1. PCET Concepts: HAT vs. EPT and Nonadiabaticity
Sharon Hammes-Schiffer
References on HAT vs. EPT and nonadiabaticity:
J. H. Skone, A. V. Soudackov, and S. Hammes-Schiffer, “Calculation of vibronic couplings for phenoxyl/phenol and benzyl/toluene
self-exchange reactions: Implications for proton-coupled electron transfer mechanisms,” J. Am. Chem. Soc. 128, (2006).
A. Sirjoosingh and S. Hammes-Schiffer, “Proton-coupled electron transfer versus hydrogen atom transfer: Generation of
charge-localized diabatic states,” J. Phys. Chem. A 115, (2011).
A. Sirjoosingh and S. Hammes-Schiffer, “Diabatization schemes for generating charge-localized electron-proton vibronic states in
proton-coupled electron transfer systems,” J. Chem. Theory and Comput. 7, (2011).
S. Hammes-Schiffer, “Proton-coupled electron transfer: Classification scheme and guide to
theoretical methods,” Energy Environ. Sci. 5, (2012).
A. V. Soudackov and S. Hammes-Schiffer, “Probing nonadiabaticity in the proton-coupled electron
transfer reaction catalyzed by soybean lipoxygenase,” J. Phys. Chem. Lett. 5, (2014).
S. Hammes-Schiffer, “Proton-coupled electron transfer: Moving together and charging forward,” J. Am. Chem. Soc. Perspective, 2015
PCET web site:
Copyright 2015, Sharon Hammes-Schiffer, University of Illinois at Urbana-Champaign

2. Definitions Relevant to PCET
SHS, Energy Environ. Sci. 2012; SHS, JACS Perspective 2015
PCET can occur by sequential or concerted mechanism (not rigorously defined but clearly
sequential if a stable intermediate can be isolated and clearly concerted if products of single
ET and PT are much less thermodynamically favorable than product of concerted PCET)
Concerted PCET includes HAT (hydrogen atom transfer) and EPT (electron-proton transfer)
HAT: electron and proton transfer from the same donor and acceptor* (not rigorous definition
but used in the field); negligible amount of electronic charge redistribution as H transfers;
electronically adiabaticǂ
EPT: electron and proton transfer between different donors and acceptors* (not rigorous);
significant amount of electronic charge redistribution as H transfers; electronically
nonadiabaticǂ
*donor/acceptor can be molecular orbitals, chemical bonds, or atoms
ǂDistinguishing HAT and EPT by electron-proton nonadiabaticity introduced in
Skone, Soudackov, SHS, JACS 2006

3. Nonadiabaticity Electron-proton subsystem wrt other nuclei e p n D
Skone, Soudackov, SHS, JACS 2006; SHS, Energy Environ. Sci. 2012
Electron-proton subsystem wrt other nuclei
Electron-proton nonadiabaticity:
determines form of vibronic coupling e p n
quantum
classical
ep nad
vib nad
nonadiabatic
adiabatic
e: electrons
p: proton(s)
n: other nuclei D

4. Electron-Proton Nonadiabaticity
Relative timescales of electronic transition and proton tunneling
- determine timescales with semiclassical formulation
- nonadiabatic when proton tunneling faster than electronic transition
Nonadiabatic coupling with respect to transferring proton coordinate
- determine from electronic wavefunctions
- nonadiabatic when character of electronic wavefunction
changes significantly with respect to proton motion
Change in electronic charge distribution as proton transfers
- determine from dipole moment, electrostatic potential, partial charges
- nonadiabatic when significant change
- hydrogen atom transfer (HAT) vs electron-proton transfer (EPT)

5. Representative Chemical Examples
Mayer, Hrovat, Thomas, Borden, JACS 2002; Skone, Soudackov, SHS, JACS 2006
benzyl/toluene
C---H---C
phenoxyl/phenol
O---H---O
SOMO
DOMO
HAT
same donor/acceptor
electronically adiabatic
EPT
different donors/acceptors
electronically nonadiabatic
HAT: hydrogen atom transfer
EPT: concerted PCET

6. Semiclassical Treatment: Timescales
Georgievskii and Stuchebrukhov, JCP 2000; Skone, Soudackov, SHS, JACS 2006 D

7. Nonadiabaticity: Relative Timescales
Skone, Soudackov, SHS, JACS 2006: CASSCF calculations
Phenoxyl-phenol: O---H---O, electronically nonadiabatic, EPT
Benzyl-toluene: C---H---C, electronically adiabatic, HAT

8. Nonadiabatic Coupling
Solid lines: phenoxyl/phenol,
electronically nonadiabatic, EPT
Dotted lines: benzyl/toluene,
electronically adiabatic, HAT
Electron-proton nonadiabatic coupling
from CASSCF states
Nonadiabatic coupling much greater for EPT than for HAT
EPT has greater molecular charge redistribution along rp
Sirjoosingh and SHS, JPC A 2011, JCTC 2011

9. Electrostatic Potential Maps
phenoxyl/phenol: electronically nonadiabatic, EPT
benzyl/toluene: electronically adiabatic, HAT
reactant
transition state
product
Red: negative charge Blue: positive charge
Sirjoosingh and SHS, JPC A 2011, JCTC 2011

10. Molecular Charge Redistribution
phenoxyl/phenol: PCET
electronically nonadiabatic
benzyl/toluene: HAT
electronically adiabatic
Sirjoosingh and SHS, JPC A 2011, JCTC 2011

11. Vibronic Couplings Phenoxyl-phenol: electronically nonadiabatic, EPT28
7.3
7.2
couplings in cm-1
Benzyl-toluene: electronically adiabatic, HAT 21 38
couplings in cm-1
vibronically nonadiabatic
Sirjoosingh and SHS, JPC A 2011, JCTC 2011

12. Nonadiabaticity in Lipoxygenase
Soudackov and Hammes-Schiffer, JPCL 2014
Model system
C-O distance 2.7 Å
Constrained DFT/ωB97X/6-31G**
Electronically nonadiabatic PT and vibronically nonadiabatic PCET

13. Spin Densities and Charge Distribution
Reactant
Product
Soudackov and Hammes-Schiffer, JPCL 2014