Potential functions medium-range

Medium-range interactions can be defined as those which dominate the dynamics when atoms interact with energies within a few eV of their molecular binding energies. These forces determine a majority of the physical and chemical properties of surface reactions which are of interest, and so their incorporation in computer simulations can be very important. Unfortunately, they are usually many-body in nature, and can require complicated functional forms to be adequately represented. This means that severe approximations are often required when one is interested in performing molecular dynamics simulations. Recently, several potentials have been semi-empirically developed which have proven to be sufficiently simple to be useful in computer simulations while still capturing the essentials of chemical bonding. 

The predominance of one species over another has been shown to be a function of a number of experimental variables including medium pH, electrode potential, solvent-electrolyte system, and electrode material [7,8]. Voltammetry studies of suitably substituted phenolate ions at elevated pH show the expected reversible one-electron process leading to formation of phenoxy radicals (V), with peak potentials on the order of —0.16 to —0.31 vs. SCE [8,9]. Oxidation of nonionized phenols in media sufficiently acidic to suppress ionization of the resulting cation radicals (II) (pXa —5 to 0 [10-13]) reveals positive peak potentials in the range of 1.3-1.65 V versus SCE [14,15]. As indicated earlier, the principal product-producing species from these oxidations are the phenoxy radical (V) and the phenoxonium ion (IV). 

Because the pH of natural water systems is a function of their dissolved compounds (including gases), these species also confer a definite electrochemical reduction potential range to the aquatic medium. Some of the pH and E values typically found in natural water systems are given in Table 6.12. 

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