PROTON–COUPLED ELECTRON TRANSFER REACTIONS: THEORETICAL INSIGHT INTO EXPERIMENTAL RESULTS
Open Access
- Author:
- Ludlow, Michelle Kathryn
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 09, 2010
- Committee Members:
- Sharon Hammes Schiffer, Dissertation Advisor/Co-Advisor
Sharon Hammes Schiffer, Committee Chair/Co-Chair
James Bernhard Anderson, Committee Member
Carsten Krebs, Committee Member
Vincent Henry Crespi, Committee Member - Keywords:
- PCET
vibronic coupling
KIE
quinol oxidation
phenoxyl/phenol
Tafel plot - Abstract:
- A theoretical formulation for vibronically nonadiabatic proton–coupled electron transfer (PCET) reactions is applied to several PCET systems. This theory allows for the calculation of rates and kinetic isotope effects (KIEs). Theoretical studies of PCET systems are presented to provide insight into experimental results. The impact of substituents on the vibronic coupling for the phenoxyl/phenol self-exchange reaction is investigated. Analysis of the results indicates that electron donating groups enhance the vibronic coupling, while electron withdrawing groups attenuate the vibronic coupling. Therefore, the electron donating groups will increase the rate, while electron withdrawing groups will decrease the rate for the phenoxyl/phenol self-exchange reaction. Theoretical calculations are performed on model systems for the quinol oxidation of ubiquinol and plastoquinol to provide an explanation for the differences in magnitudes and temperature dependences of the KIEs and an explanation for the inverse temperature dependence of the KIE for the ubiquinol. The physical properties of the system that enable the inverse temperature dependence of the KIE are a stiff hydrogen bond and small reorganization energy. The inverse temperature dependence of the KIE is observed when the (0/0) pair of reactant/product vibronic states is in the inverted Marcus region, while the (0/1) pair is in the normal Marcus region and is the dominant contributor to the overall rate. For ubiquinol, the (0/1) pair is the dominant contributor to the overall rate, while for plastoquinol other pairs of vibronic states contribute significantly to the overall rate, leading to qualitatively different temperature dependences of the KIEs for the two system. Electrochemical PCET of an osmium aquo complex attached to a self-assembled monolayer on a gold electrode is studied. Calculations indicate that the asymmetry of the Tafel plot and the deviation of the transfer coefficient at zero overpotential from one-half arise from the change in the equilibrium proton donor–acceptor distance upon electron transfer. The direction of the asymmetry and deviation from one-half is determined by the sign of this distance change, and the magnitude of these effects is determined by the magnitude of this distance change, the reorganization energy and the distance dependence of the overlap.