The Journal of Physical Chemistry A
ARTICLE
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this pathway is operative, then the intermediate OCCl2CHO
radical must decompose to C(O)Cl2 þ HC•O (as shown in
Scheme 1) since we did not observe formation of ClC(O)CHO
and there was no evidence for involvement of Cl atoms (see
Figure 3). Similarly, if OH radical addition to the terminal carbon
dominates, forming the (CH3O)2P(O)OC•HCCl2OH radical,
then the only obvious pathway leading to CO þ C(O)Cl2 þ
(CH3O)2P(O)OH is via the previously proposed8 rearrange-
ment of the (CH3O)2P(O)OCH(O•)CCl2OH radical via a
5-member ring transition state (Scheme 1). Other reaction
pathways subsequent to the initial OH radical addition are
expected to lead to C(O)Cl2 þ (CH3O)2P(O)OCHO, with
(CH3O)2P(O)OCHO possibly reacting with water to form
(CH3O)2P(O)OH þ HC(O)OH or eliminating CO. Only if
(CH3O)2P(O)OCHO rapidly eliminates CO to form CO þ
(CH3O)2P(O)OH would these pathways be consistent with our
product data. The room temperature rate constant for OH þ
dichlorvos is a factor of ∼5 higher than that for OH þ trimethyl
phosphate [(CH3O)3PO],3 consistent with our conclusion that
the major reaction pathway for OH þ dichlorvos involves OH
radical addition to the >CdC< bond to form CO þ C(O)Cl2 þ
(CH3O)2P(O)OH.
Our observation that the rate constant for the reaction of NO3
radicals with dichlorvos is approximately 3 orders of magnitude
higher than those for the alkyl phosphates and alkyl phospho-
nates previously studied8,10,16 suggests that the NO3 radical
reaction proceeds by initial addition of NO3 to the >CdC<
bond. Our API-MS analyses also suggest that (CH3O)2P(O)OH
is a product of this reaction, and a plausible reaction scheme is
one analogous to the OH radical reaction shown in Scheme 1,
with the NO3 radical adding to the terminal carbon to form the
(CH3O)2P(O)OC•HCCl2ONO2 radical. Rearrangement of the
subsequently formed (CH3O)2P(O)OCH(O•)CCl2ONO2 alkoxy
radical via a 5-member transition state would result in formation
of (CH3O)2P(O)OH þ O2NOCCI2C•O, with the latter decom-
posing to form CO þ C(O)Cl2 þ NO2.
’ ACKNOWLEDGMENT
This work was supported by ENSCO, Inc. While this research
has been funded by this agency, the results and content of this
publication do not necessarily reflect the views and opinions of
the funding agency. R.A. thanks the University of California
Agricultural Experiment Station for partial salary support.
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’ ATMOSPHERIC IMPLICATIONS
The lifetimes of gaseous dichlorvos with respect to reactions
with OH radicals, NO3 radicals, and O3 can be calculated by
combining our measured rate constants with assumed ambient
atmospheric concentrations of: OH, an average 12-h daytime
concentration of 2 ꢀ 106 molecules cm-3; NO3, a 12-h nighttime
concentration of 5 ꢀ 108 molecules cm-3; and O3, a 24-h average
concentration of 7 ꢀ 1011 molecules cm-3 (30 parts-per-billion).
The calculated dichlorvos lifetimes of 4 h for reaction with OH
radicals during daytime, 2 h for reaction with NO3 during
nighttime and 10 days for reaction with O3, together with a
photolysis lifetime of >4.6 days,11 indicates that daytime reaction
with OH radicals and nighttime reaction with NO3 radicals will be
the dominant dichlorvos chemical loss processes in the atmosphere.
Gaseous dichlorvos will therefore be transformed in the atmosphere
within typically a few hours, leading to the formation of phosgene
[C(O)Cl2] and dimethyl phosphate [(CH3O)2P(O)OH] as
major products and with little aerosol formation.
’ AUTHOR INFORMATION
Corresponding Author
* Tel: (951) 827-4191. E-mail: ratkins@mail.ucr.edu.
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dx.doi.org/10.1021/jp112019s |J. Phys. Chem. A 2011, 115, 2756–2764