Potential energy surface, parameters reaction rates from

The reactions of P-donor nucleophiles with the metal carbonyl cluster Rh4COi2 have been studied over a considerable time period.It is widely accepted that the reaction is associative. This latest investigation is aimed at quantifying the effects of the electronic and steric properties of the nucleophiles upon the kinetic parameters. A rapid substitution reaction step using an excess of the nucleophile was monitored by SF spectrophotometry. Second-order rate constants were obtained from the variation of the pseudo-first-order rate constants with nucleophile concentration. Contributions to these constants from the properties steric effect, TT-activity, and, in addition, an aryl effect of the nucleophiles were assessed in a multi-parameter equation. The outcome is a successful understanding of the relative reactivities of many P-donors toward the rhodium cluster. The data were also represented by a three-dimensional potential energy surface. 

The reaction systems on which the quantum chemical calculations were done and their results that are reported in this chapter are compared with existing experimental data that were obtained mainly in our laboratory. An important issue that should be mentioned at this point is the necessity to compare calculated rate parameters to those reduced in the experiment. One can calculate a very convincing potential energy surface, transition states, and intermediates on a surface and evaluate from them rate parameters. If, however, the calculated values cannot be anchored into experimental results, no one can be sure whether this particular surface is really the relevant one. 

Surface reaction steps involving adsorbed (chemisorbed, to be accurate) hydrogen, such as H adsorption, desorption, H-H combination, and surface-bulk transfer, play a determining role in the H cathodic reactions. The HAR and the HER most often share a common step of H electroadsorption from protons or wafer, and only a study of the overall mechanism of these reactions makes it possible to predict the conditions in which the H uptake under the surface can increase. The problem of analyzing all the data on H entry rate is that this rate, even in a pure metal, depends on many variables the nature of the metal, its thermal-mechanical history, the surface conditions (especially on iron, surface states are not easily reproducible due to the difficulty of removing oxide films on the electrodes), composition of the electrolyte, cathodic current density or electrode potential, temperature, etc. The determining factors in the kinetics of the H cathodic reactions on bare metal surfaces are the cathodic overpotential and the surface parameters, which are the density of sites for H adsorption and the free energy of adsorbed H, both dependent on the structure and the chemical composition of the surface. 

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