Ultraviolet Photodissociation Dynamics of the Cyclohexyl Radical Michael Lucas, Yanlin Liu, 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

Photodissociation of Alkyl Radicals  Prototypical organic radicals  Important intermediates in combustion  Photodissociation via Rydberg states  Our group’s previous work: methyl, ethyl, propyl

Photodissociation of Ethyl via 3s Rydberg State Bimodal H-atom distribution Fast Pathway: anisotropic (  = 0.5), high  f T , direct H-atom scission via nonclassical H-bridged structure from the 3s state to yield H + C 2 H 4 (X 1 A g ). Slow Pathway: isotropic, modest  f T , unimolecular dissociation after internal conversion. G. Amaral et al. J. Chem. Phys. 114 (2001) 5164 M. Steinbauer et al. J. Chem. Phys. 137 (2012) Conical intersection

Photodissociation of Aromatic Radicals  Our recent work: phenyl, benzyl, o-pyridyl, m- pyridyl  Important intermediates in combustion and soot formation  Photodissociation mechanisms – unimolecular dissociation following internal conversion; statistical product energy distribution I.C. Y. Song et al. J. Chem. Phys. 136 (2012)

Cyclohexyl Radical  Cycloalkanes are important component of conventional fuels  Cyclohexane model cycloalkane  Major producer of benzene  No previous photodissociation studies of cyclohexyl

Potential Energy Diagram of c-C 6 H 11 C. Franklin Goldsmith et al. J. Phys. Chem. 113 (2009) ~ ~

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) 7027 Chlorocyclohexane Bromocyclohexane

Production of Cyclohexyl Radical Beam nm VUV photoionization mass spectrometry Net mass spectrum: 193-nm radical generation radiation on minus off Radical production Precursor depletion Cl-C 6 H 11 + hv → Cl + C 6 H 11

H-atom TOF Spectra  check precursors

H-atom Product Action Spectrum  compare with absorption spectrum J. Platz et al. J. Phys. Chem. A 103 (1999) 2688

CM Product Translational Energy Distribution

Average E T Release

H-atom Product Angular Distribution β ~ Anisotropic distribution Dissociation time faster than 1 rotation period E v 

Photodissociation Mechanism I ~ ~ x x Unlikely, Unimolecular dissociation, Cannot compete with c-C 6 H 10 channel Repulsive dissociation; Similar to ethyl x

Photodissociation Mechanism I Conical intersection

Photodissociation Mechanism II  v

Summary  UV photodissociation dynamics of cyclohexyl was studied in nm for the first time  Observed: cyclohexyl → cyclohexene + H  Large translational energy release,  f T    Anisotropic distribution  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)

Acknowledgements  Prof. Jingsong Zhang  Yanlin Liu  Zhang Group