2290 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Aschmann et al.
ethylphosphonate [C2H5OP(O)(C2H5)OH] from DEEP, and
diethyl phosphate [DEP, (C2H5O)2P(O)OH] from TEP, with the
formation of DEP from TEP being confirmed by matching the
API-MS and API-MS/MS spectra and FT-IR spectra of the
reaction products with those of an authentic standard. The
products observed and quantified from the reaction of OH
radicals with TEP using in situ FT-IR analyses and GC-FID
analyses account for ∼82 ( 15% of the product carbon and
65-82% of the product phosphorus at the first analysis time
and a higher percentage if significant losses of DEP occurred
prior to the first analysis period in each of the experiments.
Our kinetic and product data are consistent with the OH
radical reactions with DEMP, DEEP, and TEP proceeding
mainly by H atom abstraction from the C-H bonds of the CH2
groups in the C2H5O moieties. Using TEP as the example
with subsequent reactions of the methyl radical leading to the
formation of HCHO plus (depending on the NO and NO2
concentrations) methyl nitrite and methyl nitrate (from reactions
of the CH3O• radical with NO and NO2, respectively, competing
with the reaction with O2 to form HCHO).15
Our product data, namely the high yields of DEP (65-82%
initial), CO2 (80 ( 10%), and HCHO (55 ( 5%) and the
observation of PAN in 8% yield, suggest that reaction 9 accounts
for ∼80-90% of the overall reaction, leading to DEP plus either
PAN or CO2 + HCHO (and methyl nitrite and methyl nitrate).
The formation of acetaldehyde, in 11 ( 2% yield, must arise
from another reaction pathway, possibly decomposition of the
(C2H5O)2P(O)OCH(O•)CH3 radical (note that the low yield of
acetaldehyde also means that a direct addition-elimination
pathway is not significant).
OH + (C2H5O)3PO f H2O + (C2H5O)2P(O)OC•HCH3 (4)
(C2H5O)2P(O)OCH(O•)CH3 f
(C2H5O)2P(O)O• + CH3CHO (13)
followed by addition of O2 and reaction with NO to form
(mainly) the alkoxy radical
While reactions analogous to reaction 9 appear to occur in
the OH radical-initiated reactions of DEMP and DEEP (as
evidenced by our API-MS analyses), a reaction analogous to
reaction 9 may not occur for P(O)OCH2O• alkoxy radicals;
rather, these radicals may form POCHO which then hydrolyzes
to form P(O)OH + HC(O)OH.
(C2H5O)2P(O)OC•HCH3 + O2 f
(C2H5O)2P(O)OCH(OO•)CH3 (5)
(C2H5O)2P(O)OCH(OO•)CH3 + NO f
(C2H5O)2P(O)OCH(O•)CH3 + NO2 (6)
Acknowledgment. This work was supported by ENSCO,
Inc., with preliminary work being funded by the California Air
Resources Board through Contract No. 99-330 (initial API-MS
product study of TEP) and by Science Applications International
Corp. While this research has been funded by these agencies,
the results and content of this publication do not necessarily
reflect the views and opinions of the funding agencies. R.A.
thanks the University of California Agricultural Experiment
Station for partial salary support.
This alkoxy radical could react with O2 or decompose
(C2H5O)2P(O)OCH(O•)CH3 + O2 f
(C2H5O)2P(O)OC(O)CH3 + HO2 (7)
(C2H5O)2P(O)OCH(O•)CH3 f
(C2H5O)2P(O)OCHO + •CH3 (8)
with (C2H5O)2P(O)OC(O)CH3 and (C2H5O)2P(O)OCHO react-
ing with water vapor to form DEP and acetic acid or DEP and
formic acid, respectively. The methyl radical formed from the
alkoxy radical decomposition reaction will form HCHO (plus
some methyl nitrite and methyl nitrate from reactions of the
CH3O• radical with NO and NO2). The lack of formation of
formic acid rules out the decomposition reaction followed by
hydrolysis of (C2H5O)2P(O)OCHO, and the formation of
significant yields of HCHO indicates that the alkoxy radical
reaction with O2 (reaction 7) is also not dominant. An additional
reaction of the alkoxy radical is via the rearrangement analogous
to that observed for the alkoxy radicals of structure RC(O)-
OCH(O•)R′ formed from esters,25,26 to form DEP + CH3C•O.
References and Notes
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In the presence of NO and NO2 the acetyl radical leads to PAN
(CH3C(O)OONO2) or HCHO + CO2 (depending on the NO/
NO2 ratio)15
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CH3C•O + O2 f CH3C(O)OO•
CH3C(O)OO• + NO2 f CH3C(O)OONO2 (PAN) (11)
CH3C(O)OO• + NO f •CH3 + CO2 + NO2
(10)
(12)