OBSERVATION OF THE A-X ELECTRONIC TRANSITION OF C 6 -C 10 PEROXY RADICALS Neal D. Kline and Terry A. Miller Laser Spectroscopy Facility The Ohio State University ~ ~

Motivation for Study of Peroxy Radicals Key combustion intermediates Atmospheric chemistry: lead to formation and destruction of ozone RO 2 OH R’CHO RO RH RO 2 NO 2 RONO 2 HO 2 CO CO 2 R’C(O)O 2 PAN ROOH O2O2 NO 2 HO 2 NO h OH NO 2 NO NO 2 h OH O2O2 O2O2 NO 2 NO  Isomerisation Multistep h NO 2 O 2 1. Lightfoot et. al. Atmos. Envir. 26A,

C 6 -C 10 Peroxy Radicals Gasoline mixtures contain a mixture of hydrocarbons: parrafins, olefins, and aromatics. Larger chain hydrocarbons(>C 5 ) make up a significant portion of gasoline mixtures; heptane and 2,2,4-trimethyl pentane (isooctane) are standards for octane rating scale with ratings of 0 and 100, respectively. Picture of refineries

Low-Lying Electronic Transitions of Peroxy Radicals B−X transition: Strong (c.f. O 2 : B 3 Σ u − ← X 3 Σ g − ) Located in UV: ca cm -1 Repulsive → little structural information, poor selectivity HOMO-1 HOMO SOMO O. J. Nielsen and T. J. Wallington, in Peroxyl Radicals, (John Wiley and Sons, New York, 1997), pp ~~ Jafri, J. A, and D. H. Phillips. J. Am. Chem. Soc. 112 (1990) R O–O X ~ A ~ B ~

Low-Lying Electronic Transitions of Peroxy Radicals A−X transition: Weak (c.f. O 2 : a 1 Δ g ← X 3 Σ g − ) Located in NIR: ca cm -1 Bound → resolved vibrational structure HOMO-1 HOMO SOMO Chung et al. J. Chem. Phys. 127 (2007) ~~ Jafri, J. A, and D. H. Phillips. J. Am. Chem. Soc. 112 (1990) R O–O X ~ A ~ B ~

Principles of CRDS time Intensity

τ abs σ Nlσ Nl + = cL)/( R1 - ( ) Principles of CRDS τ0τ0 c L )/ ( R1 - = A = L/cτ abs - L/cτ 0 L l R time Intensity A

Sirah dye laser nm Nd:YAG: 532 nm Raman cell (H 2, 300 psi) 2 nd Stokes: cm -1 Room Temperature Cavity Ringdown Setup 20 Hz ~600 mJ/pulse ~70-80 mJ/pulse ~1-2 mJ/pulse Photolysis: Excimer Laser ArF, 193 nm KrF, 248 nm Highly Reflective Mirror ( %) Highly Reflective Mirror ( %)

Production of Radicals

Origin Region COO Bending -OO Stretch

Assignment of Spectra To assign spectra for peroxy radicals, typically perform A-X origin calculations using G2 level of theory. Frequency calculations with DFT methods. Larger straight chain peroxy radicals are computationally expensive to calculate at the G2 level of theory. Assignments were deduced from spectral/structural relationships that have been determined from prior studies, and previous experience with assigning peroxy radical spectra ~ ~

Spectral/Structural Relationships of Peroxy Radicals Peroxy Radical Origin (cm -1 ) Methyl7383 Ethyl Propyl Butyl7355 Isobutyl7306 Primary Peroxy Radicals Secondary Peroxy Radicals Peroxy Radical Origin (cm -1 ) 2-Propyl Butyl Pentyl Pentyl Methyl-2- butyl7577 Tertiary Peroxy Radicals Peroxy Radical Origin (cm -1 ) t-Butyl Sharp, E. N.; Rupper, P.; and Miller, T. A. Phys. Chem. Chem. Phys., 2008, 10, 3955.

Assignments of Straight Chain Peroxy Radicals Peroxy Radical OriginCOO Bend -OO Stretch Hexyl Peroxy Heptyl Peroxy Octyl Peroxy Nonyl Peroxy Decyl Peroxy

Assignment of Isooctyl Peroxy A A’ B’B C’ BandAssignmentFrequency A Origin secondary isomer7503 A’ Origin tertiary isomer7804 B’COO bend8230 B-OO Stretch8460 C'-OO Stretch Glover, B. and Miller, T.A. J. Phys. Chem. A., 2005, 109,

Conclusions  Conclusions: A-X spectra of C 6 -C 10 straight chain peroxy radicals and isooctyl peroxy have been obtained and analyzed. Assignments were made by examining the spectral/structural relationships of peroxy radicals.

Acknowledgments Prof. Terry Miller Miller group: Funding: -US Department of Energy (DOE) -Dr. Dmitry Melnik -Dr. Mourad Roudjane -Rabi Chhantyal-Pun -Terrance Codd -Meng Huang

Assignment of Isooctyl Peroxy Peroxy Radical OriginCOO Bend -OO Stretch tert- isooctyl Peroxy Heptyl Peroxy Octyl Peroxy Nonyl Peroxy Decyl Peroxy A A’ B’B C’

Production of Radicals Production method will produce mixture of secondary and primary peroxy radical isomers. Secondary peroxy radicals will be produced preferentially.

Spectral/Structural Relationships of Peroxy Radicals Peroxy Radical Origin T1… Conformers Origin G1G2… Conformers Origin G1T2…,G1’G2...Conformers Methyl Ethyl Propyl Butyl Isobutyl Primary Peroxy Radicals Secondary Peroxy Radicals Peroxy Radical Origin G1… Conformers Origin T1… Conformers 2-Propyl Butyl Pentyl Pentyl Methyl-2- butyl Tertiary Peroxy Radicals Peroxy Radical Origin t-Butyl Sharp, E. N.; Rupper, P.; and Miller, T. A. Phys. Chem. Chem. Phys., 2008, 10, 3955.