HIGH ENERGY REACTIONS OF ATOMIC HYDROGEN

The chemistry of hot hydrogen atoms has been studied using tritium of high kinetic energy as produced by nuclear recoil. The possibilities and limitations of this technique are discussed using a collision theory for reactions of atoms having a very high initial energy. Using this theory and certain experimental data, it is concluded that hot hydrogen atoms react to combine with organic molecules at very high collision efficiency (of the order of approximately 0.2 to 0.4) in the energy range 3–10 ev. There is no indication that collisions at much higher energies lead to combination. With most systems, e.g. alkanes, a wide variety of reactions is observed. The systematics of these hot reactions is discussed and evidence on their detailed mechanism is presented. It appears that most products are formed by a fast displacement of an atom or group by the hot hydrogen. There is no evidence for the formation of a common, internally equilibrated, collision complex which decays on a statistically determined basis to the various products. Instead, the course of the reactions seems largely governed by the direction and point of impact of the hot atom. Thus, stereochemical evidence indicates that axial approach of the hot hydrogen atom along the C—H bond axis leads to abstraction while approach at large angles to this axis results in displacement without Walden inversion. In some cases sufficient excitation energy is introduced in the hot displacement process to cause further decomposition of the primary product. This model of high-energy reactions is compared with that of thermal reactions and its general implications are briefly discussed.