1. The Rovibronic Spectra of The Cyclopentadienyl Radical (C5H5)
Ketan Sharma*, Terry A. Miller*, John F. Stanton# & David J. Nesbitt^
* The Ohio State University, Columbus, OH
# University of Florida, Gainesville, FL
^ JILA CU-NIST, Boulder, CO

2. Conical intersection
Conical intersections play important role in chemical reactions
Breakdown of Born- Oppenheimer approximation
Theoretical and computational advancements have lead to better understanding of Conical intersections
Images courtesy: M A Robb and co-workers

3. Bringing experiment and theory together with Jahn teller active molecules
Conical Intersections in reaction pathways are difficult to examine spectroscopically
Jahn Teller molecules provide a possibility to experimentally study these states.

4. Cyclopentadienyl Radical
D5h point group.
It has a doubly degenerate ground state
e2’ vibration – which is linearly Jahn Teller active
e1’ vibration – which is quadratically Jahn Teller active
1965
EPR Study
of C5H5
1989
Rotationally resolved
Electronic Spectra
2001
Vibronic study of
C5H5
using LIF
2008
Vibronic level structure
using EOMIP-CCSD
2016
Infrared spectra of
CH stretch region
Theoretical study of
vibronic interactions
in C5H5
1962

5. Lian Yu et al. Chemical Physics Letters 162, 431 (1989)

6. Infrared spectra of helium solvated C5H5 radical in the C-H stretch region
E1’’ - Electronic
ground state
a1’ – vibrational
e1’ – vibrational
excited state
D. Leicht et al. J. Chem. Phys. 145, (2016)

7. Vibrational modes in state of C5H5
T. Ichino et al. J. Chem. Phys. 129, (2008)

8. Vibronic Spectra
The general spin-vibronic eigenfunction is of the form :
It is obtained by numerically solving the vibronic Jahn Teller Hamiltonian
SOCJT 2 (in house software) does that.

9. Energy Levels around C-H stretch region
e1’ has quadratic Jahn teller effect
e2’ is linearly Jahn teller active
There are other combination bands and overtones present in the region
The picture is more complicated than presented by Leicht et al.

10. Recent experiment involving C5H5 measured in David Nesbitt’s lab at JILA, University of Colorado, Boulder
Experiment
Simulation for C5H6

11. Recent experiment involving C5H5 measured in David Nesbitt’s lab at JILA, University of Colorado, Boulder
Experiment
Simulation for C5H6

12. Cyclopentadienyl Radical
Goals:
To aid and understand in detail the rovibronic structure of spectra of C5H5
Verify the assignments of experimental spectrum.
The approach:
Synthesize the spectra based on theoretical calculations.
Fit experimental data.
Use rotational structure to identify vibronic transitions.
Giving a better theoretical basis to various parameters. (instead of having them as just fitting parameters for the experimental spectrum)

13. Rotational structure of a vibronic transition
The rotational Hamiltonian1,2 for a D5h symmetry molecule is
is the Watson term given by
1 Jon T. Hougen Journal of Molecular Spectroscopy 81, 73 (1980)
2 Lian Yu et al. Chemical Physics Letters 162, 431 (1989)

14. Rotational structure of a vibronic transition
We have made a model to calculate and fit the rotational structure of any vibronic transition on any molecule with D5h Symmetry
Symmetrized Basis set used
Transforms as irreducible representation of D5h group, thereby block diagonalising the matrix
Symmetry based input for Nuclear Spin Statistics

15. Testing the model using Lian Yu et al spectra
Simulation
Experimental

16. Testing the model using Lian Yu et al spectra
Simulation
Experimental

17. Fitting parameters

18. Electronic Structure theory calculation
Summary
The picture of vibronic spectra in the C-H stretch region is more complicated than presented by Leicht et al.
HJT is sensitive to vibronic eigenstates and therefore rotational resolution of vibronic transition can be used to identify the vibronic levels involved in the transition.
To do that we have developed tools to study rovibronic spectra for Jahn-Teller active molecules in general (keeping C5H5 in mind at present)
We are presently building a bridge between experimental study and theoretical framework for Jahn Teller active molecules.
Electronic Structure theory calculation
Experimental Data
Fit data
Calculate
Parameters ?

19. Acknowledgement The Ohio State University: Dr. Terry Miller Meng Huang
Scott Garner
University of Florida:
Dr. John Stanton
University of Colorado Boulder:
Dr. David Nesbitt

20. Unanswered questions Three (e1’) C-H strectch have frequencies :
Anharmonic terms not included in the calculations
Rotational resolution of vibronic transitions, which we observe is David Nesbitt’s experiments can be used to identify these vibrations.
Discharge on
Discharge off
D. Leicht et al. J. Chem. Phys. 145, (2016)

21. 1G. R. Liebling et al. J. Chem. Phys. 42, 3931 (1965)
Vibronic study of 3
C5H5
using LIF
EPR Study 1
of C5H5
Infrared spectra of 5
CH stretch region
2008
Rotationally resolved 2
Electronic Spectra
of C5H5
Vibronic level structure 4
using EOMIP-CCSD
1965
2016
1G. R. Liebling et al. J. Chem. Phys. 42, 3931 (1965)
2Lian Yu et al. Chem. Phys. Lett. 162, 431 (1989)
3B. E. Applegate et al. J. Chem. Phys. 114, 4869 (2001)
4T. Ichino et al. J. Chem. Phys. 129, (2008)
3D. Leicht et al. J. Chem. Phys. 145, (2016)

22. Energy Levels around C-H stretch region

23. Vibrational modes in state of C5H5
T. Ichino et al. J. Chem. Phys. 129, (2008)

24. Mini summary:
HJT in needed to explain the rotational resolution of a vibronic band
We have a working code to calculate the rotationally resolved spectrum for any vibronic transition of a D5h molecule
Next: Let’s look at the vibronic spectrum