3.1.1 ESP analysis
The ESP analysis can efficiently determine potential sites of intermolecular interactions for the most stable geometric configuration of a compound from a molecular level. The stable structures of DBU, EG, DBU-EG, DBU-2EG and H2S and their corresponding electrostatic potential distributions are conducted as shown in Figure 1(b). In which the blue and red colors represent positive and negative electrostatic potentials in the colorimetric band, respectively.22,36 In the ESP map of DBU, EG and H2S, the region of nitrogen atoms (N), oxygen atom (O) and sulfur atom (S) exhibits a local negative electrostatic potential (see Figure 1(b)), respectively, while the hydrogen atom (H) attached with O in EG or H in H2S displays a local positive electrostatic potential. When DBU and EG interact to form DBU-EG or DBU-2EG multimer absorbents, O and N exhibit more local negative electrostatic potential, and H attached with O and N displays more local positive electrostatic potential in the ESP map (red and blue in those molecules deeper than DBU and EG). There are two types of N in the DBU molecule, which can interact with two EG to form a trimer molecule. Hirshfeld charge of DBU, EG, H2S, DBU-EG and DBU-2EG are shown in Table S1, the N, O and H charge values changed more negatively and positively in DBU-EG and DBU-2EG relative to DBU and EG, respectively. Therefore, it can be deduced that four potential interaction sites exist between DBU-2EG and H2S. In literature, DBU was equimolar interacts with monoalcohols.29-31 However, the interaction of two moles EG and one mole DBU hasn’t been explored yet.