Experimentally, the nucleophilic substitution to the carbon center could proceed through two different mechanisms: concerted or stepwise. Our present study focuses on the mutual interaction of the substituents in the nucleophile and substrate. The concerted single step substitution reaction shows a small negative ρ_XY value. In this work (Scheme 1), the reaction mechanism on the pyridinolysis of phenyl carbonyl isothiocyanates is investigated in the gas phase and acetonitrile solvent using the CPCM approach at the B3LYP/6-311+G(d,p) level of theory. We optimized all the reactant complexes, transition states, and product complexes to construct the potential energy surfaces (PES) for six different substituents and confirmed by frequency calculations. The reaction mechanism is discussed with Hammett constants, ρ_X, ρ_Y, β_X, cross interaction constant (CIC) ρ_XY, NBO charges and structure-energy relationships. Orbital interactions and the stereo-electronic effect between the nucleophile and the substrate are also considered to confirm the reaction mechanism.