Spectroscopic and Kinetic Studies of Atmospheric Free Radicals Elizabeth Foreman, YiTien Jou, Kara Kapnas, and Craig Murray Department of Chemistry, University of California, Irvine

Reaction of interest: CH2I+O2 CH2I + I CH2I2 + hν CH2I + I* CH2 + I2 CH2OO + I CH2I + O2 CH2O + IO CH2IOO* M=N2 CH2IOO Interested in formation of IO from CH2I+O2. 2 formation pathways: direct vs indirect. More likely to collisionally stabilize with N2. At 5-100 torr unlikely to have excited CH2IOO* decomposing to IO. CH2O +IO (low P) + I CH2IO + IO CH2O + IO + I Lee et al. Chem. Eur. J. 2012, 18, 12411-12423.

Rate Constant (cm3 molecule-1 s-1) Kinetic Evidence: Reaction Rate Constant (cm3 molecule-1 s-1) Source CH2IO2 + I → IO + HCHO + I 3.5 x 10-11 Gravestock et al. Chem. Phys. Chem. 2010, 11, 1416-1424. CH2I + O2 → IO + HCHO 2.6 x 10-14 1.28 ±0.22x 10-12 Enami et al. Bull. Chem. Soc. Jpn. 2008, 81, 1250-1257. CH2I + O2 → products 1.39±0.01 x 10-12 Eskola et al. Phys. Chem. Chem. Phys. 2006, 8, 1416-1424. CH2I + O2 → IO + HCHO 1.6 x 10-12 Masaki et al. J. Phys. Chem. 1995, 99, 13126-13131. Spectroscopic Evidence: CH2IOO Sehested et al. Int. J. Chem. Kinet. 1994, 26, 259 272. Gravestock et al. Chem. Phys. Chem. 2010, 11, 1416-1424. Sheps , L. J. Phys.Chem. Lett. 2013, 4, 4201-4205. Beames et al. J. Am. Chem. Soc. 2012, 49, 20045-20048. CH2OO Spectroscopic evidence for indirect path: CH2IOO spectrum probably contaminated (or totally) CI. Need better evidence or reject indirect mech. Kinetic evidence: rate constants vary by orders of magnitude, no consensus yet.

Technique: Cavity Ring-down Spectroscopy PMT Signal Ring down time inversely related to absorption coefficient. Absorption coefficient = sigma*N which is directly proportional to absorbance. Path length independent measurement!!

IO Formation Kinetics CH2I2 Sigma (355)=1.92 x 10-18 cm^2 #-1 IO sigma (435.5)=1.14x10^-17 cm^2 #^-1 Absorption spectrum of the precursor molecule CH2I2 Sander, S. P., J. Abbatt, J. R. Barker, J. B. Burkholder, R. R. Friedl, D. M. Golden, R. E. Huie, C. E. Kolb, M. J. Kurylo, G. K. Moortgat, V. L. Orkin and P. H. Wine, JPL Publication 10-6, Jet Propulsion Laboratory, Pasadena, 2011 http://jpldataeval.jpl.nasa.gov Absorption spectrum of the A2Π3/2←X2Π3/2 (3,0) band of the IO radical

kform=4.3±0.8 x 10-14 cm3 molecules-1 s-1 High O2 Low O2 My rate constant on order of mag with Gravestock direct mechanism estimation (and slower dependence on O2 at higher [O2]). Reason for discrepancy still unclear. More interested in formation rate (decay dominated by slow self reaction): zoom in on earlier times and shorter time step Pseudo 1st order dependence of IO formation rate on O2 number density kform=4.3±0.8 x 10-14 cm3 molecules-1 s-1 Formation and decay of IO absorption at 30 torr

Consider early times: kform=1.38±0.03 x 10-13 cm3 molecule-1 s-1 Slow rise: secondary reactions kN2=4.9 x 10-31 cm6 molecule-2 s-1 [N2]=8 x 1017 molecules cm-3  Single exp rise is a poor fit! Should not get different rate constant by fitting shorter period of time! Must model including multiple competing formation pathways. Sorting out kinetic model in progress before continuing O2/totalP kinetic studies. Same behavior exhibited from 10-60 torr. Fast rise: direct production k=10-12 cm3 molecule-1 s-1 Huang et al. J. Phys. Chem. Lett. 2012, 3, 3399-3403. Enami et al. Bull. Chem. Soc. Jpn. 2008, 81, 1250-1257. Eskola et al. Phys. Chem. Chem. Phys. 2006, 8, 1416-1424. Masaki et al. J. Phys. Chem. 1995, 99, 13126-13131.

I2 Production I2 formation: Non-photolytic I atom formation: CH2I2 + hν → CH2 + I2 λ<333 nm I+I+M → I2+M Non-photolytic I atom formation: CH2I + O2 →CH2IOO*→CH2OO + I CH2I + O2 →CH2OO + I 28% enhancement in I2 formation in the presence of O2 56% yield (lower limit) for Criegee Intermediate? Unforseen I2 production ~ 100 ppb. I2 from photodissociation not energetically accessible for us (might be for others using 248 or 266 nm). Need to be careful if using I as proxy for CI. 72% from photolysis I atoms only, 28% from I produced in rxns with O2. Interesting implication for future experiments. Absorption spectrum of I2 at 5 torr total pressure

Conclusions and Future Studies Simple pseudo first order kinetics are insufficient to fully describe IO formation Formation of IO radical occurs biexponentially Development of comprehensive kinetic model for CH2I + O2 reaction Kinetic dependence on O2 number density

Acknowledgements Undergraduates: Craig Murray Ben Toulson Kara Kapnas YiTien Jou