Experimentally Mapping Out the Ground State Potential Energy

Experimentally Mapping Out the Ground State Potential Energy Surface of 5 -phenyl-1 -pentene Nathan R. Pillsbury and Timothy S. Zwier, Department of Chemistry, Purdue University, West Lafayette, IN 47907 63 rd International Symposium on Molecular Spectroscopy, June 16 -20, 2008

R 2 PI Spectrum of 5 -Phenyl-1 -pentene 5 PPene

R 2 PI and UVHB Spectra of 5 -Phenyl-1 -pentene 000 1 60 1 1210 & 180 R 2 PI A B C D E

R 2 PI Spectrum of 5 -Phenyl-1 -pentene Origin Region A D E B C Vib. A/B Vib. C

Rotational Band Contours of 5 PPene Origins A B gg. Hg' ga. Hg' C ga. Hg D ag. Hg E aa. Hg

Structural Assignments of 5 PPene A D gg. Hg' B ag. Hg E ga. Hg' aa. Hg C ga. Hg Vib. A/B Vib. C

SEP-Population Transfer Spectroscopy 3. UV Dump A* In Practice 1. Fix Pump on A 2. Fix Probe on B 3. Tune Dump 4. Watch population come into B from A S 0 C A 5. UV Probe S 1 2. UV Pump Expt’l Protocol 1. Cool 2. Pump 3. Dump 4. Re-cool 5. Probe 4. Collisionally re-cool B Excited Vibrational Level Zero-point Level

SEP-population transfer spectroscopy can be used to obtain: 1. Direct experimental measurements of the thresholds 2. for conformational isomerization in flexible molecules 2. Experimental control over the products formed by 3. choice of dump wavelength 3. The relative energies of the conformational minima 4. 4. Insight to the dominant isomerization pathways on the potential energy surface 5. Benchmark data for ab initio, force field, and dynamics calculations: We need accurate PES’s to describe the dynamics! 6. Insight into near-threshold effects: The competition between isomerization/cooling

Upper bound Lower bound SEP-PT Spectra from Conformer A

SEP-PT Spectra from Conformer B B→C threshold

SEP-PT Spectra from Conformer C C→B threshold

Lower bound Upper bound SEP-PT Spectra from Conformer D

SEP-PT Spectrum from Conformer E

Experimental and Calculated Barrier Heights and Pathways 14 X→Y thresholds measured Expt. (cm-1) Calc. (cm-1) A B 1202 -1374 1347 X A C 1202 -1374 1340 X X A D 1202 -1374 - X X X A E 1202 -1374 1372 X X X B C <622 676 B D 1003 -1625 - C A 1001 -1622 1184 C B <621 671 C D 806 -1622 - X C E 1001 -1622 1216 X D A 1002 -1624 - X X X D B 1002 -1624 - X X X D C 1002 -1624 - X X D E 1002 -1624 1138 Cα-Cβ Cβ-Cγ Cγ-Cδ X X X d b g X X X Calculations were performed at the M 05 -2 X/6 -31+G(d) level a

Conclusions 1. 14 isomerization thresholds of 5 PPene were experimentally measured using SEP-PT spectroscopy 2. B 3 LYP and MP 2 showed systematic deficiencies, while M 05 -2 X calculations were relatively accurate at predicting both the relative energies of the minima and the barrier heights 3. Pathways to isomerization can be inferred by following the minimum energy pathway along the PES

Acknowledgements Computational Resources Professor Timothy S. Zwier Current Group Members: Dr. Christian Müller Dr. Michael Nix Tracy Le. Greve Joshua Newby William James III Chirantha Rodrigo Joshua Sebree Evan Buchanan Past Group Members: Dr. Jaime Stearns Dr. Talitha Selby * Dr. Jasper Clarkson Dr. Ching-Ping Liu Dr. V. Alvin Shubert Dr. Esteban Baquero Funding