Fascinating Aspects of Tunneling Reactions in Organic Substances

Abstract

The tunneling reaction, which is caused by the wave character of an atom or a molecule, has revealed interesting aspects in chemistry. The fascinating subjects of hydrogen-atom or hydrogen-molecule tunneling will be described in this chapter. The first subject is a highly selective reaction by use of a tunneling reaction. A selective reaction is one of the significant targets in chemical synthesis in order to obtain only one desirable product. In general we achieve the selective reaction by use of a catalyst, which changes the barrier height of the reaction (cf. Sect. 4.4). Highly selective tunneling reactions were found in solid alkane at low temperature. The selective reaction is due to the following intrinsic properties of a tunneling reaction. First, a tunneling reaction at low temperature is significantly affected by the small interaction energy with surrounding conditions around the reactants (cf Sect. 4.5). Thus, a small change of the surrounding conditions selects only one reaction among several types of tunneling reactions. Second, a tunneling reaction takes place at low temperatures, at which several competing classical reactions that take place by passing thermally over the barriers for the reactions are completely suppressed. Very selective tunneling reactions by H atoms were discovered 30 years ago by Miyazaki in alkane mixtures at 77 K (Miyazaki and Hirayama 1975). Since a large number of studies were needed in order to eliminate other possibilities and establish the selective tunneling reaction, the process up to the conclusion of the selective tunneling reaction will be described in detail in Sect. 7.2. The selective tunneling reaction by H atoms is observed also in the same alkane. The H atoms react selectively with one type of C—H bond in the same molecule, which consists of several types of C—H bonds (cf Sect. 7.3). A new model is proposed for the causes of selective reactions: the local motion of reactants in addition to the difference in the barrier heights for the reactions.