According to ESP profile analysis, in Figure 2(a), there is a strong interaction between DBU and EG, the distance of 35H and 21N in 34O−35H···21N configuration is 1.704 Å. Surprisingly, there exist two type configurations with different strengths between DBU and EG (see Figure 2(d)): type I is 34O−35H···21N configuration with 1.718 Å distance between 35H and 21N, and type II is 46O−47H···17N configuration with 2.036 Å between 47H and 17N, indicating that the intermolecular interaction of type I is stronger than that of type II. Meanwhile, combining ESP analysis, these two configurations correspond to four possible interaction sites O for H₂S absorption due to the influence of force. In Figure 2(b), one spike appeared at the region of sign (λ ₂)ρ < -0.02, indicating that there exists an H-bond,³⁵corresponding to the blue circle at the H of -OH in EG and the

in DBU (Figure 2(c)). It is worth noting in Figure 2(e) that two spikes appeared at the region of sign (λ ₂)ρ< -0.02, meaning two H-bonds with different strengths, and gradient isosurfaces information of a blue circle and a cake appear between H of -OH in EG and two Ns of in DBU (Figure 2(f)). The spikes in the region of sign (λ ₂)ρ ≈ 0 in Figure 2(b) and e as well as the green region in the gradient isosurfaces in Figure 2(c) and f mean the vdW force interaction. The calculated Enthalpy change of forming DBU-EG and DBU-2EG by DBU and EG are -10.3 and 24.9 kJ/mol (the calculation of Enthalpy change is given in Supplementary 2), respectively. Therefore, the interaction between DBU and EG can form a strong H-bond when the molar ratio is 1:1, and double H- bonds with different strengths can be formed when the molar ratio is 1:2.

As shown in Figure 2(g) and (j), the H₂S were positioned at different orientations of the absorbent under the electrostatic interaction causative by H-bond from the DBU-EG and DBU-2EG. In Figure 2(h), there are two new spikes appeared at the region of sign (λ ₂)ρ < and ≈-0.02, indicating two H-bonds of different strength occurs in DBU-EG, corresponding to the blue disc appears between 34O and 39H, and the iron blue disc between 36O and 42H in gradient isosurface map in Figure 2(i), respectively. Meanwhile, in Figure 2(k), a new spike appeared at sign (λ ₂) ρ < and ≈ -0.02 compared to Figure 2(h), indicating that there are four H-bond with different strengths formed between DBU-2EG multimer absorbent and H₂S corresponding to the blue disc appearing between 34O and 55H, and the iron blue disc between 36O and 58H, 46O and 53H, 44O and 50H in Figure 2(l), but the strength is relatively weak compared to DBU-2EG. Therefore, RDG and gradient isosurface map information analysis are consistent with the speculative result of ESP. The calculated Enthalpy change of H₂S absorption by DBU-2EG multimer absorbent is -24.1 kJ/mol (the calculation of Enthalpy change is given in Supplementary 2). However, two H-bonds are formed between DBU-EG and H₂S, meaning that the binding capacity of DBU-EG to H₂S is weaker than that of DBU-2EG. While in Figure 2(k), the H-bonds in DBU-2EG-4H₂S are slightly moved close to sign (λ ₂)ρ ≈ 0 relatives to DBU-2EG, indicating that the H-bonds in DBU-2EG become weak after the absorption H₂S through O attracting H in the absorption process. In addition, the vdW force has become stronger between DBU-2EG-4H₂S compared to DBU-EG-2H₂S, corresponding to the green region in Figure 2(i) and (l). Therefore, different from DBU-EG, the double H-bonds DBU-2EG is more beneficial for the absorption of H₂S.