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International Symposium on Molecular Spectroscopy 64 th Meeting - June 22-26, 2009,

CONFORMATIONAL ISOMERIZATION OF bis -(4-HYDROXYPHENYL)METHANE IN A SUPERSONIC JET EXPANSION, PART I: LOW BARRIER POTENTIAL ENERGY SURFACE IN THE S 0 STATE. . Presented by Chirantha P. Rodrigo , Christian W. M ü ller, William H. James III, Nathan R. Pillsbury and Timothy S. Zwier

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International Symposium on Molecular Spectroscopy 64 th Meeting - June 22-26, 2009,

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  1. CONFORMATIONAL ISOMERIZATION OF bis -(4-HYDROXYPHENYL)METHANE IN A SUPERSONIC JET EXPANSION, PART I: LOW BARRIER POTENTIAL ENERGY SURFACE IN THE S0 STATE. 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 International Symposium on Molecular Spectroscopy 64th Meeting - June 22-26, 2009, Ohio State University, Columbus, OH

  2. H H Conformations of bis-(4-hydroxyphenyl)methane (b4HPM) ? cm‒1 1100-1266 cm‒1 • Four torsional degrees of freedom. • Phenolic OH torsional barrier height is 1100-1266 cm‒1.* • what would be the phenyl torsional barrier? Diphenylmethane** • * 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).

  3. Conformations of bis-(4-hydroxyphenyl)methane (b4HPM) dd (C2) 0 cm‒1 uu (C2) 5 cm‒1 ud (C1) 11 cm‒1 Stationary points were calculated at DFT/B3LYP/6-311G(d,p) level of theory.

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

  5. x = distance from nozzle to laser beam D = nozzle diameter 2 x/D) 4 (x/D) 6 (x/D) A 7 (x/D) 11 (x/D) 18 (x/D) C B 35180 35200 -1 Energy / cm Electronic Spectrum and UV-UV Holeburning Spectra of b4HPM. A intensity / (arb. units) 35185 B (x7) * * * * C 35209 * incomplete subtraction 35150 35200 35250 35300 35350 35400 35450 35500 –1 Energy / cm

  6. Single Vibronic Level Fluorescence (SVLF) Spectroscopy Conformer C : FES (HB) S 1 CCD Camera • Provides information about ground state (S0) low frequency vibrational levels. S 0

  7. Conformer C : FES (HB) 0 20 40 60 80 100 120 140 160 180 200 Energy from ZPL / cm-1 SVLF Spectroscopy and Ground State Vibrational Frequencies: Conformers A & C S1←S0 transition S1-00 22 Conformer C : Origin SVLF 44 154 25 67 177 89 52 199 71 111 181 137 35 C conformer and its assignments.

  8. Calculated Ground state vibrational frequencies. A & C Three lowest vibrational modes. * Calculations were carried out at DFT/B3LYP/6-31+G(d) level of theory in GAUSSIAN03

  9. Pump (20Hz) SVLF: Conformer A intensity / (arb. units) SEP: Conformer A 0 100 200 300 –1 Energy above ZPL / cm Stimulated Emission Pumping (SEP) Dump (10Hz) • SEP gives specific information about low frequency ground state (S0) vibrational levels. • Prepare the molecule (conformer) with a known amount of energy. C* C B A A A B C A B A B* C C B* A C* Pump 20 Hz UV Dump 10 Hz UV Δt=0-2 ns

  10. Pump (20Hz) Dump (10Hz) A B B Probe (20Hz) A C C C B C A B A B A B Conformation-specific state preparation via SEP. S1 Fluorescence detection New conformer distribution 5. UV Probe 2. UV Pump 3. UV Dump 1. Initial Cooling Excited Vibrational Level 4. Collisional Cooling S0 B* A Zero-point Level C B Stimulated Emission Pumping (SEP) Population Transfer Spectroscopy.* Collisional re-cooling to zero-point vibrational levels C* C B A* A C A A B C C A A* B* B A C C C A C C* B* B C A A* A A A A B A C • SEP-PT Steps • Initial cooling. • Pump. (20 Hz) • Dump. (10 Hz) • Collisional re-cooling. • Probe. (20 Hz) *Brian C. Dian, Jasper R. Clarkson, and Timothy S. Zwier Science 303, 1169 (2004).

  11. SEP- A 22 26 53 45 156 SEP-PT, A-->A 45 26 intensity / (arb. units) SEP-PT, A-->C Lower Upper 0 22 bound bound SEP-PT, A-->B 0 50 100 150 200 250 300 –1 Energy above ZPL / cm SEP Population Transfer Spectroscopy of Conformer A.

  12. SEP-C 22 26 45 49 53 156 36 SEP-PT, C-->C intensity / (arb. units) 36 45 SEP-PT, C-->A Lower Upper 45 36 bound bound SEP-PT, C-->B 50 100 150 200 250 300 –1 Energy above ZPL / cm SEP Population Transfer Spectroscopy of Conformer C. There is no B SEP-PTS

  13. SEP Population Transfer Spectroscopy: Summary.

  14. 180 180 du uu 180 190 120 190 180 ud dd 120 120 ud 180 120 190 du 180 Calculated Potential Energy Surface, Ground State (S0). B3LYP / 6-311G(d,p) Relative energies: dd = 0 cm-1 uu = 5 cm-1 ud= du = 11 cm-1

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

  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. Acknowledgement • Collaborators: • Dr. David F. Plusquellic • Prof. Lyudmila Slipchenko (PURDUE) • Computational Resources: • Purdue ITap • GridChem • Funding: • Department of Energy 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

  18. p-Cresol OH Stretch Frequency* (3658) Conformer A 3659 intensity (arb. units) Conformer B 3665 Conformer C 3659 3640 3660 3680 -1 Frequency / cm S0-Fluorescence Dip InfraRed (S0-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)

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

  20. Exciton Splitting. A B 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.

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