Phenomenological manifestations of quantum-mechanical tunneling. II. Effect on Arrhenius pre-exponential factors for primary hydrogen kinetic isotope effects

The effect of quantum-mechanical tunneling on the Arrhenius pre-exponential factor AQ in reactions involving primary protium-deuterium kinetic isotope effects is investigated as a function of temperature. It is shown that large tunneling factors can either raise or lower AQ considerably or leave it essentially unchanged from its value in the absence of tunneling, depending on the details of the reaction system involved and on the temperature range under consideration. It is concluded that experimental AQ factors can be used as quantitative indicators of quantum-mechanical tunneling only in conjunction with model calculations. Experimental values of AQ inside the ``no-tunneling'' range, 2−1 ≤ AQ ≤ √ 2, do not necessarily indicate the absence of large tunneling factors.