1. 1 International Symposium on Molecular Spectroscopy 64 th Meeting - June 22-26, 2009, Ohio State University, Columbus, OH Presented by Chirantha P. Rodrigo, Christian W. Müller, William H. James III, Nathan R. Pillsbury and Timothy S. Zwier Department of Chemistry, Purdue University West Lafayette, IN 47907 CONFORMATIONAL ISOMERIZATION OF bis -(4-HYDROXYPHENYL)METHANE IN A SUPERSONIC JET EXPANSION, PART I: IN A SUPERSONIC JET EXPANSION, PART I: LOW BARRIER POTENTIAL ENERGY SURFACE IN THE S 0 STATE.

2. 2 Conformations of bis-(4-hydroxyphenyl)methane (b4HPM) Four torsional degrees of freedom. Phenolic OH torsional barrier height is 1100-1266 cm ‒ 1.* what would be the phenyl torsional barrier? * Kudchadker, S. A. et al; J. Phys. Ref. Data, 7, 2, (1978), p417 and the references therein **Nathan R. Pillsbury et al: J. Chem. Phys., 129, 114301 (2008). 1100-1266 cm ‒1 ? cm ‒ 1 HH Diphenylmethane**

3. 3 Conformations of bis-(4-hydroxyphenyl)methane (b4HPM) dd (C 2 ) 0 cm ‒ 1 uu (C 2 ) 5 cm ‒1 ud (C 1 ) 11 cm ‒1 Stationary points were calculated at DFT/B3LYP/6-311G(d,p) level of theory.

4. 4 Supersonic Jet Cooled Expansion and Spectroscopic Techniques. Collisional cooling to zero-point vibrational levels B B A C A C C B* A* C* B A C A A A B A B C C C C B A B B B Probe 20 Hz UV Δt=200 ns Pump 10 Hz UV Probe 20 Hz UV Fluorescence Excitation Spectroscopy Provides the excited state vibrational structure UV-UV Holeburning Spectroscopy Provides single conformer excitation spectra Boltzmann distribution of conformers in the pre-expansion gas mixture Probe laser

5. 5 Electronic Spectrum and UV-UV Holeburning Spectra of b4HPM. intensity / (arb. units) 3550035450354003535035300352503520035150 Energy / cm –1 ** * * * incomplete subtraction A B(x7) C 35185 35209 x = distance from nozzle to laser beam D = nozzle diameter 3520035180 Energy / cm 2 x/D) 4 (x/D) 6 (x/D) 7 (x/D) 11 (x/D) 18 (x/D) A B C

6. 6 Single Vibronic Level Fluorescence (SVLF) Spectroscopy Conformer C : FES (HB) S 1 S 0 CCD Camera Provides information about ground state (S 0 ) low frequency vibrational levels.

7. 7 22 25 35 44 52 67 71 89 111 137 154 177 181 199 S 1 -0 0 200180160140120100806040200 Energy from ZPL / cm -1 SVLF Spectroscopy and Ground State Vibrational Frequencies: Conformers A & C C conformer and its assignments. Conformer C : Origin SVLF Conformer C : FES (HB) S 1 ← S 0 transition

8. 8 Calculated Ground state vibrational frequencies. A & C Three lowest vibrational modes. Vibration mode /Symmetry Observed frequency / cm –1 Calculated frequency / cm –1 * (dd/uu) 12.511.5/14 2226/25.5 4444/44.5 * Calculations were carried out at DFT/B3LYP/6-31+G(d) level of theory in GAUSSIAN03

9. 9 Stimulated Emission Pumping (SEP) B B A C A C C B* A A A C* B A C Pump (20Hz) Dump (10Hz) intensity / (arb. units) 3002001000 Energy above ZPL / cm –1 SVLF: Conformer A SEP: Conformer A Δt=0-2 ns Dump 10 Hz UV Pump 20 Hz UV SEP gives specific information about low frequency ground state (S 0 ) vibrational levels. Prepare the molecule (conformer) with a known amount of energy.

10. 10 Stimulated Emission Pumping (SEP) Population Transfer Spectroscopy.* B B A C A C C B* A* C* A C A A A C C B A A A A C C B A C B A C Conformation- specific state preparation via SEP. New conformer distribution Collisional re-cooling to zero-point vibrational levels Probe (20Hz) A A A B A B C C C C B A B B B Pump (20Hz) Dump (10Hz) S0S0 S1S1 Zero-point Level C A B 5. UV Probe 4. Collisional Cooling 3. UV Dump 2. UV Pump Excited Vibrational Level B*B* 1. Initial Cooling Fluorescence detection SEP-PT Steps 1)Initial cooling. 2)Pump. (20 Hz) 3)Dump. (10 Hz) 4)Collisional re-cooling. 5)Probe. (20 Hz) * * Brian C. Dian, Jasper R. Clarkson, and Timothy S. Zwier Science 303, 1169 (2004).

11. 11 SEP Population Transfer Spectroscopy of Conformer A.

12. 12 SEP Population Transfer Spectroscopy of Conformer C. There is no B SEP-PTS

13. 13 SEP Population Transfer Spectroscopy: Summary. Isomerization reaction Measured barrier (Lower and Upper bounds) / cm ‒ 1 A → C26-45 A → B0-22 C → A36-45 C → B36-45

14. 14 Calculated Potential Energy Surface, Ground State (S 0 ). B3LYP / 6-311G(d,p) Relative energies: dd = 0 cm-1 uu = 5 cm-1 ud = du = 11 cm-1 uu ud du dd 120 190 180 120 ud du 180 190

15. 15 Minimum Energy Isomerization Pathway. Lowest energy isomerization pathway lies along the (asymmetric torsion) coordinate.

16. 16 Summary. b4HPM has three conformations in the jet expansion. All three conformers are nearly isoenergetic. Barriers to isomerization between conformers are within 0-45 cm –1. Theoretical calculations also indicate these unusually low barriers to isomerization. Minimum energy pathway lies along the asymmetric torsional coordinate.

17. 17 Acknowledgement Advisor: Advisor: Prof. Timothy S. Zwier Zwier Research Group Dr. Christian W. Müller (Post Doctoral fellow) William H. James III Josh J. Newby Josh Sebree Evan “Duyy” Buchanan Zachary Davis James Redwine Ryan Muir Deepali Mehta Past members Dr. Nathan R. Pillsbury Dr. Esteban E. Baquero Dr. Virgil A. Shubert Dr. Tracy A. LeGreve Dr. Jasper R. Clarkson Dr. Talitha Selby Dr. Ching-Ping Liu Collaborators: Dr. David F. Plusquellic Prof. Lyudmila Slipchenko (PURDUE) Computational Resources: Purdue ITap GridChemFunding: Department of Energy

18. 18 intensity (arb. units) 368036603640 Frequency / cm 3659 3665 3659 Conformer A Conformer B Conformer C S 0 -Fluorescence Dip InfraRed (S 0 -FDIR) Spectra of b4HPM. Hydroxyl (-OH) Stretch Region *Fujimaki, E.,Fujii, A., Ebata, T., and Mikami N.; J. Chem. Phys. 2000, 112,137 Frequency calculations were carried out at DFT/M05-2X/6-31+G(d) level of theory (GAUSSIAN03 Rev E.01) p-Cresol OH Stretch Frequency* (3658)

19. 19 uu / 5 cm-1 ud/ 11 cm-1 dd / 0.0 cm-1 Go Back

20. 20 Exciton Splitting. para-cresol* origin at 35338 cm –1 b4HPM origin at 35185 cm –1 * Oikawa, A.; Haruo, A.; Mikami, N.; Ito, M. J. Phys. Chem. 1984,88, 5180. A B