2.1 Density functional theory (DFT) Calculations
All the DFT calculations were carried out using the Gaussian 09 software package.32 The geometries of all complexes and molecules were fully optimized by using the B3LYP density functional at the 6-311+g(d,p) level until the most stable structure without imaginary frequency. Vibrational frequencies of all complexes and molecules were computed to recognize the final obtained geometries reaching a local minima. The Enthalpy change during the processes of absorbent formation and H2S absorption were calculated using the B3LYP density functional at the aug-cc-pvtz level.27 The molecular electrostatic potential (ESP) was applied to predict the most plausible interaction sites of molecules using the Gauss View program.22,33 The parts with more electrostatic potential on the surface of molecules are likely to undergo electrostatic attraction or chemical reaction with electrophilic components. To deeply study the nature of the interaction between different species, especially hydrogen bonds and weak interaction, reduced density gradient (RDG) analysis34 was conducted by Multiwfn and VMD35 software based on the results of the DFT calculation.