NO(g)+NOX3(g)2NOX2(g) \ce{NO(g) + NO3(g) -> 2NO2(g)} rate=k[NO][NOX3] \text{rate} = k[\ce{NO}][\ce{NO3}]

The reaction represented above occurs in a single step that involves the collision between a particle of NO\ce{NO} and a particle of NOX3\ce{NO3}. A scientist correctly calculates the rate of collisions between NO\ce{NO} and NOX3\ce{NO3} that have sufficient energy to overcome the activation energy. The observed reaction rate is only a small fraction of the calculated collision rate. Which of the following best explains the discrepancy?