QUANTUM MECHANICAL METHODS FOR CALCULATING PROTON TUNNELING SPLITTINGS AND PROTON-COUPLED ELECTRON TRANSFER VIBRONIC COUPLINGS
Open Access
- Author:
- Skone, Jonathan H
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 10, 2008
- Committee Members:
- Sharon Hammes Schiffer, Committee Chair/Co-Chair
James Bernhard Anderson, Committee Member
Mark Maroncelli, Committee Member
Kristen Ann Fichthorn, Committee Member - Keywords:
- electronically nonadiabatic
nonorthogonal configuration interaction
tunneling splitting
proton-coupled electron transfer
vibronic coupling
kinetic isotope effect
semiclassical method
proton transfer - Abstract:
- Nuclear quantum effects such as proton tunneling and vibrational excitations play an important role in a wide range of chemical and biological processes, including the mechanism of proton-coupled electron transfer. The vibronic coupling defined as the Hamiltonian matrix element between the reactant and product mixed electron-proton vibronic wavefunctions significantly impacts the rates, kinetic isotope effects and temperature dependences of proton-coupled electron transfer reactions. The relative timescales of the electrons and tunneling proton can span the range from adiabatic, where the electrons respond instantaneously to the motion of the proton, to nonadiabatic, where the electrons and proton move on similar time scales. To calculate quantitatively accurate proton tunneling splittings and vibronic couplings and thereby obtain meaningful insights, methods that avoid the Born-Oppenheimer separation of transferring protons and electrons are necessary. We have developed a capable molecular orbital-based nonadiabatic method within the nuclear electronic orbital framework for calculating proton tunneling splittings and proton-coupled electron transfer vibronic couplings and have also implemented an alternative semiclassical grid-based method. We have applied the semiclassical method to two hydrogen self-exchange systems that are exemplary of electronically nonadiabatic and electronically adiabatic proton transfer. The benzyl-toluene system is an example of the electronically adiabatic limit, in which the proton transfer proceeds on the adiabatic electronic ground state surface. The phenoxyl-phenol system provides an example of the nonadiabatic limit, in which the proton transfer proceeds through a nonadiabatic transition between two diabatic electronic states. Our analysis of these two systems provides insights into the fundamental physical principles underlying PCET reactions. For the phenoxyl/phenol system, quantitative agreement between the molecular orbital based nuclear electronic orbital nonothorogonal configuration interaction method and the three-dimensional semiclassical grid based approach is observed providing a level of validation for both methods. Additionally we use both methods to analyze the effects of substituents on the vibronic coupling and hence the transfer rate in the phenoxyl/phenol self-exchange reaction.