involved

#### **Figure 1.**

*Optimized structures of [BMIM]+ [NTF2]<sup>−</sup> and estrone (a), [BMIM]+ [BF4]<sup>−</sup> and estrone (b), and [BMIM]+ [PF6]<sup>−</sup> and estrone (c) complexes at the B3LYP/6-31G(d) level.*

**99**

**Table 1.**

*DFT Study on Interaction of Estrone and Imidazolium-Based Hydrophobic Ionic Liquids*

are already satisfied while, at the same time, not providing any window of opportunity for extending the coordination due to the absence of d-type orbitals in their individual atomic structure. These results suggest that electrostatic interactions and other closed-shell interactions between atoms of complimentary polarities are predominant when compared to the effects of covalent interactions and π cloud

The schematic representation for complex formation is given by Eq. (1):

*A* + *B* → *A*⋯*B* (1)

in which A, B, and A⋯B represent the estrone, ionic liquid, and estrone/ionic

Interaction energy calculations give a numerical estimate of the ability of the EDC to interact with the ionic liquid by means of chemisorptions occurring at the

∆*Eint* = *EA*⋯*<sup>B</sup>* − [*EA* + *EB*] (2)

Negative values indicate good interaction behavior of estrone in the presence

system bind the complex with stronger interaction strength contributing to the

These interaction energy values are tabulated in **Table 1**. It is to be noted that the

[PF6]<sup>−</sup> and estrone

**(Hartrees) (kcal/mol)**

−0.012152 −7.63

−0.015246 −9.57

−0.013718 −8.61

**System Interaction energy**

[NTF2]<sup>−</sup> and estrone

> [PF6]<sup>−</sup> and estrone

> [BF4]<sup>−</sup> and estrone

*DOI: http://dx.doi.org/10.5772/intechopen.86821*

effects contributed via imidazolium ring.

molecular level. These values are determined by Eq. (2):

values presented here include the zero-point correction.

highest negative binding energy of −9.57 kcal/mol.

of the ionic liquid. In comparison, it can be seen that [BMIM]+

**energy (Hartrees)**

[NTF2]<sup>−</sup> −2250.231194 [BMIM]+

[PF6]<sup>−</sup> −1363.714993 [BMIM]+

−3099.500301

−2212.987194

−1696.858564

*Interaction energy of various estrone/ionic liquid systems in Hartrees and kcal/mol.*

*Total electronic energy of various species in Hartrees is also provided for reference, all results obtained at the* 

Estrone −849.256955 [BMIM]+

[BF4]<sup>−</sup> −847.587891

*\*Denotes addition of (d) polarizable function to the 6-31G Basis set.*

**3.2 Interaction energy calculations**

liquid complex, respectively.

**Model chemistry: B3LYP/6-31G\***

[BMIM]+

[BMIM]+

[BMIM]+

[BMIM]+

[BMIM]+

[BMIM]+

*B3LYP/6-31G(d) level.*

estrone

estrone

and estrone

[NTF2]<sup>−</sup>

[PF6]<sup>−</sup> and

[BF4]<sup>−</sup> and

**Molecule/system Total electronic** 

*DFT Study on Interaction of Estrone and Imidazolium-Based Hydrophobic Ionic Liquids DOI: http://dx.doi.org/10.5772/intechopen.86821*

are already satisfied while, at the same time, not providing any window of opportunity for extending the coordination due to the absence of d-type orbitals in their individual atomic structure. These results suggest that electrostatic interactions and other closed-shell interactions between atoms of complimentary polarities are predominant when compared to the effects of covalent interactions and π cloud effects contributed via imidazolium ring.

#### **3.2 Interaction energy calculations**

*Advances in Quantum Communication and Information*

**98**

**Figure 1.**

*[BMIM]+*

*Optimized structures of [BMIM]+*

*[NTF2]<sup>−</sup> and estrone (a), [BMIM]+*

*[PF6]<sup>−</sup> and estrone (c) complexes at the B3LYP/6-31G(d) level.*

*[BF4]<sup>−</sup> and estrone (b), and* 

The schematic representation for complex formation is given by Eq. (1):

$$A \star B \rightarrow A \cdots B \tag{1}$$

in which A, B, and A⋯B represent the estrone, ionic liquid, and estrone/ionic liquid complex, respectively.

Interaction energy calculations give a numerical estimate of the ability of the EDC to interact with the ionic liquid by means of chemisorptions occurring at the molecular level. These values are determined by Eq. (2):

$$
\Delta E\_{\text{int}} = E\_{A\dots B} - \left[ E\_A + E\_B \right] \tag{2}
$$

These interaction energy values are tabulated in **Table 1**. It is to be noted that the values presented here include the zero-point correction.

Negative values indicate good interaction behavior of estrone in the presence of the ionic liquid. In comparison, it can be seen that [BMIM]+ [PF6]<sup>−</sup> and estrone system bind the complex with stronger interaction strength contributing to the highest negative binding energy of −9.57 kcal/mol.


*Total electronic energy of various species in Hartrees is also provided for reference, all results obtained at the B3LYP/6-31G(d) level. \*Denotes addition of (d) polarizable function to the 6-31G Basis set.*

#### **Table 1.**

*Interaction energy of various estrone/ionic liquid systems in Hartrees and kcal/mol.*

#### **Figure 2.**

*Optimized structures of [BMIM]+ [NTF2]<sup>−</sup> and estrone (a), [BMIM]+ [PF6]<sup>−</sup> and estrone (b), and [BMIM]+ [BF4]<sup>−</sup> and estrone (c) complexes at the B3LYP/6-31G(d) level containing (3,−1) bond critical points (in orange). Color code: white, hydrogen; red, oxygen; yellow, carbon; royal blue, nitrogen; light blue, fluorine; pink, boron; brown, phosphorous; and dark yellow, sulfur; orange, (3,−1) bond critical point.*

**101**

M]<sup>+</sup>

*DFT Study on Interaction of Estrone and Imidazolium-Based Hydrophobic Ionic Liquids*

interacting nature can alternatively be described by examining the ratio of Lagrangian kinetic energy (G(rc)) and potential energy density (V(rc)) given by [−G(rc)/V(rc)] [15]. The aforementioned ratio at (3,−1) bond critical points is used

estrone in various cases. Firstly, on introduction of [BMIM]<sup>+</sup>

(3,−1) Bond critical points (BCPs) obtained from atoms in molecule (AIM) analysis are presented as the representation index of the electronic interaction and distribution between a given bond pair in this study. Multiwfn software is used to calculate the topological properties of bond critical points and perform complete AIM analysis [9]. **Figure 2** gives a visual representation of (3,−1) BCPs of estrone/ ionic liquid complexes. Previous literature studies proposed that the non-covalent

This topological study is used as a validation tool to justify the proximity of

multiple hydrogen-bonded interactions with O and F atoms, evident with the discussion provided in the first section. Since these are hydrogen-bonded interactions and, hence, clearly non-covalent, other interactions such as the C▬F interaction between estrone carbon and F atom on [NTF2]<sup>−</sup> are investigated. At this intermolecular critical point, we have G(rc) and V(rc) values of 0.003604 and −0.002431, respectively, clearly having a ratio greater than 1. In the second case, pertaining

interactions apart from the C-H interactions between the cation of the ionic liquid and hydrogen of the EDC. The two critical points obtained bear G(rc) and V(rc) values of 0.002434, 0.002517 and −0.001707, −0.001741, respectively. In the case

simply arise out of difference in electronegativities and, hence, are completely non-

Moreover, all interactions were found to have positive Laplacian of electron

gests closed-shell interaction nature. All the three cases reveal that intermolecular interactions are completely non-covalent in nature in conjunction with the observa-

The interaction behavior of (1:1) estrone and imidazolium-based hydropho-

was studied at the B3LYP/6-31G(d) level of theory. The optimized structures were presented to study the vicinity of EDC with respect to each ionic liquid. The ZPE-corrected binding energy values were found to be negative, indicating fruitful interaction of the EDC and ionic liquid species. Based on the interaction strength, the affinity of estrone on ionic liquids can be described as [BMI

to find the nature of interactions, so as to get the electronic distribution at the intermolecular region. It was found out that all interactions were characterized by positive Laplacian of electron density and −G(rc)/V(rc) > 1 at intermolecular critical points illuminating the non-covalent nature of interactions existing

[BF4]<sup>−</sup> > [BMIM]<sup>+</sup>

[NTF2]<sup>−</sup>, [BMIM]<sup>+</sup>

density at the intermolecular critical points (in the range of 0.01eÅ<sup>−</sup><sup>5</sup>

tion made on assessing the optimized structure.

bic ionic liquids such as [BMIM]<sup>+</sup>

[PF6]<sup>−</sup> > [BMIM]<sup>+</sup>

between EDC and ionic liquids.

[PF6]<sup>−</sup>, akin to the first case, most interactions are hydrogen-bonded

[BF4]<sup>−</sup>, all intermolecular interactions are F-bonded interactions which

[NTF2]<sup>−</sup>, we can see

), which sug-

[BF4]<sup>−</sup>

[PF6]<sup>−</sup>, and [BMIM]<sup>+</sup>

[NTF2]<sup>−</sup>. AIM analYsis was carried out

*DOI: http://dx.doi.org/10.5772/intechopen.86821*

**3.3 Atoms in molecule (AIM) analysis**

to determine the nature of interactions.

to [BMIM]<sup>+</sup>

of [BMIM]+

covalent interactions.

**4. Conclusions**
