Figure 1 Energy profile diagram for IH catalyzed acetylene hydrothiolation reaction. Here zero energy value is assigned to the sum of energy of all 5 moieties present in the catalytic cycle viz., IH, IH(H)+, HCCH, MeSH and MeS-; PC indicates the product complex having stabilizing interaction between IH and vinyl sulfide.
A similar pathway for IH catalyzed acetylene hydroselenation reaction has also been unraveled. Characterized reaction points of the pathway showed only some minute geometric changes, inherent to switching methanethiol with methaneselenol. ∆G1 and ∆G2 for the two steps has been calculated to be 5.08 and 28.50 kcal/mol respectively and the trend remains the same as the ∆E calculations. As in the case of hydrothiolation, it was clear that the acetylene addition corresponded to the highest energy barrier step. Kinetic information studies identified INT and TS2 as TDI and TDTS respectively and the calculated energetic span (δE) for the reaction was 28.50 kcal/mol. By comparing with the reported ∆G value of uncatalyzed gas phase reaction (43.13 kcal/mol), it was clear that NHC lowers the free energy barrier by 14.63 kcal/mol. 48
Acetylene hydroselenation reaction is more facile as the energy barriers for both steps 1 and 2 were lowered by 5.40 and 4.40 kcal/mol respectively in comparison to acetylene hydrothiolation (Table 2). To precisely figure out the factor causing barrier lowering, stabilization energy (SE) of reaction points with respect to the corresponding reactants is computed. SE of TS1 for hydrothiolation and hydroselenation were respectively 3.32 and -0.95 kcal/mol, while that for RC were -6.22 and -5.07 kcal/mol respectively. Similarly computed SE values for TS2 were -87.34 and -82.14 kcal/mol and that for INT were -120.39 and -110.79 kcal/mol respectively (Table 3). Thus it is evident that the more TS1 stabilization in the first step and relatively less INT stabilization in the second step are the predominant factors that determine the lowering of corresponding energy barriers in hydroselenation.
Natural charge studies revealed that more stabilized hydroselenation TS1 was found with low positive charges on C2 and H4 and low negative charge on Se5 in comparison to hydrothiolation TS1(Table 4). Wiberg bond index (WBI) values suggested that in hydroselenation TS1, the breakage of Se5-H4 bond and formation of C2-H4 bond happen to a lesser extent indicating an early TS in comparison to hydrothiolation (Hydroselenation TS1: WBI of Se5-H4= 0.515 and C2-H4= 0.442; Hydrothiolation TS1 WBI of S5-H4= 0.404 and C2-H4= 0.533). AIM studies showed the laplacian of the electron density, ∇2ρ, -0.026 and 0.027 for Se-H and S-H bond respectively in TS1, indicating the prevalence of a covalent nature in hydroselenation TS1. Thus the computed natural charges and WBI values suggested that the hydroselenation TS1 with geometry close to the reactants proceeds with lower energy barrier. Comparison of reduced natural charges on INT in hydrothiolation and hydroselenation cases do not shed much light into the destabilization of hydroselenation INT (Table, supplementary information). However the WBI values revealed that C2-Se5 (WBI= 0.792) interaction in INT was weaker than C2-S5 interaction (WBI= 0.840) (Figure 2). Weaker C2-Se5 interaction implies destabilization of INT in hydroselenation in comparison to hydrothiolation, thus substantiating lower ∆E2value.