A simplified model of a chemical reaction with pathways for H and D isotopes of hydrogen. The positions on the energy wells are based on the quantum harmonic oscillator. Note the lower energy state of the heavier isotope and the higher energy state of the lighter isotope. Under equilibrium conditions, the heavy isotope is favored in the products as it is more stable. Under kinetic conditions, like an enzymatic reaction, the lighter isotope is favored because of a lower activation energy.