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We describe experiments on the temperature dependence of the rate of the ferrous-ferric electron transfer reaction at a gold electrode and compare them with a detailed molecular dynamics simulation which is used to predict the rate. We find from the experiments that the temperature dependence of the rate has the Arrhenius form over the temperature range from 25 to 275~ and that the transfer coefficient is independent of temperature in this range. The molecular dynamics simulations are used in two ways to extract activation energies and transfer coefficients for comparison with experiment. In one of these methods, we assume parabolic dependence of the energies for the product and reactant in a reaction coordinate which is not specified a priori. In the other method, we use a quantum mechanical calculation extrapolated from the very short molecular dynamics time scale to times characteristic of the electron transfer rate. The assumption of parabolic dependence of the energies gives an estimate for the activation energy which is consistent with experiment. The transfer coefficient calculated using this assumption is also consistent with experiment. The activation energy and the transfer coefficient from the quantum mechanical calculation are both lower than the experimental values. The quantum mechanical method, together with a molecular orbital calculation of the electron transfer matrix element, permits a theoretical estimate of the absolute value of the rate, which is also compared with the experimental result. These results show that the ferrous-ferric reaction, which is a single-step outer-sphere charge-transfer reaction, follows the classical Butler-Volmer equation at temperatures up to 275~ and that earlier results on other reactions giving a temperature dependent transfer coefficient are likely to arise from elementary steps other than outer-sphere charge transfer.

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abc # 0, eq 6 represents the linearly forced harmonic oscillator. Pechukas and Child3 showed analytically that the (pf,c@, curve for this system was a circle displaced from the phase space origin. Now, trajectory calculations with c = 0 and abd # 0 yield an elliptical @f,gf),i curve with center at the origin and rotated axes for the quadratically forced harmonic oscillator.% Such a phase space picture is isomorphous to that obtained for the overlap integral between harmonic oscillators with identical equilibrium positions but different force constants, kl = 1 and k2 # 1. The functional form for these overlap integrals is well-known, and the phase space pictures suggest that it may be related to the less tractable expressions for the transition probabilities obtained by Lewis and Riesenfeld% for the harmonic oscillator with time-dependent force constant. Such an oscillator is a possible model for experimentally observed30 non-Landau-Teller behavior in Ar + CsI scattering.Whether used to explain results from sophisticated CID calculations or to suggest links between simple theoretical models, classical S-matrix phase space pictures appear to be an easily employed theoretical tool worthy of continued application and development. P.M.H. is indebted to the DowThe results of investigations into the association reactions occurring in polar gases by analysis of the pressure and temperature dependence of thermal conductivity are reviewed. The gases studied include water, methanol, ethanol, 2-propanol, 2-methyl-2-propano1, 2,2,2-trifluoroethanol, acetone, acetonitrile, pyridine, acetic acid, trifluoroacetic acid, as well as the binary mixtures methanol-water, trifluoroethanol-water, and acetic acid-water. The prewure and temperature ranges of these studies are 100-2000 torr and 33&440 K, respectively. Expressions for the thermal conductivity of reacting gases derived by Butler and Brokaw, based on the work of Hirschfelder, give calculated thermal conductivities which fit the observed data very well for all the types of observed pressure dependence. The fits yield information on the associated species present (e.g., dimer, trimer, tetramer, etc.), on the concentrations of the species, and on their enthalpies and entropies of association. The thermodynamic results, in conjunction with ab initio molecular orbital calculations, provide information on the strength and structure of hydrogen bonding occurring in the cluster species.