Talitha M. Selby, Jasper R. Clarkson, H. Daniel Lee, and Timothy S. Zwier Isomer Specific Spectroscopy and Conformational Energetics of ortho-, meta-, and para-Ethynylstyrenes 60th Annual International Symposium on Molecular Spectroscopy Funding by DOE Diane Mitchell, James A. J. Fitzpatrick, and David W. Pratt J. Phys. Chem. A, 2005, 109, 4484

Introduction: Ethynylstyrenes I.Structural-Isomer Specific Spectroscopy C 4 H 2 * + Robinson, A. G.; Winter, P. R.; Zwier, T. S. J. Phys. Chem. A 2002, 106, II.Conformational-Isomer Specific Spectroscopy and Dynamics ? E a =?  E=? E a =?  E=? C 10 H 8 cis trans cis trans ortho meta ortho meta para

Techniques Resonance enhanced two photon ionization (R2PI) Ultraviolet holeburning spectroscopy (UV HB) Light Sources UV: Nd:YAG pumped dye lasers ( nm) and fourth harmonic (266 nm) of Nd:YAG Ionization continuum 1CR2PI Ionization continuum S1S1 S0S0 2CR2PI S1S1 S0S0 UV HB S1S1 Ionization continuum hb (10 Hz) probe (20 Hz) tuned S 0 (v=0) Boltzmann distribution of the vibrational population prior to expansion Collisional cooling to zero-point vibrational level B* C E C C B A D A C A A B C C A A E B B B B B D UV C A Boltzmann distribution of the vibrational population prior to expansion Collisional cooling to zero-point vibrational level B* C E C C B A D A C A A B C C A A E B B B B B D UV C A Supersonic-jet Spectroscopy

Overview R2PI Spectra of the Ethynylstyrenes 1CR2PI 2CR2PI (?) Ionization Potentials: pES below 8.29 eV | mES 8.48 eV | oES eV

UV Holeburning Spectrum of ortho-Ethynylstyrene Only trans-oES present cis-oES only 5% RT population  E=1.8 kcal/mol  E=2.03 kcal/mol  E=0.0 kcal/mol Intensity (arbitrary units) Wavenumbers(cm -1 ) Para impurity CR2PI ortho UVHB ortho B3LYP/6-31+G* c-oES has 1600 cm -1 internal energy pre-expansion k isomerization (1600 cm -1 ) ~10 11 s -1 k collision early in expansion ~ s -1 Rate of isomerization faster than collision rate.

UVHB Spectra of meta-Ethynylstyrenes 0.00 kcal/mol E a ≈1200 cm-1 *B3LYP/6-31+G* level of theory 0.08 kcal/mol B3LYP/6-31+G* Wavenumbers/(cm -1 ) Intensity (arbitrary units) A B A(0 0 0 ) B(0 0 0 ) a) 2CR2PI mES b) UVHB mES(A) c) UVHB mES(B) B3LYP/6-31+G*

Rotationally Resolved Fluorescence Excitation a-type transitions B3LYP/6-31+G* red-shifted conformer of mES TDM: ~90% A type mES (A)

Rotationally Resolved Fluorescence Excitation a-type transitions pES(0 0 0 ) TDM: ~90% A type pES B3LYP/6-31+G*

Stimulated Emission Pumping –Population Transfer Spectroscopy II. UV Pump, pump V. UV Probe, probe III. UV Dump, dump IV. Collisional Cooling, isomerization k isom k cool Dian, B. C.; Clarkson, J. R.; Zwier, T. S. Science 2004, 303, A(v=0) A* B(v=0) (A) (B) A* Expt’l protocol: 1.Cool 2.Pump 3.Dump 4.Re-cool 5.Probe Population transfer spectroscopy: Fix Pump on A, Probe on B; Tune Dump: Watch population come into B from A I. Cooling Prepare ground state A with a well defined amount of energy

a) SEP of cis-meta d) SEP-PTS trans cis b) SEP-PTS cis trans SEP and SEP-PTS of mES c) SEP of trans -meta

Near-threshold population transfer intensity determined by the competition between isomerization, cooling k isom (E) k cool (E ) Harmonic RRKM estimate: At threshold, k isom (E) = 2.6X10 9 sec -1 and k coll = 1.0X10 9 sec -1

Bounds on the barrier and relative energies of minima B A E(A→B) E(B→A)  E=E(A→B) - E(B→A) Hollas, J. M.; Musa, H.; Ridley, T.; Turner, P. H.; Weisenberger, K. H.; Fawcett, V. J. Mol. Spectrosc. 1982, 94, 437

Comparison of Methods

Also gives the form of the entire potential energy function along the torsional coordinate. Requires spectroscopic detection of the torsional energy levels Assumes the torsional coordinate is the only coordinate involved in isomerization Torsional Potential Fitting Comparison of Methods

Also gives the form of the entire potential energy function along the torsional coordinate. Requires spectroscopic detection of the torsional energy levels Assumes the torsional coordinate is the only coordinate involved in isomerization Not reliant on assignment of normal mode to reaction coordinate Relies on the spacing of the SEP transitions, but yes/no question Relies on isomerization occurring on a time scale that can successfully compete with collisional cooling Apply to cases where many conformers: Breaks into specific A→B pairs Torsional Potential Fitting Comparison of Methods SEP-PTS

Summary of the Ethynylstyrenes Only one isomer of oES was observed in the expansion. From calculated energy differences the observed conformer was assigned to the trans conformer. Two isomers of mES were observed. The red-shifted conformer was identified as the cis conformer from the rotationally resolved fluorescence excitation spectrum. The barrier to cis→trans isomerization in mES is ~1000 cm -1 and the two conformations are nearly isoenergetic, in qualitative agreement with calculations

Acknowledgments Prof. Timothy S. Zwier The Zwier Group -Jasper R. Clarkson H. Daniel Lee Funding: Department of Energy Prof. David W. Pratt The Pratt Group -Diane Mitchell -James A. J. Fitzpatrick

UVHB spectrum of trans-ortho-Ethynylstyrene Evidence of vibronic coupling Intensity of transitions No overtones False origin S 1 (A′)←S 0 (A′)

Nature of the S 0 -S 1 Transitions: Transition Dipole Moment Directions The direction of the TDM in disubstituted benzenes depends both on the nature of the substituents and their relative positions. All the meta disubstituted benzenes shown have the TDM along the a-axis. In para disubstituted benzenes it appears the nature of the substituents does matter for the TDM direction. In pES, the vinyl group has a larger influence on the transition T.V. Nguyen, J.W. Ribblett, and D.W. Pratt, Chem. Phys 283,279,2002 J.A. Stearns and T.Z. Zwier, J Phys Chem. A, 107,10723,2003 Ribblett, J. W.; Borst, D. R.; Pratt, D. W. J. Chem. Phys. 1999, 111, Stearns, J. A.; Zwier, T. S. J. Phys. Chem. A 2003, 107,

Electronic Origin Shifts in Vinyl and Ethynyl Substituted Benzenes Electronic origin shifts are additive Trends in ortho, meta, para Trends in cis and trans mDVB and mES As substituents become closer together they further red shift T.V. Nguyen, J.W. Ribblett, and D.W. Pratt, Chem. Phys 283,279,2002 J.A. Stearns and T.Z. Zwier, J Phys Chem. A, 107,10723,2003 J.A. Syage, F. Al Adel, and A.H. Zewail, Chem Phys Lett. 103,15,1983 K. Narayanan, G.C. Chang, K.C. Shieh, C.C. Tung, and W.B. Tzeng, Spectochim Acta A. 52,1703,1996