Theory of Atom Tunneling Reactions in the Gas Phase

Abstract

Although the study of quantum mechanical tunneling has a long history, the importance of tunneling in chemical reactions has been the subject of misunderstanding. Traditionally, the curvature in an Arrhenius plot of reaction rate constants has been explained by tunneling. However, it is generally very difficult to experimentally observe such clear curvature in Arrhenius plots for reactions in the gas phase. On the other hand, a clear curvature and low temperature limits were observed in many condensed phase reactions at low temperature below 100 K. Nevertheless, it has been theoretically pointed out that tunneling still plays a very important role even for reactions in the gas phase, for which the corresponding Arrhenius plots show nearly linear behavior. For example, Schatz (1987, 1988) showed that tunneling contributes more than 75% of the total rate constants for the simplest H + H₂ → H₂ + H reaction and its isotopic variants even at room temperature, where no significant curvature in Arrhenius plots is seen (cf. Sect. 12.1.1). We should notice that this important conclusion was derived from the accurate three-dimensional quantum reactive scattering calculations on a very accurate potential energy surface. In other words, it is usually difficult to conclude, only from the experimental results, whether tunneling is playing a significant role in the reaction studied. Thus, quantum mechanical theory is very important for understanding the importance of tunneling in chemical reactions in the solid phase.