1. Manifestation of Nonadiabatic Effects in the IR Spectrum of para-Benzoquinone Radical Cation Krzysztof Piech, Thomas Bally Department of Chemistry, University of Fribourg Takatoshi Ichino, John F. Stanton Department of Chemistry and Biochemistry The University of Texas at Austin June 20, 2013

2. para-Benzoquinone as ubiquitous biological unit e.g., Coenzyme Q (ubiquinone) —Electron transfer and proton translocation in mitochondria A number of studies have been conducted to characterize the pBQ radical anion. (optical absorption, resonance Raman, ESR, photodetachment, etc.) pBQpBQ ●− hydroquinone How about the oxidized para-benzoquinone, i.e., pBQ ●+ ? → Photoelectron spectroscopy of pBQ

3. Photoelectron spectroscopy of p-benzoquinone Allan, Bally, Stanton, Gauss and coworkers, J. Chem. Phys. 115, 1 (2001) Spectral simulation using the quasidiabatic model Hamiltonian technique — Linear Vibronic Coupling (LVC) model pBQ orbitals The four lowest electronic states of the pBQ radical cation are observed. Koopmans’ theorem does not predict the correct energy ordering. Nonadiabatic interaction between the 2 B 3g and 2 B 2u states. 2 B 3g pBQ ●+ (ground state) 2 B 2u 2 B 1g, 2 B 3u nearly degenerate b 3u −11.22 −11.36 −12.02 −12.65 b 1g b 3g b 2u orbital energy (eV) Experiment Simulation

4. Low-temperature matrix isolation spectroscopic study Dr. Krzysztof Piech and Professor Thomas Bally University of Fribourg, Switzerland X-ray irradiation of pBQ-doped Ar matrix at 10K Ar Ar ●+ + e − Ar ●+ + pBQ Ar + pBQ ●+ e − + pBQpBQ ●− ( Ar ●+ + DABCO Ar + DABCO ●+ ) DABCO: 1,4-diazabicyclo[2.2.2]octane “hole scavenger” Measurements of the IR spectra of the irradiated matrix → more detailed information on the nonadiabatic interaction in pBQ ●+ It helps differentiation between pBQ ●+ and pBQ ●−. X-ray

5. IR spectrum of X-irradiated, pBQ-doped Ar matrix pBQ-doped matrix before X-ray irradiation After X-ray irradiation Followed by UV photolysis + pBQ ●+ pBQ ●−

6. IR spectrum of X-irradiated, pBQ-doped Ar matrix pBQ-doped matrix before X-ray irradiation After X-ray irradiation Followed by UV photolysis + pBQ ●+ pBQ ●−

7. IR spectrum of X-irradiated, pBQ-doped Ar matrix C: pBQ ●+ A: pBQ ●− C C C C, A C C A A A A A

8. Asymmetric CO stretch in pBQ 11 (b 1u ) asymmetric CO stretch The most intense IR absorption for pBQ

9. Asymmetric CO stretch in pBQ ●+ and pBQ ●− 11 in pBQ ●+ 11 in pBQ ●− 11 (b 1u ) asymmetric CO stretch 11 is the most intense for pBQ ●−, analogous to pBQ, but it is rather weak for pBQ ●+. 11 in pBQ

10. Molecular orbitals of pBQ energy (eV) −11.22 −11.36 −12.02 −12.65 b 1g b 3u b 3g b 2u b 2g +0.13

11. Intense b 1u fundamental peaks in pBQ ●+ 11 12 13 14 The lower-frequency b 1u modes ( 12, 13, and 14 ) of pBQ ●+ have intense fundamental transitions, unlike pBQ and pBQ ●−. b 3g × b 2u = b 1u the ground state the excited state

12. Quasidiabatic model Hamiltonian technique H. Köppel, W. Domcke, and L. S. Cederbaum, Adv. Chem. Phys. 57, 59 (1984) EOMIP-CCSD/TZ2P calculations are employed to construct the Hamiltonian in terms of the reduced normal coordinates of pBQ. model potential for pBQ ●+ : quadratic vibronic coupling (QVC) model X : 2 B 3g A : 2 B 2u B, C : 2 B 1g, 2 B 3u Parametrization of the model potential: J. Chem. Phys. 125, 084312 (2006) J. Chem. Phys. 130, 174105 (2009) where Diagonal block Off-diagonal block Diabatic coupling: X ↔ A B ↔ C

13. Photoelectron spectrum of pBQ Adiabatic simulation Nonadiabatic simulation Experiment reasonable agreement between the experiment and the model Hamiltonian simulation Substantial nonadiabatic interaction between the X and A states and between the B and C states

14. Electronic spectrum of pBQ ●+ in the IR region Adiabatic simulation Nonadiabatic simulation Nonadiabatic interaction distributes the electronic transition intensity among a large number of the vibronic states of b 2u symmetry. A 2 B 2u ← X 2 B 3g transition The transition dipole moment 3.32 D The conical intersection between the X and A states is located 1860 cm −1 above the ground level. Conical Intersection

15. Nonadiabatic mixing Projections of wavefunctions for the vibronic states of pBQ ●+ along reduced normal coordinates of pBQ Conical intersection As it approaches the conical intersection, vibronic mixing becomes substantial. → No pure 11 fundamental level exists for the X 2 B 3g state of pBQ ●+.

16. IR spectrum of pBQ ●+ Experiment Simulation H2OH2O pBQ ●− 14 13 12 11 Why is the band associated with the 11 fundamental transition relatively weak? The vibrational states of b 1u symmetry in the X 2 B 3g state gain the character of the A 2 B 2u state through nonadiabatic interaction. The strong A ← X electronic transition is carried over to the b 1u fundamental transitions ( 11 – 14 ).

17. Quasidiabatic picture of the  11 fundamental transition in pBQ ●+ Vibronic state  electronic component nuclear component Transition dipole matrix element dipole derivative contribution within the X state Contribution of the A ← X electronic transition The two contributions have comparable magnitudes with opposite phases for the 11 fundamental transition, largely canceling each other.

18. Assignments of the IR spectrum

19. Summary The IR spectrum of the para-benzoquinone radical cation (pBQ ●+ ) has been analyzed with the quasidiabatic model Hamiltonian technique. EOMIP- CCSD/TZ2P calculations have been performed to construct the model Hamiltonian. The model potential has been expanded up to the second order in terms of the reduced normal coordinates of pBQ. The nonadiabatic coupling between the X 2 B 3g and A 2 B 2u states leaves its signature in the IR spectrum of pBQ ●+. –Three b 1u fundamental transitions of pBQ ●+ in the X 2 B 3g state have large intensities, which derive from the A ← X electronic transition. –The fundamental level of the b 1u mode that represents asymmetric CO stretch is strongly coupled with other nearby vibronic states of b 2u symmetry. Consequently, no distinct peak appears for the fundamental transition in the IR spectrum, and instead, it has been transformed into a weak broad band. The small intensity reflects cancellation of the two contributions to the transition dipole matrix element; one is from the dipole derivative within the X state, and the other is from the A ← X electronic transition. –Beyond 1900 cm −1 in the IR spectrum, a numerous vibronic states of b 2u symmetry appear as a number of broad bands.

20. Acknowledgments Professor Thomas Bally Dr. Krzysztof Piech Professor John F. Stanton Swiss National Science Foundation US National Science Foundation US Department of Energy The Robert A. Welch Foundation

21. IR spectrum of X-irradiated, pBQ-doped Ar matrix