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.