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

Titan’s Atmosphere Waite, J. H., Jr. et al; Science 2007, 316, 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

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,

Alkylated Benzenes Selby, T. M.; Zwier, T. S. J. Phys. Chem. A 2005, 109, (RA08)

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

R2PI of 4-phenyl-1-butene Toluene like ring modes Complex origin region may suggest several conformers are present in expansion

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

UVHB of 4-phenyl-1-butene Three conformers present in jet with origins at 37525, and cm -1

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 )

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

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,

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, *calculations ran using G** level of theory

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

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-

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

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 ?

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

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 Müller Dr. Mike Nix Professor Timothy S. Zwier National Aeronautics and Space Administration