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