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

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

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.

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

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

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, 074304 (2016)

Vibrational modes in state of C5H5 T. Ichino et al. J. Chem. Phys. 129, 084310 (2008)

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.

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.

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

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

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)

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)

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

Testing the model using Lian Yu et al spectra Simulation Experimental

Testing the model using Lian Yu et al spectra Simulation Experimental

Fitting parameters

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 ?

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

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, 074304 (2016)

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 1988-89 2008 Rotationally resolved 2 Electronic Spectra of C5H5 Vibronic level structure 4 using EOMIP-CCSD 1965 2000-01 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, 084310 (2008) 3D. Leicht et al. J. Chem. Phys. 145, 074304 (2016)

Energy Levels around C-H stretch region

Vibrational modes in state of C5H5 T. Ichino et al. J. Chem. Phys. 129, 084310 (2008)

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