We report experimental evidence for the formation of C5-hydroperoxyaldehydes (HPALDs) from 1,6-H-shift isomerizations in peroxy radicals formed from the hydroxyl radical (OH) oxidation of 2-methyl-1,3-butadiene (isoprene). At 295 K, the isomerization rate of isoprene peroxy radicals (ISO∙_2) relative to the rate of reaction of (ISO∙_2) + HO_2 is (k^(295)_(isom))/(k^(295)_((ISO∙_2) + HO_2) = (1.2±0.6) × 10^8 mol cm^(-2), or k^(295)_(isom)≃0.002 s^(-1). The temperature dependence of this rate was determined through experiments conducted at 295, 310 and 318 K and is well described by (k_(isom))T))/(k_((ISO∙_2)+HO_2) (T) = 2.0 × 10^(21) exp(-9000/T)mol cm^(-3). The overall uncertainty in the isomerization rate (relative to k_((ISO∙_2)+HO_2) is

estimated to be 50%. Peroxy radicals from the oxidation of the fully deuterated isoprene analog isomerize at a rate of ~15 times slower than non-deuterated isoprene. The fraction of isoprene peroxy radicals reacting by 1,6-H-shift isomerization is estimated to be 8–11% globally, with values up to 20% in tropical regions

Crounse, John D.

Kjaergaard, Henrik G.

Paulot, Fabien

Wennberg, Paul O.

Caltech Authors - Main

Electronic Supporting Information (ESI)
Peroxy radical isomerization in the oxida-
tion of isoprene
John D. Crounse,∗,a Fabien Paulot,b Henrik G. Kjaergaard,c
and Paul O. Wennbergb,d
1 CIMS sensitivity.
We estimate the CIMS sensitivity for the hydroxyhydroperoxides
(ISOPOOH) and hydroperoxyaldehydes (HPALD) from the ion-mol-
ecule collision rate, calculated using the parameterization of Su and
Chesnavich [1]. This requires knowledge of the average dipole mo-
ment and polarizibility of the neutral species. We obtain these pa-
rameters for the species of interest from ab initio calulations. For
accurate results, all conformers with significant populations at the
temperature of interest must be considered. We calculate conformer-
specific dipoles for all conformers with a relative population of >1%
at T=298K, as estimated from a Boltzman weighting to the con-
former energies. The conformer-specific dipoles, weighted by their
relative population, are averaged to yield the overall dipole moment
for the species of interest. This methodology has been described in
more detail by Garden et al. [2].
Table S1 lists the isomer-specific average dipole moments and
polarizibilities for the four important ISOPOOH isomers and both
a Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, CA 91125, USA. E-mail: crounjd@caltech.edu; Fax: +01 626 585 1917; Tel: +01
626 395 8551
b Division of Engineering and Applied Science, California Institute of Technology,
Pasadena, CA 91125, USA
c Department of Chemistry, DK-2100 Copenhagen Ø, University of Copenhagen, Copen-
hagen, Denmark.
d Division of Geological and Planetary Sciences, California Institute of Technology,
Pasadena, CA 91125, USA
1
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This journal is © The Owner Societies 2011
HPALD isomers. In this work we use values calculated at the
B3LYP/cc-pVTZ level. The same values calculated at the lower
B3LYP/6-31G(d) level, used in the study of Paulot et al. [3], are
listed for comparison.
The CIMS instrument measures the sum of isomers appearing at
a specific mass. Thus to calculate the average CIMS sensitivity, we
weight the four major ISOPOOH isomers according to the relative
initial ISO2 distribution (at 303 K) as calculated by Peeters et al.
[4] (1-OH-2-OO: 0.45, Z-1-OH-4-OO: 0.21, 4-OH-3-OO: 0.23, 4-OH-
1-OO: 0.11). We give the HPALD isomers equal weight. Weighting
the ISOPOOH sensitivity by the near-equilibrium ISO2 distribution
predicted for our experimental conditions at 295 K (1-OH-2-OO:
0.67, Z-1-OH-4-OO: 0.04, 4-OH-3-OO: 0.27, 4-OH-1-OO: 0.02) [4]
changes the inferred sensitivity by only ∼3%. As both HPALD
isomers have similar dipole moments, our determinations are insen-
sitive to this weighting scheme.
Absolute sensitivities for both ISOPOOH and HPALD are de-
termined from the calculated ion-molecule collision rates using the
average between the ratios of the experimentally determined sen-
sitivities for glycolaldehyde and hydroxyacetone to their respective
calculated-collision rate:
RGH =
SGLYCexpt
kGLYCcoll
+
SHACexpt
kHACcoll
2
SXcalc = k
X
coll ×RGH
The sensitivity for the isoprene hydroxynitrates is determined
relative to nitric acid following Paulot et al. [5].
2 Rate of 1,5-H-shift (from alcohol).
Fig. S1 shows CIMS observations of MVK+MACR and isoprene ni-
trates as a function of time for experiment #3 (main paper, Table 1,
T=318 K). Also shown are results from a kinetic model simulation
for this isoprene oxidation experiment for MVK+MACR and iso-
prene nitrates using several values for the 1,5-H-shift isomerization
rate (k1,5-isom) within the model. Simulations were run using the
rate for the 1,6-H-shift determined in this study, isomer-dependant
nitrate yields from Paulot et al. [5], the temperature dependance
2
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for nitrate yields from Carter and Atkinson [6], with several the-
oretical values for the 1,5-H-shift isomerization rates reported by
Peeters et al. [4], da Silva et al. [7], Peeters and Muller [8]. The
rate calculated by Peeters et al. [4](k3181,5-isom = 0.026 s−1) and in-
creased by 5× in Peeters and Muller [8] (k3181,5-isom = 0.13 s−1) pre-
dicts that the isoprene chemistry at the long ISO2• lifetimes and
elevated temperature in this experiment should be dominated by
this mechanism. This however, is not consistent with the CIMS ob-
servations. The geometric average of the rates for the two 1,5-H-shift
isomerization channels calculated by da Silva et al. [7] (k3181,5-isom =
0.005 s−1) is consistent with our results, but also is not well con-
strained, as errors in the assumed ISO2 distribution, ISO2 + ISO2
reaction rates, and isomer-specific nitrate yields, all can impact the
modeled MVK+MACR and ISONO2.
References
[1] T. Su and W. J. Chesnavich, J. Chem. Phys., 1982, 76, 5183–
5185.
[2] A. L. Garden, F. Paulot, J. D. Crounse, I. J. Maxwell-Cameron,
P. O. Wennberg and H. G. Kjaergaard, Chem. Phys. Lett., 2009,
474, 45–50.
[3] F. Paulot, J. D. Crounse, H. G. Kjaergaard, A. Kuerten, J. M.
St Clair, J. H. Seinfeld and P. O. Wennberg, Science, 2009, 325,
730–733.
[4] J. Peeters, T. L. Nguyen and L. Vereecken, Phys. Chem. Chem.
Phys., 2009, 11, 5935–5939.
[5] F. Paulot, J. D. Crounse, H. G. Kjaergaard, J. H. Kroll, J. H.
Seinfeld and P. O. Wennberg, Atmos. Chem. Phys., 2009, 9,
1479–1501.
[6] W. P. L. Carter and R. Atkinson, J. Atmos. Chem., 1989, 8,
165–173.
[7] G. da Silva, C. Graham and Z.-F. Wang, Environ. Sci. Technol.,
2010, 44, 250–256.
[8] J. Peeters and J.-F. Muller, Phys. Chem. Chem. Phys., 2010, 12,
14227–14235.
3
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Table S1 – Average dipole moments and polarizibilities for ISOPOOH and
HPALD isoprene oxidation products, for T = 298K. Collision rates have units of
10−9 cm3 molec.−1 s−1.
B3LYP/6-31G(d) B3LYP/cc-pVTZ
Dipole Polarizability kxcoll Dipole Polarizability k
x
coll
Molecule Structure (D) (Å3) (D) (Å3)
β1-ISOPOOH
OH
OOH
2.19 9.44 1.85 2.32 10.8 1.96
β4-ISOPOOH
OH
OOH
2.20 9.44 1.85 2.29 11.0 1.95
δ1-ISOPOOH
HO OOH
2.85 9.63 2.23 2.54 11.0 2.09
δ4-ISOPOOH
OHHOO
3.34 9.66 2.49 3.22 11.06 2.47
HPALD1
O OOH
2.51 9.5 2.03 2.54 10.9 2.10
HPALD2
OHOO
1.90 9.6 1.70 2.36 10.9 2.00
Glycolaldehyde
O
OH
2.3 4.5 2.0 2.33 4.64 2.06
Hydroxyacetone
O
OH
3.1 5.5 2.5 3.08 6.40 2.49
4
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Figure S1 – CIMS observations (black circles) of MVK+MACR (top) and
ISORO2 (bottom) during isoprene oxidation experiment (#3 - main paper, T
= 318 K). Kinetic model results are shown using a range of 1,5-H-shift isomer-
ization rates.
0 1 2 3 4 5 6
10
−2
10
−1
10
0
10
1
M
V
K
+
M
A
C
R
 (
pp
bv
)
Experiment #3: T=318.2K
Extra lights→
0 1 2 3 4 5 6
0
0.1
0.2
0.3
0.4
0.5
Time since lights on (hrs)
IS
O
N
O
2 
(p
pb
v)
 
 
Observations
k
1,5−H−shift
=0
k
1,5−H−shift
=da Silva (2010)
k
1,5−H−shift
=Peeters (2009)
k
1,5−H−shift
=Peeters (2010)
5
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Peroxy radical isomerization in the oxidation of isoprene

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