ABSTRACT Shephard’s interpolation potential energy surface and isotopic substitution were used to understand the mechanisms of chemical reactions; the NH + C2 reaction potential energy surface was constructed in this research and evaluated using ab-initio calculations. Classical trajectory simulations were used to calculate the reaction probability and cross-section. In addition, the rate constant by the nonlinear least-squares fitting was calculated. These were determined as a function of cross-section. In non-reactive trajectories for inelastic collisions, the scattering angle was estimated. The effects of the impact parameter and the relative velocity of particles on the reaction probability, transmitted energy, and dispersion angle were investigated. In addition, Isotopic substitution in the NH radical was evaluated for the effect of the mass of the target and incident particles on the cross-section, reaction probability, dispersion angle, and reaction rate. To calculate the kinetic isotope effect, the influence of the mass of colliding molecules on the observable properties was studied. Deuterated imidogen reacted faster than hydrogenated imidogen. These observations provided valuable insights into the details of the mechanisms, possible reactions of ND and NH + C2 , and deuterium fractionation.