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

*Advances in Quantum Communication and Information*

**100**

**Figure 2.**

*[BMIM]+*

*Optimized structures of [BMIM]+*

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

*pink, boron; brown, phosphorous; and dark yellow, sulfur; orange, (3,−1) bond critical point.*

*(in orange). Color code: white, hydrogen; red, oxygen; yellow, carbon; royal blue, nitrogen; light blue, fluorine;* 

*[BF4]<sup>−</sup> and estrone (c) complexes at the B3LYP/6-31G(d) level containing (3,−1) bond critical points* 

*[PF6]<sup>−</sup> and estrone (b), and* 

(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 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 to determine the nature of interactions.

This topological study is used as a validation tool to justify the proximity of estrone in various cases. Firstly, on introduction of [BMIM]<sup>+</sup> [NTF2]<sup>−</sup>, we can see 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 to [BMIM]<sup>+</sup> [PF6]<sup>−</sup>, akin to the first case, most interactions are hydrogen-bonded 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 of [BMIM]+ [BF4]<sup>−</sup>, all intermolecular interactions are F-bonded interactions which simply arise out of difference in electronegativities and, hence, are completely noncovalent interactions.

Moreover, all interactions were found to have positive Laplacian of electron density at the intermolecular critical points (in the range of 0.01eÅ<sup>−</sup><sup>5</sup> ), which suggests closed-shell interaction nature. All the three cases reveal that intermolecular interactions are completely non-covalent in nature in conjunction with the observation made on assessing the optimized structure.
