CONFORMATION-SPECIFIC ELECTRONIC SPECTROSCOPY OF JET-COOLED 5-PHENYL-1-PENTENE NATHAN R. PILLSBURY, TALITHA M. SELBY, AND TIMOTHY S. ZWIER, Department of Chemistry, Purdue University, West Lafayette, IN 47907
5-phenyl-1-pentene 5-phenyl-1-pentyne Motivation for Studying 5-phenyl-1-pentene What differences arise from replacing the ethynyl group with a vinyl group? Barriers to exciplex formation from different starting structures could be reflected in different lifetimes as a function of energy above the origin Ho, C. D.; Morrison, H. J. Am. Chem. Soc.2005, 127, 2114-2124.
pulsed valve ground plate manual gate valve 2 stage ion acceleration steering plates Einzel lens mass gate pulser laser port microchannel plate detector pneumatic gate valve cryocooler to roughing pump to roughing pump diffusion pump Schematic Diagram of TOF Mass Spectrometer
Resonant Two-Photon Ionization Spectroscopy (R2PI) • Molecules are cooled to zero point vibrational levels in the free jet expansion • Mass selection gives confirmation that the spectrum is due to the molecule of interest 5-phenyl-1-pentene+ + e- 5-phenyl-1-pentene* (S1) 5-phenyl-1-pentene (S0)
B A B C C A B B C B B A B A A B UV-UV Hole-burning spectroscopy Records the UV spectrum of a single conformation free from interference from others present in the expansion UV Hole-burn laser fixed: Provides l selectivity UV probe laser tuned Conformer A Conformer B 5-phenyl-1-pentene + + e- UV A C A C B* B* B C B* A C B* Boltzmann distribution of conformers in the pre-expansion UV 5-phenyl-1-pentene * (S1) Collisional cooling to zero-point vibrational level Probe Hole-burn Hole-burn Probe 50-500 nsec 5-phenyl-1-pentene (S0) UV Hole-burn UV probe Laser Timing
00 0 60 1 120 & 180 1 1 UV-UV Hole-burning Spectra
Calculated Structures and Relative Energies 0.0 kcal/mole 0.41 kcal/mole 0.68 kcal/mole Cδ Cε Cγ H H’ Cα Cβ C(1) anti-gauche-eH’ anti-anti-eH gauche-anti-eH 0.99 kcal/mole 0.77 kcal/mole 0.80 kcal/mole 1.64 kcal/mole gauche-anti-eH’ anti-anti-eC anti-gauche-eH gauche-anti-eC Dihedral Angle Definitions t2 = C(1)-Cα-Cβ-Cγ t3 = Cα-Cβ-Cγ-Cδ t4 = Cβ-Cγ-Cδ-Cε Dihedral Labels t2 (t3) = 1800 = anti t2(t3) = ±600 = gauche t4 = 00 = eC (eclipsed with Cβ) t4 = 1200 = eH t4 = -1200 = eH’
Origin Region of 5-phenyl-1-pentene gauche (t2) anti (t2)
Transition Dipole Moment Sensitivity The TDM in monosubstituted benzenes has been found to be very sensitive to the nature and orientation of the substituent According to Pratt and Simons*, the TDM in gauche conformations swings about 30 degrees from the anti (trans) conformations Surprisingly, CIS calculations correctly predict the transition moment direction in these gauche structures * Kroemer, R. T. L., K. R; Dickinson, J. A.; Robertson, E. G.; Simons, J. P.; Borst, D. R.; Pratt, D.W. J. Am. Chem. Soc. 1998, 120, 12573.
Experimental Best Fit Rotational Band Contour Fits %A:%B:%C 67:0:33 0:36:64 0:12:88 28:16:56 16:48:36
Structural Assignments A aa(eH) ga(eC) D ag(eH’) ga(eH) E B ag(eH) C ga(eH’) Vib A/B Vib C
Comparison of Electronic Frequency Shifts 5-phenyl-1-pentyne (37601 cm-1) 5-phenyl-1-pentene (37580 cm-1) Vinyl group red shifts the spectrum by a about 20 cm-1 to the red and adds two more conformations; however the shifts between ag and ga are held roughly constant
14ns 13ns 56ns 14ns 62ns 83ns 73ns 11ns 96ns 51ns 67ns 88ns 12ns 75ns 12ns 7ns 14ns 10ns 35ns 83ns 87ns 7ns 85ns 14ns 80ns 10ns 76ns Lifetime Study of 5-phenyl-1-pentene
Possible Reasons for Lack of Conformation Specificity • Barrier to exciplex structures is too high and therefore not probed in the FC region • Lifetime shortening may be determined by something other than exciplex formation (e.g. internal conversion or intersystem crossing) • IVR may be fast relative to isomerization S1 Lifetimes of 5-phenyl-1-pentyne (nsec) ~80 ~50
Future Work • Dispersed Fluorescence • Shows where strong SEP transitions are located • May show conformation-specific IVR effects • Probe isomerization in the S1 state (may see a difference between gauche vs. anti) • SEP-Population Transfer Spectroscopy • Measures the barriers to isomerization experimentally • Jasper Clarkson • Talitha Selby
Acknowledgements • The Zwier Group • Dr. Timothy Zwier • Department of Energy (DOE)
Optically active ring modes of mono-substituted alkylbenzenes Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys.1980, 72, 5039.
S1 I. UV Pump, l1 II. UV Dump, l2 IV. UV Probe, l3 Excited vibrational Level A* S0 Zero-point level Collisional cooling, isomerization B III. C A SEP-Population Transfer Spectroscopy
Resonant Ion-Dip Infrared Spectroscopy (RIDIRS) Records IR spectrum of single species free from interference from others present in the expansion S0 RIDIRS S1 RIDIRS Hydrocarbon+ + e- Hydrocarbon+ + e- Hydrocarbon *(S1) Hydrocarbon *(S1) Hydrocarbon (A) (S0) Hydrocarbon (A) (S0)