1. Electronic Spectroscopy of DHPH Revisited: Potential Energy Surfaces along Different Low Frequency Coordinates Leonardo Alvarez-Valtierra and David W. Pratt Department of Chemistry University of Pittsburgh Pittsburgh, PA 15260 9,10-Dihydrophenathrene (DHPH)

2. Low-Frequency Modes in Molecules Low frequency modes are the major contributors to the entropy of a system. They promote vibrational energy flow in molecules. The density of low-frequency vibrational states is huge! They play an important role coupling higher electronic states in molecules. Why are they important…? ~21.7°~8.4° c b S0S0 c b S1S1 Theoretical DHPH structures MP2/6-31G** CIS/6-31G

3. Theoretical Study of Low Frequency Vibrations in DHPH HF/6-31G**CIS/6-31G Ab initio calculations have revealed the existence of the following low frequency (< 350 cm -1 ) vibrational modes in both S 0 and S 1 electronic states of DHPH.

4. LIF Spectrum of 9,10-Dihydrophenanthrene (DHPH) The “a” progressionThe “b” progressionThe “c” progression

5. Some Rotationally Resolved Electronic Spectra of the “a” Progression. 34155.734157.6Frequency (cm -1 ) +487 (a5) ~0.1 cm -1 33961.233963.6Frequency (cm -1 ) +293 (a3)

6. Inertial Parameters of the High Resolution Fits Parameter+293+487 A"/ MHz 1526.3 (1)1526.1 (1) B"/ MHz 545.5 (1) C"/ MHz 412.6 (1) ΔI"/ amu*Å 2 -32.5 (1) ΔA/ MHz -35.6 (1)-35.8 (1) ΔB/ MHz 0.4 (1) 0.1 (1) ΔC/ MHz -6.4 (1)-6.2 (1) ΔI'/ amu*Å 2 -20.5 (1)-21.1 (1)

7. Experimental Inertial Defects in S 1 Excited electronic state inertial defect ( ΔI' ) related to the ring twisting angle ( φ )? c b φ Transition∆I’/ amu*Å 2 DHPH+0 (origin)-18.7 (1) DHPH+98-19.0 (1) DHPH+196-19.6 (1) DHPH+293-20.5 (1) DHPH+390-20.8 (1) DHPH+487-21.1 (1) Theoretical Model to Predict Inertial Defect Values in both, S 0 and S 1 Transition∆I”/ amu*Å 2 DHPH+0 (origin)-32.3 (1) DHPH+98-32.2 (1) DHPH+196-32.2 (1) DHPH+293-32.5 (1) DHPH+390-32.4 (1) DHPH+487-32.5 (1) S 0 (HF/6-31G**) S 1 (CIS/6-31G*) Experimental values Inertial defect (amu Ǻ 2 )

8. Mode Assignment and Potential Energy Surfaces S0S0 S1S1 Q 2 (φ/deg) 21.58.5 Highest intensity transition φ “Symmetric ring twisting mode” Theoryν = 83.7 cm -1 Experimental*v = 97.5 cm -1 Theoryν = 140.1 cm -1 Experimental**v = 104.0 cm -1 * This work. ** J. M. Smith and J. L. Knee. J. Chem. Phys. 99(1), 1993, 38. V(Q 2 )

9. Some Rotationally Resolved Electronic Spectra of the “b” Progression. 34192.634193.5Frequency (cm -1 ) +523 (b3) 34384.334385.1Frequency (cm -1 ) +714 (b5) ~0.1 cm -1

10. ~0.03 cm -1 Some Rotationally Resolved Electronic Spectra of the “c” Progression. 34395.5 34397.6Frequency (cm -1 ) +727 (c5) 34300.034301.7 Frequency (cm -1 ) +631 (c4) 145 MHz 27 MHz ~0.03 cm -1

11. Transition“b” Progression“c” Progression ∆I’/ amu*Å 2 +427 (b2)-19.7 (1) +523 (b3)-20.0 (1) +535 (c3) -20.0 (1) +619 (b4)-20.2 (1) +631 (c4)-20.3 (1)-21.6 (1) +714 (b5)-21.1 (1) +727 (c5)-20.5 (1)-21.3 (1) Important observations: - Inertial defect values in S 1 follow similar trend as in the “a” progression (but less steep). Experimental Inertial Defects in S 1 - In the “c” progression, the c3 inertial defect follows the trend of the red- shifted c4 and c5 subbands. - On the other hand, the blue-shifted subbands in c4 and c5 manifest the opposite behavior. Symmetric Antisymmetric

12. α 2 γ 5 β 3 Vib. Mode Mode Assignments for the “b” Progression Assignments corrected from the experimental studies performed in the ground electronic state by Disperse Fluorescence Spectroscopy* *Zgierski et al. J. Chem. Phys. 96(10), 1992, 7229. α β γ Separation (cm -1 )

13. Mode Assignments for the “c” Progression Vib. Mode 6 2 62

14. Potential Energy Surfaces “b” progression“c” progression Q3Q3 Q5Q5 V(Q 2,Q 3,Q 5 ) = 2650 ± 50 cm -1 P o t e n t i a l E n e r g y

15. Conclusions The main “a” FC progression has been assigned to the “symmetric out-of-plane ring twisting” mode, the “b” progression to “in-plane stretching” + “in-plane bending” + “ring twisting” modes, and the “c” progression to “CH 2 -CH 2 bridge deformation” + “ring twisting” modes. The c4 and c5 subband splitting is due to inversion tunneling upon the combination of the two modes involved. The potential barrier estimation are 2650 cm -1 (for the c4 band) and 2150 cm -1 (for the c5 band). The potential barrier decreases upon excitation of further quanta of the ring twisting mode (Q 2 )! Potential energy surfaces along different low frequency coordinates have been obtained from analyses of the experimental data for each progression of transitions.

16. Acknowledgements Many thanks to: * Dr. John Yi (WSSU) and Dr. David Borst (INTEL) for helpful contributions on the data analysis. * To the current Pratt group members at the University of Pittsburgh. * To the National Science Foundation (NSF) for its financial support (CHE-0615755). * And thank YOU again, for your attention!