Among the various NHC catalysts considered, IH has been chosen for detailed substitution studies. N,N-disubstitution with an electron donating (-NH₂) and withdrawing group (-CN) has been done to see how these affect the energetics of the reaction. On comparing the ∆E values of N,N-disubstituted systems with parent system, electron donating –NH₂ has been found to lower the energy barriers (as already observed for IMe) while electron withdrawing -CN increases both ∆E₁ and ∆E₂ ( Table 2). The variation of ∆E₁ in acetylene hydro- thiolation and selenation reactions for INH₂/ICN with respect to IH is caused by quantitatively predominant TS1 stabilization/destabilization (Table 2, 3). WBI and AIM studies were performed on these TSs and the results supports our conclusion of an early TS1 favoring the reaction (Table 4 and supplementary information). Similarly the variation of ∆E₂ in these systems is due to INT’ destabilization/INT stabilization respectively (Table 2, 3). Computed WBI values also support these results (Figure 2).

Effect of a widely studied sterically bulky substituent (Mes) on the mechanism and energetics of the reaction is also analyzed and it was clear that the geometry of INT’ and TS2 are the same as in the case of electron donating groups, with lowered energy barriers as compared to the parent system but higher with respect to IMe and INH₂. Earlier conclusion of early TS nature and much reduced C2-S/Se5 interaction in INT’ (destabilized INT’) substantiate the observed lowering of energy barriers for either steps in both reactions. SE studies are in good agreement with the other electron donating N,N-disubstitution cases.