The Structure and Spectra of Organic Peroxy Radicals Erin N. Sharp, Patrick Rupper, Terry A. Miller The Laser Spectroscopy Facility Department of Chemistry The Ohio State University Columbus, OH 43210

Motivations for Studying Peroxy Radicals Atmospheric Chemistry:  Key components in low temperature oxidation of hydrocarbons  RO 2 upsets the balance of NO and NO 2 and yields an increase in O 3 production Combustion Chemistry:  Key reaction intermediates in low temperature combustion of hydrocarbons  Their lack of formation is believed to be responsible for the negative temperature coefficient (NTC) from K 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 Lightfoot et al., Atmos. Envir. 26A, 1805 (1992). Robertson et al., in Low-Temperature Combustion and Autoignition (Elsevier, Amsterdam, 1997).

Alkyl Peroxy Radicals (RO 2 ) methyl peroxy ethyl peroxy 1-propyl peroxy2-propyl peroxy 1-butyl peroxy2-butyl peroxy isobutyl peroxy t-butyl peroxy R = -CH 3 R = -C 2 H 5 R = -C 3 H 7 R = -C 4 H 9 R = -C 5 H 11 1-pentyl peroxy2-pentyl peroxy3-pentyl peroxy 2-methylbutyl peroxy 3-methylbutyl peroxy 3-methyl-2- butyl peroxy t-pentyl peroxy neopentyl peroxy 62 nd MSS: MH06, MH0762 nd MSS: TB03 62 nd MSS: TB04 61 st MSS: RF04 59 th MSS: TI08 59 th MSS: TI09 A-X Room Temp Cavity Ringdown Spectra ~~

B-X Transition ~~ very strong, σ ~ cm 2 /molecule located in the UV, used in kinetics studies B state - repulsive broad, lack of selectivity ~ CH 3 O 2 a) Jafri et al., J. Am. Chem. Soc. 112, 2586 (1990). b) O. J. Nielsen and T. J. Wallington, in Peroxyl Radicals, (John Wiley and Sons, New York, 1997). Spectroscopy of Alkyl Peroxies weak, σ ~ cm 2 /molecule A state - bound selective requires a tunable light source in the IR ~ A-X Transition ~ ~ a b ~ ~ ~

Ringdown Cell YAG 532 nm PD High Reflectivity Mirrors ArF 193 nm 650 – 700 mJ mJ 1 – 3 mJ 100 – 200 mJ  m 2 nd Stokes Filters Sirah Dye Laser H 2 Raman Cell psi H 2 CRDS Experimental Setup Method 1: Direct Photolysis Mechanism 1a) RX + hν (193 nm or 248 nm) → R + X 1b) RCOR’ + hν (193 nm) → R + R’ + CO 2) R + O 2 + M → RO 2 + M Method 2: H-atom Abstraction Mechanism 1) (COCl) 2 + hν (193 nm) → 2CO + 2Cl 2) RH + Cl → R + HCl 3) R + O 2 + M → RO 2 + M

Cavity Ringdown Spectroscopy (CRDS) R L A = σ Nl A = L/c τ absorber - L/c τ 0 Time Intensity 00  absorber Sensitivity of Technique: If R = % and L = 54 cm then τ 0 = 36 µs L eff = 10.8 km If R = % and l = 13 cm then τ abs = 9 µs l eff = 2.6 km τ abs σ Nlσ Nl + = cL)/( R1 - ( ) τ0τ0 c L )/ ( R1 - = l

Experimental A-X Spectra ~~  A-X electronic transition yields sharp structure both in the origin and COO bend and O-O stretch vibrational bands ~~  A-X electronic transition is selective among different RO 2 and even for different conformers of a given RO 2 ~~ Methyl Peroxy (CH 3 O 2 ) Ethyl Peroxy (C 2 H 5 O 2 ) TG

Looking at the Bigger Picture With the “complete” data set for the A-X electronic transition of the alkyl peroxy radicals: C 1 -C 5, we can compare and contrast their CRDS spectra to establish relationships between molecular structure and A-X spectral properties. We have selected the A-X origin frequency for investigating these spectra/structure relationships, and we will look at the following structural effects on the transition frequency: –Effect of changing the number of carbons –Effect of branching at the α carbon yielding primary, secondary, tertiary peroxy radicals –Effect of branching at the β carbon yielding iso- and neo- peroxy radical isomers –Effect of T 1 /G 1 orientation (OOCC dihedral angle) –Effect of T 2 /G 2 /G 2 ' orientation (OCCC dihedral angle) ~~ ~~ ~~ T1T2T1T2 G1T2G1T2 T1G2T1G2

Looking at the Trends A-X Origin Frequencies for Alkyl Peroxy Isomers (T 1 … conformers) ~~ Increasing the number of carbons (n) decreases the transition energy (~50 cm -1 from CH 3 O 2 to 1-C 3 H 7 O 2 ) Branching at the α carbon increases the transition energy (~ cm -1 from primary to secondary isomers) Branching or substitution at the β carbon decreases the transition energy (~ cm -1 from 1- isomers to iso-/neo- isomers)

Looking at the Trends - 2 Changing OOCC dihedral angle from T 1 to G 1 increases transition energy by ~ cm -1 (compare positions of squares to circles) Changing OCCC dihedral angle from G 2 to T 2 increases transition energy by ~50 cm -1 (compare positions of circles to triangles) A-X Origin Frequencies for Primary Alkyl Peroxy Isomers ~~

Can we rationalize/explain/understand the observed spectral dependence on the peroxy radical’s structure by way of chemical intuition or electronic structure (MO theory)? Calculate HOMO (highest fully occupied molecular orbital) and SOMO (singly occupied molecular orbital) energies for peroxy radicals, as their energy difference should replicate trends observed in electronic transition frequencies. Investigate this for two of the five observed trends: Increasing the # carbons (n) in the radical decreases the transition frequency by ~50 cm -1 OOCC dihedral angle conformation (T 1, G 1 ) increases the transition frequency by ~ cm -1 from T 1 to G 1 Explain the Trends

Calculated MO Energies for Increasing n Trend  Both HOMO and SOMO increase asymptotically, but HOMO rise is faster  SOMO-HOMO difference reproduces the trend that as the number of carbons increases in RO 2, the transition fre- quency decreases BLYP/6-31+G(d) 0 cm -1 = cm -1

Calculated MO Energies for T/G Trend  HOMO increases in energy going from G to T conformations and the SOMO decreases  SOMO-HOMO difference from G to T reproduces trend observed in transition frequencies, with G lying higher than T BLYP/6-31+G(d) 0 cm -1 = cm -1

Benchmarking Calculations with Experimental Results Peroxy isomer Peroxy conformer SymmetryExp (cm -1 ) Theor. (G2) (cm -1 ) Δ(Exp.-Theor.) methylTCsCs ethylTCsCs ethylGC1C propylT1T2T1T2 CsCs propylT1G2T1G2 C1C propylG1G2G1G2 C1C propylG1T2G1T2 C1C propylG1′G2G1′G2 C1C propylGC1C propylTCsCs butylT1T2T3T1T2T3 CsCs isobutylT1T2T1T2 CsCs pentylT1T2T3T4T1T2T3T4 CsCs neopentylT1T2T1T2 CsCs

Using the room temperature CRDS spectra of the A-X electronic transition for the alkyl peroxy radicals, we were able to draw some overall conclusions on their structure/spectral relationships. We were able to use the simple one-electron hop model of the A-X electronic transition to justify most of these. We determined that the G2 computational method typically predicts the A-X origin frequencies of alkyl peroxy radicals to within 1%. Begin assembling a new data series on the alkenyl peroxy radicals and on the cycloalkyl peroxy radicals (RI11). Look at the A-X electronic transition for many of the alkyl peroxy radicals with high resolution CRDS (WG08, WG09). Conclusions and Future Work ~~ ~~ ~~ ~~

Dr. Terry A. Miller, Dr. Patrick Rupper Miller group: past and present members DOE for Funding Acknowledgments