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

2. 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 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,

3. 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,
CH2I + O2 → IO + HCHO
2.6 x 10-14
1.28 ±0.22x 10-12
Enami et al. Bull. Chem. Soc. Jpn. 2008, 81,
CH2I + O2 → products
1.39±0.01 x 10-12
Eskola et al. Phys. Chem. Chem. Phys , 8,
CH2I + O2 → IO + HCHO
1.6 x 10-12
Masaki et al. J. Phys. Chem. 1995, 99,
Spectroscopic Evidence:
CH2IOO
Sehested et al. Int. J. Chem. Kinet. 1994, 26, Gravestock et al. Chem. Phys. Chem. 2010, 11, Sheps , L. J. Phys.Chem. Lett. 2013, 4, Beames et al. J. Am. Chem. Soc. 2012, 49,
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.

4. 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!!

5. IO Formation Kinetics
CH2I2 Sigma (355)=1.92 x 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
Absorption spectrum of the A2Π3/2←X2Π3/2 (3,0) band of the IO radical

6. 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 cm3 molecules-1 s-1
Formation and decay of IO absorption at 30 torr

7. Consider early times: kform=1.38±0.03 x 10-13 cm3 molecule-1 s-1
Slow rise: secondary reactions
kN2=4.9 x 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 torr.
Fast rise: direct production
k=10-12 cm3 molecule-1 s-1
Huang et al. J. Phys. Chem. Lett. 2012, 3,
Enami et al. Bull. Chem. Soc. Jpn. 2008, 81,
Eskola et al. Phys. Chem. Chem. Phys , 8,
Masaki et al. J. Phys. Chem. 1995, 99,

8. I2 Production I2 formation: Non-photolytic I atom formation:
CH2I2 + hν → CH2 + I 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

9. 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

10. Acknowledgements Undergraduates: Craig Murray Ben Toulson Kara Kapnas
YiTien Jou