Contour plot of the potential energy surfaces describing reactions X + H₂ → XH + Η (with X = C, N, O, and S) as a function of r, the H₂ internuclear distance, and R, the distance between the Ν and H₂ center-of-mass in a collinear H–H–X configuration. Contours correspond to the vibrational energies of H₂ from ν = 0 to 7. The thin green line shows the saddle point energy contour. The thick black arrow is a graphical representation of the reaction coordinate at the top of the barrier.