Reactions of diatomic molecules

In this section, we review the comparitively small field of energy disposal in the elementary bimolecular reactions of diatomic molecules with other diatomic or polyatomic species. Because of the smaller number of reactions studied, they are summarised in a single table (Table 9). 

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In section 3.1, reactions of diatomic molecules with metal surfaces are discussed. These studies, although perhaps not sufficiently complicated to directly address processes of technological interest, have produced considerable insight into the dynamics of gas-surface reactions. Simulations of metal surfaces where more i istic interactions are required than are used in the gas-surface studies are presented in section 3.2. This is followed in section 3.3 by a discussion of simulations of reactions on the surfaces of covalently bonded solids. These final studies are particularly suited for addressing technologically relevant processes due to the importance of semiconductor technology. 

The chemical reaction is the most chemical event. The first application of symmetry considerations to chemical reactions can be attributed to Wigner and Witmer [2], The Wigner-Witmer rules are concerned with the conservation of spin and orbital angular momentum in the reaction of diatomic molecules. Although symmetry is not explicitly mentioned, it is present implicitly in the principle of conservation of orbital angular momentum. It was Emmy Noether (1882-1935), a German mathematician, who established that there was a one-to-one correspondence between symmetry and the different conservation laws [3, 4],... 

The maximum hardness principle also demands that hardness will be minimum at the transition state. This has been found to be true for different processes including inversion of NH3 [147] and PH3 [148], intramolecular proton transfer [147], internal rotations [149], dissociation reactions for diatomics [150,151], and hydrogen-bonded complexes [152]. In all these processes, chemical potential remains either constant or passes through an extremum at the transition state. The maximum hardness principle has also been found to be true (a local maximum in hardness profile) for stable intermediate, which shows a local minimum on the potential energy surface [150]. The energy change in the dissociation reaction of diatomic molecules does not pass through a... 

The thermal dissociation and recombination reactions of diatomic molecules in the gas phase are important for several reasons (1) they may be the initiation and termination steps for complex reaction mechanisms,... 

The homogeneous gas phase exchange reactions of diatomic molecules are described by activation energies that are much lower than the decomposition activation energies. The finding is that isotopic equilibrium is reached under reaction conditions in which the contributions from three centre atom—molecule reactions are negligible. 

The relative importance of vibrational and translational energy in promoting chemical reactions is of both theoretical and practical interest. In reactions of diatomic molecules with atoms it has been substantiated both experimentally and theoretically that for endothermic reactions vibrational energy is more important, while for exothermic reactions the opposite is true. For polyatomic molecules, however, there is insufficient experimental and theoretical evidence to draw conclusions. The major work on laser-excited polyatomic reactions has involved the vibrational excitation of ozone in its exothermic reaction with nitric oxide. Although the vibrational energy increased the reaction rate, comparison with statistical models and the temperature dependence of the thermal reaction indicate about equal importance for vibrational and translational energy. On the other hand, a molecular beam study of the temperature dependence of the reaction of potassium with sulfur hexafluoride" has shown a definite preference for vibrational energy of the SF. ... 

We start by considering possible reactions of diatomic molecules with each other, e.g.. 

The same conclusion of forbiddenness results if we consider addition reactions of diatomic molecules, such as. 

Like other reactions of diatomic molecules this reaction is forbidden by symmetry to occur by a four-center transition state. In terms of the bond symmetry rules... 

Precursors of these rules are the Wigner - Wittmer rules [23], concerning the symmetry of chemical reactions. The Wigner- Wittmer rules deal with the conservation of spin and orbital angular momentum in the course of the reaction of diatomic molecules. 

The first application of spin and symmetry considerations to chemical reactions was presented by Wigner and Witmer, who published rules concerned with the conservation of spin and orbital angular momentum for reactions of diatomic molecules. Later extensions to this work by Hoffmann and by Fukui earned them the Nobel Prize for Chemistry in 1981. 

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