Atmospheric chemistry of diethyl methylphosphonate, diethyl ethylphosphonate, and triethyl phosphate

Article abstract of DOI:10.1021/jp0446938

Rate constants for the reactions of OH radicals and NO₃ radicals with diethyl methylphosphonate [DEMP, (C₂H₅O) ₂P(O)CH₃], diethyl ethylphosphonate [DEEP, (C ₂H₅O)₂P(O)C₂H₅], and triethyl phosphate [TEP, (C₂H₅O)₃PO] have been measured at 296 ± 2 K and atmospheric pressure of air using relative rate methods. The rate constants obtained for the OH radical reactions (in units of 10^-11 cm³ molecule^-1 s^-1 were as follows: DEMP, 5.78 ± 0.24; DEEP, 6.45 ± 0.27; and TEP, 5.44 ± 0.20. The rate constants obtained for the NO₃ radical reactions (in units of 10^-16 cm³ molecule^-1 s^-1) were the following: DEMP, 3.7 ± 1.1; DEEP, 3.4 ± 1.4; and TEP, 2.4 ± 1.4. For the reactions of O₃ with DEMP, DEEP, and TEP, an upper limit to the rate constant of <6 × 10 ^-20 cm³ molecule^-1 s^-1 was determined for each compound. Products of the reactions of OH radicals with DEMP, DEEP, and TEP were investigated using in situ atmospheric pressure ionization mass spectrometry (API-MS) and, for the TEP reaction, gas chromatography with flame ionization detection (GC-FID) and in situ Fourier transform infrared (FT-IR) spectroscopy. The API-MS analyses show that the reactions are analogous, with formation of one major product from each reaction: C₂H₅OP(O)(OH)CH₃ from DEMP, C ₂H₅OP(O)(OH)C₂H₂ from DEEP, and (C₂H₅O)₂P(O)OH from TEP. The FT-IR and GC-FID analyses showed that the major products (and their molar yields) from the TEP reaction are (C₂H₅O)₂P(O)OH (65-82%, initial), CO₂ (80 ± 10%), and HCHO (55 ± 5%), together with lesser yields of CH₃CHO (11 ± 2%), CO (11 ± 3%), CH₃C(O)OONO₂ (8%), organic nitrates (7%), and acetates (4%). The probable reaction mechanisms are discussed.

Full text of DOI:10.1021/jp0446938

2282
J. Phys. Chem. A 2005, 109, 2282-2291
Atmospheric Chemistry of Diethyl Methylphosphonate, Diethyl Ethylphosphonate, and
Triethyl Phosphate
Sara M. Aschmann, Ernesto C. Tuazon, and Roger Atkinson*,†
Air Pollution Research Center, UniVersity of California, RiVerside, California 92521
ReceiVed: NoVember 20, 2004; In Final Form: January 18, 2005
Rate constants for the reactions of OH radicals and NO3 radicals with diethyl methylphosphonate [DEMP,
(C2H5O)2P(O)CH3], diethyl ethylphosphonate [DEEP, (C2H5O)2P(O)C2H5], and triethyl phosphate [TEP,
(C2H5O)3PO] have been measured at 296 ( 2 K and atmospheric pressure of air using relative rate methods.
The rate constants obtained for the OH radical reactions (in units of 10-11 cm3 molecule-1 s-1) were as
follows: DEMP, 5.78 ( 0.24; DEEP, 6.45 ( 0.27; and TEP, 5.44 ( 0.20. The rate constants obtained for the
NO3 radical reactions (in units of 10-16 cm3 molecule-1 s-1) were the following: DEMP, 3.7 ( 1.1; DEEP,
3.4 ( 1.4; and TEP, 2.4 ( 1.4. For the reactions of O3 with DEMP, DEEP, and TEP, an upper limit to the
rate constant of <6 × 10-20 cm3 molecule-1 s-1 was determined for each compound. Products of the reactions
of OH radicals with DEMP, DEEP, and TEP were investigated using in situ atmospheric pressure ionization
mass spectrometry (API-MS) and, for the TEP reaction, gas chromatography with flame ionization detection
(GC-FID) and in situ Fourier transform infrared (FT-IR) spectroscopy. The API-MS analyses show that the
reactions are analogous, with formation of one major product from each reaction: C2H5OP(O)(OH)CH3 from
DEMP, C2H5OP(O)(OH)C2H5 from DEEP, and (C2H5O)2P(O)OH from TEP. The FT-IR and GC-FID analyses
showed that the major products (and their molar yields) from the TEP reaction are (C2H5O)2P(O)OH (65-
82%, initial), CO2 (80 ( 10%), and HCHO (55 ( 5%), together with lesser yields of CH3CHO (11 ( 2%),
CO (11 ( 3%), CH3C(O)OONO2 (8%), organic nitrates (7%), and acetates (4%). The probable reaction
mechanisms are discussed.
Introduction
for the gas-phase reactions of OH radicals, NO3 radicals, and
O3 with diethyl methylphosphonate [DEMP, (C2H5O)2P(O)CH3],
diethyl ethylphosphonate [DEEP, (C2H5O)2P(O)C2H5], and
triethyl phosphate [TEP, (C2H5O)3PO], and we investigated the
products of the atmospherically dominant OH radical reactions.
Volatile organic compounds emitted into the troposphere can
undergo photolysis (at wavelengths >290 nm), react with OH
radicals, react with NO3 radicals, and react with O3.1 Alkyl and
aryl phosphates [(RO)3PO] and alkyl phosphonates [(RO)2-
P(O)R], where R ) aryl or alkyl], are used as plasticizers, flame
retardents and fire-resistant fluids and lubricants, and organo-
phosphorus compounds of structure (RO)3PO and (RO)3PS are
widely used as pesticides.2,3 These compounds and their
precursors may be released into the atmosphere where they can
undergo transport and chemical transformations. While the
kinetics of the atmospheric reactions of a number of simple
“model” alkyl phosphates, alkyl phosphorothioates, and alkyl
Experimental Methods
Most experiments were carried out at 296 ( 2 K and 740
Torr total pressure of purified air at ∼5% relative humidity in
two ∼7000 L volume Teflon chambers, each equipped with two
parallel banks of blacklamps for irradiation and a Teflon-coated
fan to ensure rapid mixing of reactants during their introduction
into the chamber. One of the Teflon chambers was interfaced
to a PE SCIEX API III MS/MS direct air sampling, atmospheric
pressure ionization tandem mass spectrometer (API-MS). Ex-
periments utilizing in situ Fourier transform infrared (FT-IR)
spectroscopy were carried out at 298 ( 2 K and 740 Torr total
pressure of synthetic air (80% N2 + 20% O2) in a 5870 L
Teflon-coated, evacuable chamber equipped with a multiple
reflection optical system interfaced to a Mattson Galaxy 5020
FT-IR spectrometer.8 Irradiation was provided by a 24 kW
xenon arc lamp, with the light being filtered through a 6 mm
thick Pyrex pane to remove wavelengths <300 nm. IR spectra
were recorded with 32 scans per spectrum (corresponding to a
1.2 min averaging time) at a full width at half-maximum
resolution of 0.7 cm-1 and a path length of 62.9 m.
phosphonates of structure (RO)nP(O)(SR)3-n, (RO)nP(S)(SR)3-n
,
and (RO)2P(O)X (R ) CH3 or C2H5 and X ) H or CH3) have
been studied,4-8 the only compounds with R * CH3 for which
data have been reported are triethyl phosphate [TEP, (C2H5O)3-
PO] and diethyl methylphosphonate [DEMP, (C2H5O)2P(O)-
CH3].6,7 For the alkyl phosphates and alkyl phosphorothioates
studied to date, reaction with the OH radical is the dominant
atmospheric loss process,4,5 and only upper limits to the rate
constants have been reported for the NO3 radical and O3
reactions.4,5
To further investigate the reactivity of the alkyl groups
attached directly to the phosphorus atom in alkyl phosphonates
(i.e., of X in (RO)2P(O)X), we have measured rate constants
Kinetic Studies. Rate constants for the reactions of OH and
NO3 radicals with DEMP, DEEP, and TEP were measured using
relative rate techniques in which the concentrations of the
organophosphorus compound and a reference compound (whose
† Also Department of Environmental Sciences and Department of
Chemistry.
* To whom correspondence should be addressed: e-mail
ratkins@mail.ucr.edu; Tel (951) 827-4191.
10.1021/jp0446938 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/22/2005

Atmospheric Chemistry of DEMP, DEEP, and TEP
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2283
OH radical or NO3 radical reaction rate constant is reliably
known) were measured in the presence of OH or NO3 radicals.5,9
studies (see below), acetaldehyde, 100 cm3 volume gas samples
were collected from the chamber onto Tenax-TA solid adsor-
bent, with subsequent thermal desorption at ∼250 °C onto a 30
m DB-1701 megabore column held at -40 or 0 °C and then
temperature-programmed to 200 °C at 8 °C min-1. Based on
replicate analyses in the chamber in the dark, the analytical
uncertainties for the organophosphorus compounds and the
reference compounds used were typically e3%.
The rate constants, or upper limits thereof, for the reactions
of DEMP, DEEP, and TEP with O3 were determined by
monitoring the decay of the organophosphorus compound in
the presence of a known concentration of O3,9 with cyclohexane
being present to scavenge any OH radicals formed. Assuming
that under these conditions the only loss process for DEMP,
DEEP or TEP is by reaction with O3, then
OH
NO3
+ organophosphorus compound f products (1)
}
OH
+ reference compound f products
(2)
}
NO3
Providing that the organophosphorus compound and the refer-
ence compound reacted only with OH radicals or NO3 radicals,
then
[organophosphorus]t
0
ln
- Dt )
(
)
[organophosphorus]t
[reference compound]t
k
1 ln
- Dt (I)
0
ln([organophosphorus]t /[organophosphorus]t) - Dt )
0
(
)
k2
[reference compound]t
k3[O3](t - t0) (II)
where [organophosphorus]t and [reference compound]t are the
0
0
where [organophosphorus]t and [organophosphorus]t are the
concentrations of the organophosphorus compound and reference
compound, respectively, at time t0, [organophosphorus]t and
[reference compound]t are the corresponding concentrations at
time t, Dt is a factor to account for dilution caused by any
additions to the chamber during the experiments (Dt ) 0 for
the OH radical reactions and Dt ) 0.0014 per N2O5 addition to
the chamber in the NO3 radical reactions), and k1 and k2 are the
rate constants for reactions 1 and 2, respectively.
0
concentrations of DEMP, DEEP, or TEP at times t0 and t,
respectively, Dt () 0.0025) is the small amount of dilution
caused by the initial addition of O3 to the chamber, and k3 is
the rate constant for reaction 3.
O3 + organophosphorus compound f products (3)
Hydroxyl radicals were generated in the presence of NO by
the photolysis of methyl nitrite in air at wavelengths >300 nm.5,9
The initial reactant concentrations (molecules cm-3) employed
for the OH radical reactions were CH3ONO, ∼2.4 × 1014; NO,
∼2.4 × 1014; and organophosphorus compound(s) and reference
compound(s), ∼2.4 × 1013 each. The reference compounds used
were R-pinene, 1,3,5-trimethylbenzene, and, in selected experi-
ments as a check on the system, di-n-butyl ether, and irradiations
were carried out at 20% of the maximum light intensity for
2.5-13 min. Experiments were also conducted to investigate
the importance of dark decay and photolysis of ∼2.4 × 1013
molecules cm-3 of DEMP, DEEP, and TEP in the chamber,
with 8.0 × 1015 molecules cm-3 of cyclohexane also being
present in the photolysis experiments to scavenge any OH
radicals formed. While most experiments were carried out in
air (21% O2 content) at ∼5% relative humidity, experiments
were also carried out at 6%, 77%, and 78% O2 content (with a
relative humidity of <5%) and in air at 47% and 50% relative
humidity.
The initial reactant concentrations (molecules cm-3) were
DEMP, DEEP, or TEP, ∼2.4 × 1013; O3, (4.64-5.17) × 1013;
and cyclohexane, (0.80-1.60) × 1016. O3 concentrations were
measured during the ∼5.0 h duration reactions by ultraviolet
absorption using a Dasibi model 1003-AH ozone analyzer, and
the concentrations of DEMP, DEEP, and TEP were measured
by GC-FID as described above.
Products of the OH Radical Reactions. Products of the
reactions of OH radicals with DEMP, DEEP, and TEP were
investigated using in situ API-MS analyses. In addition, products
of the TEP reaction were investigated using in situ FT-IR
spectroscopy and GC-FID. Experiments with GC-FID analyses
for TEP and CH3CHO were carried out in a ∼7000 L Teflon
chamber as described above for the kinetic experiments, except
that the reference compound was not present.
Experiments with API-MS Analyses. In these experiments,
the chamber contents were sampled through a 25 mm diameter
× 75 cm length Pyrex tube at ∼20 L min-1 directly into the
API mass spectrometer source. The operation of the API-MS
in the MS (scanning) and MS/MS [with collision activated
dissociation (CAD)] modes has been described previously.11,12
Use of the MS/MS mode with CAD allows the “product ion”
or “precursor ion” spectrum of a given ion peak observed in
the MS scanning mode to be obtained.11 Both positive and
negative ion modes were used in this work, with the majority
of the data obtained being in the negative ion mode. In the
positive ion mode, protonated water hydrates (H3O+(H2O)n)
generated by the corona discharge in the chamber diluent air
were responsible for the protonation of analytes. Ions are drawn
by an electric potential from the ion source through the sampling
orifice into the mass-analyzing first quadrupole or third quad-
rupole. Neutral molecules and particles are prevented from
entering the orifice by a flow of high-purity nitrogen (“curtain
gas”), and as a result of the declustering action of the curtain
gas on the hydrated ions, the ions that are mass analyzed are
mainly protonated molecular ions ([M + H]+) and their
protonated homo- and heterodimers.11
Nitrate radicals were produced from the thermal decomposi-
tion of N2O5,10 and NO2 was also included in the reactant
mixtures. The initial reactant concentrations (molecules cm-3
)
were DEMP, DEEP, or TEP and methacrolein, propene, or
1-butene (the reference compounds), ∼2.4 × 1013 each; NO2,
(4.8-9.6) × 1013, and two additions of N2O5 (each addition
corresponding to (1.1-1.2) × 1014 molecules cm-3 of N2O5 in
the chamber) were made to the chamber during an experiment.
The concentrations of the organophosphorus compounds and
the reference compounds were measured during the experiments
by gas chromatography with flame ionization detection (GC-
FID). For the analyses of propene and 1-butene, gas samples
were collected from the chamber into 100 cm3 volume all-glass
gastight syringes and transferred via a 1 cm3 gas sampling loop
onto a 30 m DB-5 megabore column initially held at -25 °C
and then temperature-programmed to 200 °C at 8 °C min-1
.
For the analyses of methacrolein, R-pinene, 1,3,5-trimethylben-
zene, di-n-butyl ether, DEEP, DEMP, TEP, and, in product

2284 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Aschmann et al.
In the negative ion mode, negative ions are generated by the
negative corona around the discharge needle. The superoxide
ion (O2-), its hydrates, and O2 clusters are the major negative
ions in the chamber diluent air. Other reagent ions, for example,
-
NO2 and NO3-, are then formed from reactions between the
primary reagent ions and neutral molecules such as NO2, and
instrument tuning and operation were designed to induce cluster
formation.12 The initial concentrations of CH3ONO and NO,
and of DEMP, DEEP, or TEP, were ∼2.4 × 1013 molecules
cm-3 each, and irradiations were carried out for up to 2 min,
resulting in up to ∼45% reaction of the initially present DEMP,
DEEP, or TEP.
Experiments with FT-IR Analyses. Irradiations of CH3ONO
[or (CH3)2CHONO)]-NO-TEP mixtures in dry and humidified
(5% relative humidity) air were carried out in the evacuable
chamber, and both FT-IR and GC-FID analyses were employed.
To investigate the formation of HCHO, the photolysis of
2-propyl nitrite8,13 was used as the OH radical source (photolysis
of methyl nitrite forms HCHO, while photolysis of 2-propyl
nitrite leads to acetone formation8,13). To coordinate FT-IR and
GC analyses, the irradiations were carried out intermittently,
each with 3-8 min duration, for up to a total irradiation time
of 16 min, with GC-FID analyses during the intervening dark
periods and FT-IR spectra of the reaction mixture being recorded
before and after each irradiation period. The initial concentra-
tions (in units of 1013 molecules cm-3) employed were TEP,
2.4-6.4; CH3ONO, 24.6, or (CH3)2CHONO, 19.7; and NO,
24.6. A weighed amount of liquid TEP in a sample tube was
introduced into the chamber by heating and flushing with a
stream of heated N2 gas. Partial pressures of CH3ONO (or
(CH3)2CHONO) and NO in calibrated 2 L Pyrex bulbs were
measured in a vacuum manifold with a 100 Torr MKS Baratron
and flushed into the chamber with N2.
Figure 1. Band envelopes of diethyl phosphate (DEP) in dichlo-
romethane solution (2.36 × 10-3 mol L-1, 1.09 mm path length cell),
cyclopentane solution (2.20 × 10-3 mol L-1, 1.09 mm path length cell),
and vapor phase (calculated to be 8.4 × 1012 molecules cm-3, 62.9 m
path). Cyclopentane has a narrower transmission window in this
frequency range than dichloromethane. The baseline-corrected integrated
absorption coefficients measured for the corresponding 990-1130 cm-1
band segments of the two solution spectra are in excellent agreement.
(99+%), R-pinene (99+%), methacrolein (95%), and triethyl
phosphate (99+%), Aldrich Chemical Co.; 1,3,5-trimethylben-
zene, Eastman; diethyl phosphate (98.3%), Chem Service; and
NO (g99.0%), propene (g99.0%), and 1-butene (g99.0%),
Matheson Gas Products. Methyl nitrite, 2-propyl nitrite, and
N2O5 were prepared and stored as described previously,8,10,13,14
and O3 in O2 diluent was generated using a Welsbach T-408
ozone generator. NO2 was prepared as needed by reacting NO
with an excess of O2.
Vapor spectra of (C2H5O)2P(O)OH (diethyl phosphate, DEP)
were obtained by heating the liquid sample and flushing into
the chamber with a stream of heated N2 gas. However, the
calibration for these spectra could not be determined directly
from the known amount injected due to the rapid loss of the
compound to the chamber walls. Attempts at calibration with
the use of a heated, 15 cm path length Pyrex cell with NaCl
windows attached by epoxy resin failed. Therefore, an estimate
of the gas-phase absorption coefficient of the strongest IR band
of DEP was obtained from measurements of the IR spectra of
DEP in CH2Cl2 solution using a sealed 1.09 mm path length
NaCl liquid cell. Molar concentrations of DEP in the range
0.001-0.005 were used, resulting in an average integrated
coefficient (baseline corrected) for the absorption band(s) in the
922-1128 cm-1 region of 8.2 × 10-17 cm molecule-1 (base
10). Measurements using solutions in cyclopentane, a nonpolar
solvent, were in excellent agreement with those from CH2Cl2
solutions for comparable segments of the band envelope. The
above absorption coefficient was then used to estimate the
concentrations which corresponded to the spectra recorded for
a sample of DEP introduced as vapor into the evacuable chamber
as described above. One of these quantified gas-phase DEP
spectra was then employed for analysis of DEP in irradiated
CH3ONO-NO-TEP-air mixtures. The use of this indirect
calibration of the DEP IR absorption coefficient has a probable
error of ∼(20%. Figure 1 illustrates the absorption band
contours of DEP in the vapor state and in dichloromethane and
cyclopentane solutions.
Results
Photolysis and Dark Reactions. No decays (<5%) of gas-
phase DEMP, DEEP, or TEP were observed in a ∼7000 L
Teflon chamber in either the dark over a period of 5.6 h or
during 60 min of photolysis using the same light intensity and
spectral distribution as used in the OH radical rate constant
determinations in the same chamber. (The total durations of the
photolysis experiments were 3.5-3.6 h.) These results show
that dark losses of DEMP, DEEP, and TEP to the walls of the
Teflon chamber were of negligible importance during all
experiments and that any photolysis of DEMP, DEEP, and TEP
over the e13 min irradiations of CH3ONO-NO-organophos-
phorus compound-reference compound-air mixtures was also
negligible.
Rate Constants for the Reactions of OH Radicals with
DEMP, DEEP, and TEP. A series of CH3ONO-NO-
organophosphorus compound-reference compound-air irradia-
tions were carried out in the ∼7000 L Teflon chamber, with
R-pinene and 1,3,5-trimethylbenzene as the reference com-
pounds, and the data obtained are plotted in accordance with
eq I in Figures 2 and 3. No effects of O2 content (6-78%) or
of relative humidity (5-50%) on the rate constant ratios were
observed. R-Pinene and 1,3,5-trimethylbenzene, and R-pinene
and di-n-butyl ether (the latter to serve as an additional check
on the system), were present together as reference compounds
Chemicals. The chemicals used, and their stated purities, were
diethyl ethylphosphonate (98%), Lancaster; acetaldehyde
(99.5+%), diethyl methylphosphonate (97%), di-n-butyl ether

Atmospheric Chemistry of DEMP, DEEP, and TEP
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2285
Figure 4. Plots of eq I for the reactions of OH radicals with 1,3,5-
trimethylbenzene and di-n-butyl ether, with R-pinene as the reference
compound.
Figure 2. Plots of eq I for the reactions of OH radicals with diethyl
methylphosphonate (DEMP), diethyl ethylphosphonate (DEEP), and
triethyl phosphate (TEP), with R-pinene as the reference compound.
The data for DEMP and DEEP have been displaced vertically by 0.25
and 0.50 units, respectively, for clarity.
Figure 5. Plots of eq I for the reactions of OH radicals with diethyl
ethylphosphonate (DEEP) and triethyl phosphate (TEP), with diethyl
methylphosphonate (DEMP) as the reference compound. The data for
DEEP have been displaced vertically by 0.20 units for clarity.
Figure 3. Plots of eq I for the reactions of OH radicals with diethyl
methylphosphonate (DEMP), diethyl ethylphosphonate (DEEP), and
triethyl phosphate (TEP), with 1,3,5-trimethylbenzene as the reference
compound. The data for DEMP and DEEP have been displaced
vertically by 0.25 and 0.50 units, respectively, for clarity.
1, and it can be seen that the rate constants obtained with 1,3,5-
trimethylbenzene and R-pinene as the reference compounds are
in excellent agreement.
in three experiments each, and hence rate constants for the
reactions of OH radicals with 1,3,5-trimethylbenzene and di-
n-butyl ether could be obtained relative to that for R-pinene
(Figure 4). Similarly, in a number of experiments two or three
of the organophosphorus compounds were present in the reactant
mixtures, and hence rate constant ratios for the reactions of
DEEP and TEP relative to DEMP and for the reaction of TEP
relative to DEEP could be obtained (see, for example, the data
plotted in Figure 5). Least-squares analyses of the experimental
data lead to the rate constant ratios k1/k2 given in Table 1. These
rate constant ratios k1/k2 are placed on an absolute basis by use
of rate constants k2 for the reactions of OH radicals with
R-pinene and 1,3,5-trimethylbenzene at 296 K of 5.28 × 10-11
Rate Constant for the Reactions of NO3 Radicals with
DEMP, DEEP and TEP. Reactions of N2O5-NO3-NO2-
organophosphorus compound-methacrolein-air mixtures and
of an N2O5-NO3-NO2-TEP-propene-1-butene-air mixture
were carried out in the same Teflon chamber as used for the
OH radical rate constant determinations. The percentage losses
of the organophosphorus compound were significantly less than
those of the reference compounds (with maximum reactive
losses of DEMP, DEEP, and TEP of 21% compared to up to
86% for methacrolein and a maximum reactive loss of TEP of
8% compared to up to 88% and 94% for propene and 1-butene,
respectively). Least-squares analyses of these data lead to the
rate constant ratios k1/k2 given in Table 2. These rate constant
ratios are placed on an absolute basis by use of rate constants
at 296 K for the NO3 radical reactions of methacrolein, 3.4 ×
cm3 molecule-1 s-1 and 5.67 × 10-11 cm3 molecule-1 s-1
,
respectively.1 The resulting rate constants k1 are given in Table

2286 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Aschmann et al.
TABLE 1: Rate Constant Ratios k1/k2 and Rate Constants k1 for the Reactions of OH Radicals with DEMP, DEEP, and
Triethyl Phosphate at 296 ( 2 K
a
organophosphorus compound
reference compound
k1/k2
1011k1 (cm3 molecule-1 s-1
)
diethyl methylphosphonate (DEMP)
R-pinene
1.14 ( 0.07
0.990 ( 0.053
1.22 ( 0.06
1.14 ( 0.08
1.05 ( 0.06
0.951 ( 0.042
1.12 ( 0.02
0.578 ( 0.014
1.09 ( 0.04
0.958 ( 0.012
0.866 ( 0.026
6.02 ( 0.37
5.61 ( 0.31
6.44 ( 0.32
6.46 ( 0.46
5.54 ( 0.32
5.39 ( 0.24
5.91 ( 0.11
3.05 ( 0.08
1,3,5-trimethylbenzene
R-pinene
diethyl ethylphosphonate (DEEP)
triethyl phosphate (TEP)
1,3,5-trimethylbenzene
R-pinene
1,3,5-trimethylbenzene
R-pinene
R-pinene
DEMP
DEMP
DEEP
1,3,5-trimethylbenzene
di-n-butyl ether
DEEP
TEP
TEP
a Indicated errors are two least-squares standard deviations. b Placed on an absolute basis by use of rate constants at 296 K of k2(R-pinene) )
5.28 × 10-11 cm3 molecule-1 s-1 and k2(1,3,5-trimethylbenzene) ) 5.67 × 10-11 cm3 molecule-1 s-1 1
.
TABLE 2: Rate Constant Ratios k1/k2 and Rate Constants k1 for the Reactions of NO3 Radicals with DEMP, DEEP, and TEP
at 296 ( 2 K
1016k1 (cm3 molecule-1 s-1
)
a
organophosphorus compound
reference compound
k1/k2
diethyl methylphosphonate (DEMP)
diethyl ethylphosphonate (DEEP)
triethyl phosphate (TEP)
methacrolein
methacrolein
methacrolein
propenec
0.11 ( 0.03
0.10 ( 0.04
0.07 ( 0.04
0.038 ( 0.009
0.029 ( 0.007
3.7 ( 1.1
3.4 ( 1.4
2.4 ( 1.4
3.5 ( 0.9c
3.8 ( 1.0c
1-butenec
a Indicated errors are two least-squares standard deviations. b Placed on an absolute basis by use of rate constants at 296 K of k2(methacrolein)
) 3.4 × 10-15 cm3 molecule-1 s-1 1,15
,
k2(propene) ) 9.24 × 10-15 cm3 molecule-1 s-1,1 and k2(1-butene) ) 1.32 × 10-14 cm3 molecule-1 s-1 1
.
c From a single experiment with propene and 1-butene as reference compounds.
10-15 cm3 molecule-1 s-1 1,15
;
propene, 9.24 × 10-15 cm3
molecule-1 s-1;1 and 1-butene, 1.32 × 10-14 cm3 molecule-1
s-1.1 The resulting rate constants k1 are also given in Table 2.
Rate Constants for Reactions of O3 with DEMP, DEEP,
and TEP. No decays (<5%) of gas-phase DEMP, DEEP, or
TEP were measured in the ∼7000 L Teflon chamber in the
presence of (4.64-5.17) × 1013 molecules cm-3 of O3 over
297-305 min reaction periods. Use of an upper limit of 5%
for the amount of DEMP, DEEP, or TEP reacted over these
time periods results in upper limits to the rate constants for
reaction of O3 with DEMP, DEEP, and TEP of k3 <6 × 10-20
cm3 molecule-1 s-1 at 296 ( 2 K.
Products of the Reaction of OH Radicals with DEMP,
DEEP, and TEP. API-MS Analyses. Irradiations of CH3-
ONO-NO-DEMP, DEEP, or TEP-air mixtures were carried
out in a second ∼7000 L Teflon chamber at ∼5% relative
humidity with analyses by in situ API-MS. In the positive ion
mode, API-MS spectra of these mixtures prior to irradiation
showed the protonated organophosphorus compound dimer at
305 u (DEMP), 333 u (DEEP), and 365 u (TEP), with no
additional ion peaks being observed after irradiation due to
reaction products.
In the negative ion mode, no ion peaks attributed to DEMP,
DEEP, or TEP were observed prior to irradiation. Negative ion
API-MS spectra of irradiated CH3ONO-NO-air mixtures of
DEMP, DEEP, and TEP after 1 min irradiation are shown in
Figure 6. Addition of 1.7 × 1013 molecules cm-3 of NO2 to the
chamber changed the relative intensity of certain of these ion
peaks (for example, those at 280 and 294 u in the DEMP
reaction, at 308 and 322 u in the DEEP reaction, and at 340
and 354 u in the TEP reaction) because of differing relative
concentrations of O2-, NO2-, and NO3- reagent ions. API-MS/
MS “product ion” and “precursor ion” spectra of the ion peaks
shown in Figure 6 were obtained and, as an example, API-MS/
MS “product ion” spectra of the 294, 322, and 354 u ion peaks
observed in the API-MS spectra of the DEMP, DEEP, and TEP
reactions, respectively, are shown in Figure 7. For the DEMP,
Figure 6. Negative ion API-MS spectra of irradiated CH3ONO-NO-
diethyl methylphosphonate-air, CH3ONO-NO-diethyl ethylphos-
phonate-air and CH3ONO-NO-triethyl phosphate-air mixtures after
1 min of irradiation (see text for ion peak assignments).
DEEP, and TEP reactions, all of the ion peaks due to reaction
product(s) in the API-MS spectra shown in Figure 6 can be
attributed as arising from a single product from each reaction,
of molecular weight 124 (DEMP reaction), 138 (DEEP reaction),

Atmospheric Chemistry of DEMP, DEEP, and TEP
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2287
Figure 8. Negative ion API-MS spectra of diethyl phosphate (DEP)
introduced into the chamber in the presence of NOx and of an irradiated
CH3ONO-NO-triethyl phosphate-air mixture after 1 min of irradia-
tion.
Figure 7. Negative ion API-MS/MS “product ion” spectra of the 294
u (diethyl methylphosphonate), 322 u (diethyl ethylphosphonate), and
354 u (triethyl phosphate) ion peaks present in the API-MS spectra
shown in Figure 6.
and 154 (TEP reaction), and these are attributed to C2H5-
OP(O)(CH3)OH (ethyl methylphosphonate) from the DEMP
reaction, C2H5OP(O)(C2H5)OH (ethyl ethylphosphonate) from
the DEEP reaction, and (C2H5O)2P(O)OH (diethyl phosphate,
DEP) from the TEP reaction. As an example, the assignments
of the ion peaks shown in Figure 6 for the TEP reaction are
153 u, [154 - H]- (i.e., (C2H5O)2P(O)O-); 200 u, [154 +
NO2]-; 216 u, [154 + NO3]-; 307 u, [154 + 154 - H]-; 340
u, [154 + 154 + O2]-; 354 u, [154 + 154 + NO2]-; 370 u,
[154 + 154 + NO3]-; 386 u, [154 + 154 + NO2 + O2]-; and
461 u, [154 + 154 + 154 - H]-, with analogous assignments
for the ion peaks in the DEMP and DEEP reactions.
An API-MS spectrum of diethyl phosphate (DEP) introduced
into the chamber in the presence of NOx is shown in Figure 8
together with the API-MS spectrum of an irradiated CH3ONO-
NO-TEP-air mixture. Identical ion peaks are present in both
spectra, and API-MS/MS product ion spectra of the ion peaks
observed in the API-MS spectra are also identical, as shown
for the 354 u ion peaks in Figure 9. These data confirm that
DEP [(C2H5O)2P(O)OH] is the product observed from the OH
radical-initiated reaction of TEP, and by analogy the products
of the DEMP and DEEP reactions are C2H5OP(O)(CH3)OH and
C2H5OP(O)(C2H5)OH, respectively.
FT-IR and GC-FID Analyses of the Reaction of OH
Radicals with TEP. These experiments were carried out in a
5870 L Teflon-coated evacuable chamber. The quantitative
analysis of products and reactants by FT-IR spectroscopy was
carried out by a subtractive procedure. Components were
successively subtracted from the spectrum of the mixture using
calibrated spectra of the gaseous reactants and known products,
Figure 9. Negative ion API-MS/MS “product ion” spectrum of the
354 u ion peaks present in the API-MS spectra shown in Figure 8.
which have been recorded previously with the same instrument
and identical spectral parameters. As routinely carried out, the
absorption bands of CH3ONO and NO and of their irradiation
products NO2, HCHO, CH3ONO2, HNO3, and HONO were
subtracted first to reveal more clearly the products from the
main reactant. Readily identified products were CO2 (a major
product), CO, and CH3C(O)OONO2 (PAN, peroxyacetyl nitrate).
Characteristic group frequencies also suggested the formation
of organic nitrates, RONO2, at 1680, 1295, and 827 cm-1 and
an organic ester, most likely ROC(O)CH3, at 1795 and 1210
cm-1 as minor products. HCHO was also determined by FT-IR

2288 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Aschmann et al.
spectroscopy as a major product in irradiations employing
(CH3)2CHONO instead of CH3ONO as the OH radical precur-
sor.
CH3CHO, a minor product, has a weak infrared absorption
and was measured by GC-FID. Experiments carried out in a
∼7000 L Teflon chamber with GC-FID analyses showed that
the formation yield of CH3CHO in air (21% O2 content) at ∼5%
relative humidity and in synthetic air with 6% O2 content (at
e2% relative humidity) was 11 ( 2%, where the indicated
errors are two least-squares standard deviations combined with
estimated uncertainties in the GC-FID response factors for TEP
and CH3CHO of (5% each, and the measured CH3CHO
concentrations have been corrected for secondary reaction with
OH radicals (the maximum correction being 18%).
Subtraction of the spectrum of the unreacted TEP revealed
(C2H5O)2P(O)OH (DEP) as a product by comparison with the
IR spectrum of DEP introduced into the chamber. The analyses
of TEP and its major product DEP were not straightforward.
Their major infrared absorption bands, found in the 900-1100
cm-1 region, severely overlap, and both compounds exhibited
significant decay to the wall of the evacuable chamber. Because
of its significant decay in this evacuable chamber, the FT-IR
calibrations for TEP were referenced to the GC-FID analyses
of samples taken contemporaneously with the spectra recorded.
GC-FID calibrations of TEP were performed independently and
more accurately in a 7000 L Teflon chamber where TEP decay
was minimal (see above). It should be noted that the inside
surface of the evacuable chamber consists of Teflon-coated
aluminum together with quartz windows and gold-coated mirror
surfaces of the multiple reflection optical system and with a
number of “O”-ring seals for various flanges (including the end-
window assemblies). It is possible that the dark decays of the
organophosphorus compounds studied here involved absorption
into these “O”-rings.
Figure 10. (A) Triethyl phosphate (TEP) reference spectrum. (B)
Diethyl phosphate (DEP) reference spectrum. Numbers in parentheses
are concentrations in molecules cm-3. (C) Product spectrum from an
irradiated CH3ONO-NO-TEP-air mixture, with 1.2 × 1013 molecules
cm-3 of TEP consumed and 5.4 × 1012 molecules cm-3 of DEP present.
Symbols indicate the characteristic band positions of the minor products
PAN (*), RONO2 (+), and ROC(O)CH3 (#). (D) From (C) after
subtraction of the DEP absorption band. See text for details.
starting compounds, HCHO, and other products from CH3ONO
and NO (see above) have been subtracted in Figure 10C,
together with the weak CH3CHO absorptions per the GC-FID
analyses. Thus, the presence of 5.4 × 1012 molecules cm-3 of
DEP is shown in Figure 10C, with the weaker but distinct signals
from the minor products PAN, RONO2, and RC(O)OCH3 also
identified in this spectrum and in Figure 10D where the
absorption bands of DEP have been subtracted. The absorption
bands of CO2, a major product, are outside the range of the
plots. The measured concentrations of HCHO and CH3CHO
were corrected for secondary reaction with the OH radical (with
maximum corrections of 7% and 8%, respectively) using OH
radical reaction rate constants for HCHO and CH3CHO (cm3
molecule-1 s-1) of 8.5 × 10-12 and 1.5 × 10-11, respectively.15
The cited uncertainties are two least-squares standard deviations
combined with estimated uncertainties in the TEP, CO2, HCHO,
CH3CHO, and CO concentrations of TEP, (5%; CO2, (7%
(which includes calibration errors and errors due the nonlinearity
inherent to the finely structured CO2 band at 2350 cm-1);
HCHO, (5%; CH3CHO, (5%; and CO, (7% (including the
calibration error and nonlinearity inherent to its highly resolved
band at 2145 cm-1). The effect of ambient CO2 on the product
CO2 measurements was minimized by stabilizing the N2 purge
of the FT-IR instrument for a period of at least 90 min prior to
the measurements. CO2 contributions from the photolysis of
CH3ONO and (CH3)2CHONO were negligible, as determined
from the CO2 yields from separate photolyses of CH3ONO-
NO-air and (CH3)2CHONO-NO-air mixtures.
Since wall losses of TEP occurred during the dark periods
when GC-FID analyses were conducted (∼40 min on average),
corrections to the actual amount of TEP consumed during each
irradiation period were obtained from the decay rate of TEP
which was established for each experiment by relative IR
intensity measurements prior to the irradiation. The measured
initial decay rates of TEP after injection into the evacuable
chamber depended unpredictably on wall conditioning, with
initial rates of (1-1.3) × 10-3 min-1 being observed for the
chamber injected with TEP after numerous other types of
experiments have been carried out and for TEP runs with diluent
air at 5% relative humidity. However, for the majority of the
experiments where TEP remained in the chamber for ∼1 h or
more prior to irradiation, a slower decay rate in the range (4-
6) × 10-4 min-1 was established. Calculations using these later
decay rates for the individual runs resulted in 8-14% corrections
to the amounts of TEP consumed by the reaction as obtained
from the GC-FID analyses alone.
Using these procedures, the products observed and their yields
were DEP, 65-82% (initial, see below); CO2, 80 ( 10%;
HCHO, 55 ( 5%; CH3CHO, 11 ( 3% (by GC-FID analyses
from the evacuable chamber experiments, in excellent agreement
with the 11 ( 2% yield obtained in the Teflon chamber, see
above); CO, 11 ( 3%; PAN, 8%; RONO2, 7%; and ROC(O)-
CH3, 4%.
These product yields were derived from four irradiation
experiments carried out with dry diluent air. A number of
experiments were carried out in diluent air at 5% relative
humidity to see whether the presence of water vapor leads to
HC(O)OH as a secondary product. HC(O)OH was not positively
detected in either dry or humidified diluent air, and its upper
limit yield was estimated to be <2% in both dry air and in air
at 5% relative humidity. The 65% and 82% yields of DEP given
The formation of several products from a CH3ONO-NO-
TEP-air irradiation is shown in Figure 10. Traces A and B of
Figure 10 are reference spectra of TEP and DEP, respectively.
Figure 10C is the product spectrum after 1.2 × 1013 molecules
cm-3 of TEP had reacted. The absorption bands of the unreacted

Atmospheric Chemistry of DEMP, DEEP, and TEP
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2289
TABLE 3: Room Temperature Rate Constants k1 for the Reactions of OH Radicals with DEMP, DEEP, and TEP, Compared
with Literature Values
1011k1 (cm3 molecule-1 s-1
)
organophosphorus compound
this worka
literature
reference
diethyl methylphosphonate (DEMP)
diethyl ethylphosphonate (DEEP)
triethyl phosphate (TEP)
5.78 ( 0.24
6.45 ( 0.27
5.44 ( 0.20
3.23 ( 0.11b
Kleindienst and Smith7
5.53 ( 0.35c
Atkinson et al.6
a Weighted averages of the two rate constants measured relative to those for 1,3,5-trimethylbenzene and R-pinene (Table 1). b At ∼298 K.
Relative to the rate constants for reactions of OH radicals with n-hexane, propene, and 1,3-butadiene. The rate constant used for the reaction of OH
radicals with n-hexane was 3% higher than a more recent recommendation.1 c At 296 ( 2 K. Relative to the reaction of OH radicals with propene,
using a rate constant of k2(propene) ) 2.66 × 10-11 cm3 molecule-1 s-1 1
.
above are results from the first irradiation periods of two separate
experiments. These two values are in agreement within the
overall uncertainties of the measurements, i.e., the estimated
20% calibration error for DEP, the 5% uncertainty associated
with each FT-IR measurement of TEP as referenced to the
nominal (5% error of the GC-FID analysis of TEP, and the
additional error associated with the correction for TEP decay
in the evacuable chamber which occurred between the GC-FID
measurements. More importantly, a large uncertainty would arise
from the decay of DEP to the chamber walls which occurred
while it was being formed and monitored, as evidenced by the
drastically lower yields of DEP which were measured during
the subsequent irradiation periods for both experiments (with
measured yields of 82 and 44% in one experiment and 65%,
49%, and 30% in the second experiment). While DEP is also
expected to react with OH radicals, no data are available for
the kinetics of this reaction, and hence no corrections for
secondary reaction of DEP with OH radicals were made.
However, the observed decreases in DEP formation yield with
increasing extent of reaction were almost certainly largely due
to wall losses and not secondary reactions.
(CH3O)2P(O)SCH3, (CH3S)2P(O)OCH3, and (CH3O)2P(S)-
SCH3.4,5 This study is the first to observe a reaction of NO3
radicals with an alkyl phosphate, alkyl phosphorothioate, or alkyl
phosphonate, with previous studies of the reactions of NO3
radicals with (CH3O)3PS, (CH3O)2P(O)SCH3, (CH3S)2P(O)-
OCH3, and (CH3O)2P(S)SCH3 having obtained only upper limits
to the rate constants, of <(1-30) × 10-15 cm3 molecule-1 s-1 5
,
factors of 2.5-77 higher than the actual rate constants measured
here for DEMP, DEEP, and TEP.
The rate constants measured here for the reactions of OH
radicals with DEMP, DEEP, and TEP are given in Table 3 (these
being weighted averages of the two individual measurements
with 1,3,5-trimethylbenzene and R-pinene as reference com-
pounds), and those for DEMP and TEP are compared with the
available literature values.6,7 Our present rate constant for TEP
is in excellent agreement with our previous measurement6
determined relative to the rate constant for the reaction of OH
radicals with propene. Furthermore, combining the present rate
constant ratio of k1(TEP)/k2(1,3,5-trimethylbenzene) ) 0.951
( 0.042 with the rate constant ratio k2(1,3,5-trimethylbenzene)/
k2(propene) ) 2.16 ( 0.11 at 296 ( 2 K23 results in k1(TEP)/
k2(propene) ) 2.05 ( 0.14, in excellent agreement with our
previous value of this rate constant ratio of 2.08 ( 0.13.6
However, for the reaction of OH radicals with DEMP our room
temperature rate constant of 5.78 × 10-11 cm3 molecule-1 s-1
is a factor of 1.8 higher than that reported by Kleindienst and
Smith,7 for reasons that are presently not known.
The quantitative estimate of the yield of organic nitrates
(RONO2) was made by measuring the area of the 1680 cm-1
band and applying the average integrated absorption coefficient
of 2.5 × 10-17 cm molecule-1, derived from the corresponding
absorption bands of a series of organic nitrates.16 Likewise, the
quantitative estimate of the formation yield of acetate (ROC(O)-
CH3) species was obtained from the 1795 cm-1 band using the
There is a large increase in the room temperature OH radical
reaction rate constant when the methyl groups in trimethyl
phosphate (7.4 × 10-12 cm3 molecule-1 s-1)4 are replaced by
ethyl groups in triethyl phosphate (5.5 × 10-11 cm3 molecule-1
s-1 [Table 3]). However, as evident from Table 1, there is only
a 9 ( 4% increase in replacing the methyl group bonded directly
to the P atom in DEMP by the ethyl group in DEEP, and DEEP
is actually more reactive than TEP (by 15 ( 4%). It therefore
appears that the reaction of OH radicals with DEMP, DEEP,
and triethyl phosphate proceed mainly by H atom abstraction
from the CH2 portion of the CH3CH2O- groups. Using the
approach of Kwok and Atkinson24 and assuming that the partial
OH radical rate constants for the two CH3CH2O- groups in
DEEP and DEMP are identical, then the 9 ( 4% increase in
rate constant from DEMP to DEEP suggests that ∼9% of the
overall OH radical reaction with DEEP proceeds by H atom
abstraction from the C2H5 group attached directly to the P atom.
The room temperature OH radical reaction partial rate constants
per CH3CH2O- group are then 2.9 × 10-11 cm3 molecule-1
s-1 for DEEP and DEMP and 1.8 × 10-11 cm3 molecule-1 s-1
for TEP.
integrated absorption coefficient of 2.3 × 10-17 cm molecule-1
,
an average derived from the corresponding absorption band areas
of ethyl acetate, allyl acetate, and 3-chloropropyl acetate (with
the individual absorption coefficients agreeing within 10% of
each other).
Discussion
Our rate constant ratio of k(OH + 1,3,5-trimethylbenzene)/
k(OH + R-pinene) ) 1.12 ( 0.02 is in excellent agreement
with the value of 1.07 derived from the literature OH radical
reaction rate constants.1 Furthermore, the rate constant obtained
here for the reaction of OH radicals with di-n-butyl ether of
3.05 × 10-11 cm3 molecule-1 s-1 is in excellent agreement with
the literature room temperature rate constants of (in units of
10-11 cm3 molecule-1 s-1) 2.78 ( 0.36,17 2.68 ( 0.32,18 2.72
( 0.02,19 2.68 ( 0.12,19 3.22 ( 0.10,20 2.91 ( 0.15,20 2.69 (
0.22,21 and 3.26 ( 0.2522 (where relative rate measurements
have been reevaluated using the most recent recommendations
for the reference compounds used).
The rate constants measured here for the reactions of NO3
radicals and O3 with DEMP, DEEP, and TEP and for the
reaction of OH radicals with DEEP are the first reported. The
observed lack of reaction with O3 is consistent with previous
data for the reactions of O3 with (CH3O)3PO, (CH3O)3PS,
Our product study of the reactions of OH radicals with
DEMP, DEEP, and TEP in air at ∼5% relative humidity using
in situ API-MS observed product ion peaks attributed to ethyl
methylphosphonate [C2H5OP(O)(CH3)OH] from DEMP, ethyl

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.
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(10)
(12)

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