Computing the energetic component of the charge-transfer symmetry factor

• Main Author: Drechsel-Grau, Christof
• Published: University of Cambridge 2010
• Subjects: 541.2
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.541939

The oxidation half-reaction of the aqueous ferrous ion serves as a model to investigate electron-transfer dynamics. The present classical model consists of two empirical valence bond states and a control parameter that effectively determines the reaction free energy. The model mimics an outer-sphere electron-transfer reaction that obeys Marcus theory to a good approximation. This theory uses the energy difference between the two empirical valence bond states as the reaction coordinate and quantitatively predicts the location of the transition state and activation parameters. The knowledge of the reaction coordinate is exploited in two ways: to compute activation parameters from umbrella integration (UI) and Marcus theory (MT) based simulations assuming linear response and to test the accuracy of transition path sampling (TPS) for the calculation of activation energies. Activation energies from transition path sampling (10.2 kJ/mol) agree within statistical uncertainty with reference calculations (UI: 15.2 kJ/mol; MT: 15.7 kJ/mol) and are lower than activation free energies (UI: 25.8 kJ/mol; MT: 31.8 kJ/mol), indicating substantial activation entropies. The variation of the activation free energy with the reaction free energy defines the charge-transfer symmetry factor (UI: 0.47; MT: 0.49). The latter is larger than its energetic (TPS: 0.39; UI: 0.23; MT: 0.38) and entropic (UI: 0.25; MT: 0.13) components, given by the variation of the activation energy and entropy with the reaction free energy. The charge-transfer symmetry factor also describes the location of the transition state, which is verified by a committor analysis, thereby supporting the validity of Marcus theory.