Computational methods
The reaction points of the catalytic pathway has been characterized using molecular orbital theory program package, Gaussian 0932 and the visualization tool, Gauss view 5.0.33 The gas phase reaction mechanism and energetics of reaction involving acetylene (HCCH) and chalcogenol (MeSH/MeSeH) catalyzed by selected NHCs have been investigated at M06-2X/aug-cc-pVTZ//B3LYP/6-31+G(d) level. 34-36Initial characterizations were done using B3LYP, a popular hybrid GGA functional. The wide spread use of this functional in exploring various catalytic mechanisms have prompted us to select this functional.37-41 Single point calculations were performed using M06-2X, which is a highly parametrized functional known to give good performance for main group thermochemistry, kinetics and non-covalent interactions along with a correlation-consistent basis set.42 Regio- and stereoselectivity of the reaction were studied by employing propyne (MeCCH). The computed transition structures were confirmed by Intrinsic Reaction Coordinate (IRC) analysis. Natural Bond Orbital (NBO) and Bader’s Quantum Theory of Atoms in Molecules (QTAIM) analyses were performed to analyze the nature of bonding.43-44 Solvent effects were taken into account using the SMD solvation model with solvents - tetrahydrofuran (THF), dimethylsulfoxide (DMSO) and methanol (MeOH) as the dielectric continuums. SMD is a widely used solvation model and can be applied to any charged or uncharged solute in any solvent or liquid medium.45 Energetic span model is applied to determine the rate-determining states and energetic span of various NHC catalyzed pathways employing AUTOF program. 46 Energetic span (δE) is calculated as the energy difference between the TDI (rate -determining intermediate) and the TDTS (rate -determining transition state), i.e., δE= TTDTS - ITDI, if TDTS appears after TDI and TTDTS - ITDI + ∆Gr, if TDTS appears before TDI.
Different NHCs employed in the study and their abbreviations are listed in Table 1