Characterized pathway and energetics discussion
The computed pathway unraveled for acetylene hydrothiolation reaction (Figure 1) proceeded through an initial reaction complex (RC) stabilized by 6.22 kcal/mol with respect to the separated reactants (IH and MeSH). Abstraction of thiolic hydrogen by IH is then followed through transition structure1 (TS1). The energy barrier for this first step (∆E1) is computed to be 9.53 kcal/mol. After thiol activation, acetylene insertion proceeds through an intermediate (INT, Figure 1) which is characterized by having a cyclic interaction between acetylene, thiolate ion (MeS-) and imidazolium ion (IH(H)+). The notable interaction was the C2-S5 bonding interaction (WBI of C2-S5 bond=0.840) which obviously caused pyramidalization at the carbene carbon (C2). Finally, the desired product is generated via simultaneous addition of MeS-and imidazolium hydrogen to each of the acetylenic carbon atoms through TS2 (∆E2 = 33.06 kcal/mol) (Figure 1). IRC calculation confirmed this C2-S5 interaction in TS2 as well. Free energy of activation, ∆G1 and ∆G2 for the two steps had been calculated to be 7.47 and 29.99 kcal/mol respectively and the trend remained the same as the ∆E calculations. From the whole catalytic pathway it was clear that the acetylene addition corresponded to the highest energy barrier step. Kinetic information studies using AUTOF program identified INT as TDI and TS2 as TDTS and the calculated energetic span (δE) for the reaction was 29.99 kcal/mol. When compared with the reported (∆G) values for uncatalyzed gas phase acetylene hydrothiolation reaction (49.61 kcal/mol), it was clear that NHC lowered this free energy barrier by 19.62 kcal/mol. 47