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