Ultraviolet Photodissociation Dynamics of the 3-Cyclohexenyl Radical Michael Lucas, Yanlin Liu, Jasmine Minor, Raquel Bryant, Jingsong Zhang Department of Chemistry University of California, Riverside 69 th International Symposium on Molecular Spectroscopy 6/17/2014

Photodissociation of Free Radicals  Free radicals  Open shell  Highly reactive  Important in many areas of chemistry  Combustion, atmospheric, plasma, interstellar  Dissociation depends on potential energy surfaces  Provide benchmarks for theory

Cyclohexyenl Radical  Cycloalkanes are important component of conventional fuels  Cyclohexane model cycloalkane  Major producer of benzene  Previous Research: cyclohexyl, phenyl  What effect does the double bond have on the photochemistry?

Potential Energy Diagram of c-C 6 H 9 ~ ~ ● ● ● K. Furukawa et al. Int. J. Chem. Kin. 6 (1974) 337 NIST Chemistry WebBook

High-n Rydberg H-atom Time-of-Flight (HRTOF) H Lyman-  Probe nm Photolysis Pulsed Valve Rydberg Probe nm Detector Skimmer 193 nm H transitions 1 2 nH+H+ H (n) H (2 2 P) nm Lyman-  nm K. Welge and co-workers, J Chem Phys 92 (1990) chlorocyclohexene 3-bromocyclohexene

Production of Cyclohexenyl Radical Beam  nm VUV photoionization mass spectrometry  Net mass spectrum: 193-nm radical generation radiation on minus off Radical production Precursor depletion

H-atom TOF Spectra  check precursors

H-atom Product Action Spectrum  compare with absorption spectrum R. Schuler et al. Chem. Phys. Lett. 27 (1974) 369; D. Pratt et al. J. Am. Chem. Soc. 96 (1974) 5588

CM Product Translational Energy Distribution

Average E T Release

H-atom Product Angular Distribution E v  Major: β ~ 0 Isotropic distribution Dissociation time slower than 1 rotational period (ps) Minor: β < 0 Anisotropic distribution Dissociation time faster than 1 rotational period *

H-atom Product Angular Distribution Major: β ~ 0 Isotropic distribution Dissociation time slower than 1 rotational period (ps) E v  Minor: β < 0 Anisotropic distribution Dissociation time faster than 1 rotational period

Photodissociation Mechanism ~ ~ ● ● ● Repulsive dissociation I.C. Unimolecular Dissociation

Summary  UV photodissociation dynamics of cyclohexenyl was studied in nm for the first time  Observed: cyclohexenyl → cyclohexadiene + H  Modest translational energy release,  f T  ~ 0.15  Two components  Major: Isotropic distribution, β ~ 0 Dissociation mechanism: internal conversion from excited electronic state followed by unimolecular dissociation on ground electronic state  Minor: Anisotropic distribution, β < 0 Dissociation mechanism: direct dissociation from excite state or repulsive part of ground state

Comparison With Cyclohexyl

 Moderate translational energy release  Two component  Major  Isotropic distribution  Statistical distribution  Hot radical dissociation mechanism  Minor  Anisotropic distribution, β < 0  Repulsive dissociation mechanism  Large translational energy release  Anisotropic distribution, β > 0  Non-statistical distribution  Dissociation mechanism: direct dissociation from the excited state and/or on the repulsive part of the ground state (possibly via conical intersection). CyclohexenylCyclohexyl

Acknowledgements  Prof. Jingsong Zhang  Yanlin Liu  Jasmine Minor  Raquel Bryant  Zhang Group