text: In some cases, rates of reaction decrease with increasing temperature. When following an approximately exponential relationship so the rate constant can still be fit to an Arrhenius expression, this results in a negative value of E.
  Elementary reactions exhibiting negative activation energies are typically barrierless reactions, in which the reaction proceeding relies on the capture of the molecules in a potential well. Increasing the temperature leads to a reduced probability of the colliding molecules capturing one another (with more glancing collisions not leading to reaction as the higher momentum carries the colliding particles out of the potential well), expressed as a reaction cross section that decreases with increasing temperature. Such a situation no longer leads itself to direct interpretations as the height of a potential barrier.
  Some multistep reactions can also have apparent negative activation energies. For example, the overall rate constant k for a two-step reaction A  B, B → C is given by k = kK, where k is the rate constant of the rate-limiting slow second step and K is the equilibrium constant of the rapid first step. In some reactions, K decreases with temperature more rapidly than k increases, so that k actually decreases with temperature corresponding to a negative observed activation energy.
  An example is the oxidation of nitric oxide which is a termolecular reaction . The rate law is  with a negative activation energy. This is explained by the two-step mechanism:  and .
  Certain cationic polymerization reactions have negative activation energies so that the rate decreases with temperature. For chain-growth polymerization, the overall activation energy is , where i, p and t refer respectively to initiation, propagation and termination steps. The propagation step normally has a very small activation energy, so that the overall value is negative if the activation energy for termination is larger than that for initiation. The normal range of overall activation energies for cationic polymerization varies from .