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

para-Benzoquinone as ubiquitous biological unit

e.g., Coenzyme Q (ubiquinone)—Electron transfer and proton translocation in mitochondria

O

O

H3CO

H3CO H

n

O

O

O

O

OH

OH

e e, 2H+

A number of studies have been conducted to characterize the pBQ radical anion.(optical absorption, resonance Raman, ESR, photodetachment, etc.)

pBQ pBQ●− hydroquinone

How about the oxidized para-benzoquinone, i.e., pBQ●+?→ Photoelectron spectroscopy of pBQ

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 2B3g and 2B2u states.

2B3g pBQ●+

(ground state)

2B2u

2B1g, 2B3u nearly degenerate

b3u

−11.22

−11.36

−12.02

−12.65

b1g

b3g

b2u

orbi

tal e

nerg

y (e

V)

Experiment

Simulation

Low-temperature matrix isolation spectroscopic study

Dr. Krzysztof Piech and Professor Thomas BallyUniversity of Fribourg, Switzerland

X-ray irradiation of pBQ-doped Ar matrix at 10K

Ar Ar●+ + e−

Ar●+ + pBQ Ar + pBQ●+

e− + pBQ pBQ●−

( Ar●+ + DABCO Ar + DABCO●+ )

DABCO: 1,4-diazabicyclo[2.2.2]octane

N

N “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

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

pBQ-doped matrixbefore X-ray irradiation

After X-ray irradiation

Followed by UV photolysis

O

O

O

O

O

O

+

pBQ●+ pBQ●−

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

pBQ-doped matrixbefore X-ray irradiation

After X-ray irradiation

Followed by UV photolysis

O

O

O

O

O

O

+

pBQ●+ pBQ●−

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

O

O

C: pBQ●+

A: pBQ●−

CC

C

C, A

CC

AAA

A

A

Asymmetric CO stretch in pBQ

O

O

n11 (b1u)asymmetric CO stretch

The most intense IR absorptionfor pBQ

Asymmetric CO stretch in pBQ●+ and pBQ●−

n11 in pBQ●+

n11 in pBQ●− n11 (b1u)asymmetric CO stretch

n11 is the most intensefor pBQ●−, analogous topBQ, but it is ratherweak for pBQ●+.

n11 in pBQ

Molecular orbitals of pBQen

ergy

(eV

)

−11.22

−11.36

−12.02

−12.65

b1g

b3u

b3g

b2u

b2g+0.13

Intense b1u fundamental peaks in pBQ●+

O

O

n11

n12n13n14

The lower-frequency b1u

modes (n12, n13, and n14)of pBQ●+ have intensefundamental transitions,unlike pBQ and pBQ●−.

b3g × b2u = b1u

the ground state

the excited state

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 Hamiltonianin terms of the reduced normal coordinates of pBQ.

model potential for pBQ●+: quadratic vibronic coupling (QVC) model

X : 2B3g

A : 2B2u

B, C : 2B1g, 2B3u

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 ↔ AB ↔ C

Photoelectron spectrum of pBQ

Adiabaticsimulation

Nonadiabaticsimulation

Experiment

reasonable agreementbetween the experimentand the model Hamiltoniansimulation

Substantial nonadiabatic interactionbetween the X and A states

andbetween the B and C states

Electronic spectrum of pBQ●+ in the IR region

Adiabaticsimulation

Nonadiabaticsimulation

• Nonadiabatic interaction distributes the electronic transition intensity among a large number of the vibronic states of b2u symmetry.

A 2B2u ← X 2B3g transition

The transition dipole moment3.32 D

• The conical intersection between the X and A states is located 1860 cm−1 above the ground level.

ConicalIntersection

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 n11 fundamental level exists for the X 2B3g state of pBQ●+.

IR spectrum of pBQ●+

Experiment

Simulation

H2O

pBQ●−

n14

n13

n12

n11

Why is the band associated withthe n11 fundamental transitionrelatively weak?

The vibrational states of b1u symmetry in the X 2B3g stategain the character of the A 2B2u

state through nonadiabaticinteraction. The strong A ← X electronic transition is carriedover to the b1u fundamentaltransitions (n11 – n14).

C

C

C

C

O

O

H

H

Quasidiabatic picture of the n11 fundamental transition in pBQ●+

Vibronic state a

electronic component

nuclear component

Transition dipole matrix elementdipole derivative

contribution withinthe X state

Contribution of the A ← X electronic transition

The two contributions have comparable magnitudes with opposite phasesfor the n11 fundamental transition, largely canceling each other.

Assignments of the IR spectrum

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 2B3g and A 2B2u states leaves its signature in the IR spectrum of pBQ●+.

– Three b1u fundamental transitions of pBQ●+ in the X 2B3g state have large intensities, which derive from the A ← X electronic transition.

– The fundamental level of the b1u mode that represents asymmetric CO stretch is strongly coupled with other nearby vibronic states of b2u 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 b2u symmetry appear as a number of broad bands.

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

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

O

O