Slide 1

Conformation specific spectroscopy of jet- cooled 4-phenyl-1-butene Joshua A. Sebree, Josh J. Newby, Nathan R. Pillsbury, Timothy Zwier Department of Chemistry, Purdue University West Lafayette, IN Slide 2 Titans Atmosphere http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1512 Waite, J. H., Jr. et al; Science 2007, 316, 870-875. Molecular Nitrogen and Methane Photo-dissociation reactions C 2 H 2, C 2 H 4, C 2 H 6, HCN Ionization reactions C 2 H 5 +, CH 5 +, C 4 N 5 +, HCNH + Sunlight Energetic Particles C 6 H 6 isomers including benzene Other Complex organics (100~350 Da) Negative Organic Ions (20~8000 Da) Tholins Slide 3 Radical Chemistry Buildup of resonance stabilized radicals Propargyl Recombination Adducts can be formed from other resonance stabilized radicals Miller, J. A.; Klippenstein, S. J. J. Phys. Chem. A 2001, 105, 7254-7266. Slide 4 Alkylated Benzenes Selby, T. M.; Zwier, T. S. J. Phys. Chem. A 2005, 109, 8487-8496. (RA08) Slide 5 Time-of-Flight Mass Spectrometer/ Resonant 2 Photon Ionization Spectroscopy R2PI S0S0 S1S1 Ion 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 Slide 6 R2PI of 4-phenyl-1-butene Toluene like ring modes Complex origin region may suggest several conformers are present in expansion Slide 7 UV holeburning Holeburn (10Hz) Probe (20Hz) Conformer AConformer B S0S0 S1S1 Ion Holeburn (10Hz) Probe (20Hz) B* C E C C B A D A C A A B C C A A E B B B B B D Probe C A Holeburn Laser Timing ~700 nsec UV Hole-burn UV probe -Records the UV spectrum of a single conformations in the expansion using active baseline subtraction Active Baseline Subtraction Wavenumbers (cm -1 ) Probe only Probe + HB Difference Slide 8 UVHB of 4-phenyl-1-butene Three conformers present in jet with origins at 37525, 37528 and 37580 cm -1 Slide 9 Structures of PB Three dihedral angles C2-C1-C -C ~90 o for all structures C1-C -C -C Gauche (60 o ) Anti (180 o ) C -C -C -C Eclipsed (0 o ) Syn (120 o ) Slide 10 Structures of PB Results from B3LYP/6-311G** level of theory g/a C C 60/180 e/s C C 0/120 gauche-syn(+120) (gs+) E rel =3.98 kJ/mol A= 2368 MHz B= 835 MHz C= 766 MHz gauche-sync(-120) (gs-) E rel =2.48 kJ/mol A= 2565 MHz B= 828 MHz C= 774 MHz anti-eclisped (ae) E rel =2.43 kJ/mol A= 4089 MHz B= 608 MHz C= 568 MHz anti-sync( 120) (as) E rel =0.00 kJ/mol A= 4021 MHz B= 591 MHz C= 543 MHz Slide 11 Conformer Assignments High resolution (.08 cm -1 ) scans can resolve the rotational band contours of the origin peaks Rotational band contours can be simulated (JB95) using the rotational constants of both the ground and excited states and the transition dipole moment A B C g/a C C 60/180 e/s C C 0/120 gs+ gs- as ae Plusquellic, D. F.; Suenram, R. D.; Mate, B.; Jensen, J. O.; Samuels, A. C. J. Chem. Phys. 2001, 115, 3057-3067. Slide 12 Simulating Rotational Band Contours Ground state and excited state rotational constants are obtained from HF and CIS calculations respectively* Transition dipole moments can be obtained from the CIS calculations Transition dipole moments are conformer specific Anti: perpendicular (b-type) Gauche: hybrid (a-, b-, c-type) Mixing of 1 L a / 1 L b states AntiGauche Kroemer, R. T. L. et al.; J. Am. Chem. Soc. 1998, 120, 12573-12582. *calculations ran using 6-311+G** level of theory Slide 13 RBCs of PB Rotational band contours were simulated using JB95 gs+ g/a C C 60/180 e/s C C 0/120 A B C Hybrid band indicates gauche structure Q-branch implies a-character in hybrid band a : b : c TDM: 37:52:11 Slide 14 RBCs of PB Rotational band contours were simulated using JB95 g/a C C 60/180 e/s C C 0/120 A B C Perpendicular-type transition with b- and c-type mixing indicating gauche structure a : b : c TDM: 1:72:27 gs- Slide 15 RBCs of PB Rotational band contours were simulated using JB95 g/a C C 60/180 e/s C C 0/120 A B C Strong perpendicular-type transition indicative of anti structure a : b : c TDM: 8:83:9 as Slide 16 RBCs of PB g/a C C 60/180 e/s C C 0/120 A B C gs+ gs- as ae TDM: 8:83:9 E ref : 0.00 KJ/mol TDM: 1:72:27 E ref : 2.48 KJ/mol TDM: 37:52:11 E ref : 3.98 KJ/mol TDM: 0:0:100 E ref : 2.43 KJ/mol ? Slide 17 Conclusions R2PI and UVHB revealed three conformers of 4-phenyl-1-butene in the supersonic expansion. These conformers were assigned through rotational band spectroscopy as gauche- syn(+120), gauche-syn(-120), and anti-syn. The anti-eclipsed conformer that was also predicted by calculations was not observed Possible low barrier between anti-eclipsed and anti-synclinal conformations that could result in cooling out g/a C C 60/180 e/s C C 0/120 Slide 18 Acknowledgements The Zwier Group Tracy LeGreve Nathan Pillsbury* William James* Josh Newby* Chirantha Rodrigo Evan Buchanan Dr. Esteban Baquero Dr. V. Alvin Shubert Dr. Christian Mller Dr. Mike Nix Professor Timothy S. Zwier National Aeronautics and Space Administration http:www.gridchem.org