**Abstract**

The calix[4]arenes (abbreviated as CX[4]) are characterized by a specific hydrophobic cavity formed by a four cyclically phenol groups to encapsulate a gas or small molecules. Recently, the CX[4] molecule is used in a specific media and in pharmaceutical drug delivery. The pollution problem will be a vital subject in the future because the increase of the explosions of the gaseous pollutants in the environment. In this report, we have encapsulated the polluting gases NO3, NO2, CO2 and N2 by the calix[4]arene molecule. In this work, The binding energies of the CX[4]-gas has been calculated including the BSSE (Basis Set Superposition Error) counterpoise (CP). The red-shift of the O-H bonding interactions obtained by adding the gas in the sensitive area of calix[4]arene is clearly explained by the infrared spectrum analysis. The Molecular electrostatic potential (MEP) of the stable CX[4]-gas complexes have been investigated in the endo-vs. exo-cavity regions. Finally, the non-covalent interactions analyses of the stable host-guests complexes have been estimated by using DFT calculations.

**Keywords:** complexation, specific gases, H-bonding, molecular electrostatic potential and binding energy

## **1. Introduction**

The recognition of the electrostatic and magnetic properties in the selective guest in microscopic systems facilitates to know several anionic cationic or neutral guests' complexes [1–5]. In this work, the CX[4] molecule is specified by its own chemical composition and the hydrophobic cavity form [6]. This molecule is characterized by specific parameters such as the diameter and the height, which facilitates the encapsulation of the cationic, anionic, neutral guests or small molecules [7–9]. Also, the cavity of the CX[4] molecule have attracted the researchers to test a new guest materials to be functional in the medical [10–12] or micro-biological field [13, 14]. In literature, we have noted that there are some works which discuss the encapsulation of the CX[4] with gases molecules (CH4, NH3 and C2H2) [15–18]. In our work, we have studied the photo-physical properties of the CX[4]-gas complexes (CX[4]-NO3, CX[4]-NO2, CX[4]-CO2 and CX[4]-N2). We discuss the

possibility of the encapsulation of these gases by the CX[4] molecule to show the sensibility of this molecule to the polluting gases outside or inside the cavity. We have chosen the NO3, NO2, CO2 and N2 gas as guest's because they can be formed a dipole-dipole interactions and a CH…π hydrogen-bonding with the CX[4] molecule. By using the density functional theory (DFT) calculations, we have described the dynamic stabilities of the endo-vs. exo-cavity of the CX[4]-gas complexes. The vibrational properties of the CX[4]-gas complexes have been studied. The Molecular electrostatic potential studies of these host-guests complexes have been performed. The Non-covalent interaction via RDG function are very important to know the nature of the interactions between the specific guests and the CX[4] molecule.
