The definition and possible determination of an absolute standard reduction potential, SRP, is an intriguing subject, which continuously attracts the interest of researchers both from theoretical and experimental points of view. Experimental values of half-cell reduction potentials are generally anchored to the standard hydrogen electrode, SHE, in water to which the conventional value of exactly 0 V has been assigned. Thus, the experimentally measured values are relative SRPs and these are adequate for many applications of standard potentials. However, the growing development of computational efforts and the need of making comparisons between theoretical and experimental values set the question of absolute SRPs into relevant actuality. Another important redox process for which reliable SRPs are not available is the reduction of the halogen atoms. EoX•/X- plays a crucial role in the evaluation of the SRPs of many alkyl halides that are involved in several important processes such as reductive dehalogenation of recalcitrant pollutants, atom transfer radical polymerization and various other processes of synthetic importance. This paper reports on the calculation of the SRPs of H+, X• and RX. The absolute potential of the standard hydrogen electrode, SHE, was calculated on the basis of a thermodynamic cycle involving H2(g) atomization, ionization of H•(g) to H+(g) and hydration of H+. The most up-to-date literature values on the free energies of these reactions have been selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e- and H+ are fermions. Unlike almost all previous estimations of SHE, Goaq(H+) was used instead of the real potential, aq(H+). This choice was made to obtain an SHE value based on chemical potential, which is the appropriate reference to be used in theoretical computations of SRPs. More complicated thermochemical cycles were used for the calculation of EoX•/X (vs SHE) in water as well as in MeCN and DMF. Last, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other processes such as pollution abatement have been calculated in MeCN and DMF. This has been done by using a thermochemical cycle involving gas phase homolytic dissociation of the C-X bond, solvation of RX, R• and X•, and reduction of X• to X- in solution.
Evaluation of the standard reduction potentials of some electrochemical processes of primary importance
GENNARO, ARMANDO;AHMED ISSE, ABDIRISAK
2010
Abstract
The definition and possible determination of an absolute standard reduction potential, SRP, is an intriguing subject, which continuously attracts the interest of researchers both from theoretical and experimental points of view. Experimental values of half-cell reduction potentials are generally anchored to the standard hydrogen electrode, SHE, in water to which the conventional value of exactly 0 V has been assigned. Thus, the experimentally measured values are relative SRPs and these are adequate for many applications of standard potentials. However, the growing development of computational efforts and the need of making comparisons between theoretical and experimental values set the question of absolute SRPs into relevant actuality. Another important redox process for which reliable SRPs are not available is the reduction of the halogen atoms. EoX•/X- plays a crucial role in the evaluation of the SRPs of many alkyl halides that are involved in several important processes such as reductive dehalogenation of recalcitrant pollutants, atom transfer radical polymerization and various other processes of synthetic importance. This paper reports on the calculation of the SRPs of H+, X• and RX. The absolute potential of the standard hydrogen electrode, SHE, was calculated on the basis of a thermodynamic cycle involving H2(g) atomization, ionization of H•(g) to H+(g) and hydration of H+. The most up-to-date literature values on the free energies of these reactions have been selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e- and H+ are fermions. Unlike almost all previous estimations of SHE, Goaq(H+) was used instead of the real potential, aq(H+). This choice was made to obtain an SHE value based on chemical potential, which is the appropriate reference to be used in theoretical computations of SRPs. More complicated thermochemical cycles were used for the calculation of EoX•/X (vs SHE) in water as well as in MeCN and DMF. Last, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other processes such as pollution abatement have been calculated in MeCN and DMF. This has been done by using a thermochemical cycle involving gas phase homolytic dissociation of the C-X bond, solvation of RX, R• and X•, and reduction of X• to X- in solution.Pubblicazioni consigliate
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