Kinetic and product studies of the reactions of selected glycol ethers with OH radicals

Article abstract of DOI:10.1021/es010831k

Glycol ethers are widely used as solvents and are hence liable to be released into the atmosphere where they may contribute to the formation of photochemical air pollution in urban and regional areas. The dominant reaction of glycol ethers in the atmosphere has been previously shown to be with OH radicals. Using a relative rate method, rate constants have been measured at 296 ± 2 K for the gasphase reactions of the OH radical with 1-butoxy-2propanol [CH₃CH₂CH₂CH₂OCH₂CH (OH)CH₃], diethylene glycol ethyl ether [CH₃CH₂OCH₂ CH₂OCH₂CH₂OH], and diethylene glycol n-butyl ether [CH₃CH₂CH₂CH₂OCH₂ CH₂OCH₂CH₂OH] of (in units of 10^-11 cm³ molecule^-1 s^-1) 3.76 ± 0.54, 5.72 ± 0.85, and 7.44 ± 0.94, respectively, where the error limits include the estimated overall uncertainties in the rate constants for the reference compounds. Products of the OH radical-initiated reactions of these glycol ethers have been investigated using gas chromatography with flame ionization detection (GC-FID), combined gas chromatography-mass spectrometry(GC-MS), in situ Fourier transform infrared (FT-IR) spectroscopy, and in situ atmospheric pressure ionization tandem mass spectrometry (API-MS). The products identified and quantified account for 102 ± 11% of the reaction products from 1-butoxy-2-propanol, 87 ± 9% of those from diethylene glycol ethyl ether, and 83 ± 12% of those from diethylene glycol n-butyl ether. An empirical estimation method for calculating reaction rates of alkoxy radicals under atmospheric conditions appears to fairly well predict the products formed and their yields. Detailed reaction schemes after the initial OH radical reactions are formulated for each of these glycol ethers, with the majority of the reactions involving H-atom abstraction from the CH₂ groups adjacent to the ether linkage.

Full text of DOI:10.1021/es010831k

Environ. Sci. Technol. 2001, 35, 4080-4088
NO3 radicals, and O3 (1, 2), with the OH radical reaction
Kinetic and Product Studies of the
Reactions of Selected Glycol Ethers
with OH Radicals
being an important, and often dominant, atmospheric loss
process (1, 2). Glycol ethers are widely used as solvents and
are hence liable to be released into the atmosphere where
they may contribute to the formation of photochemical air
pollution in urban and regional areas (3). To date, there have
been a number of studies of the atmospheric chemistry of
glycol ethers (4-15), mostly concerned with the measurement
of rate constants for their reactions with OH radicals (4-8,
11, 12, 14), with product studies of glycol ether reactions
having been reported for 2-ethoxyethanol (9), 2-propoxy-
ethanol (14), 2-isopropoxyethanol (15), 2-butoxyethanol (10,
13), and 1-methoxy-2-propanol (13).
In this work, we have extended our recent studies of the
atmospheric chemistry of 1-methoxy-2-propanol and 2-bu-
toxyethanol (12, 13) to the glycol ethers 1-butoxy-2-propanol
[CH3CH2CH2CH2OCH2CH(OH)CH3], diethylene glycol ethyl
ether [CH3CH2OCH2CH2OCH2CH2OH], and diethylene glycol
n-butyl ether [CH3CH2CH2CH2OCH2CH2OCH2CH2OH].
S A R A M . A S C H M A N N , # P I L A R M A R T I N , † , #
E R N E S T O C . T U A Z O N , #
J A N E T A R E Y, ‡ , # A N D
R O G E R A T K I N S O N * , ‡ , § , #
Air Pollution Research Center, Interdepartmental Graduate
Program in Environmental Toxicology, Department of
Environmental Sciences, and Department of Chemistry,
University of California, Riverside, California 92521
Glycol ethers are widely used as solvents and are hence
liable to be released into the atmosphere where they
may contribute to the formation of photochemical air pollution
in urban and regional areas. The dominant reaction of
glycol ethers in the atmosphere has been previously shown
to be with OH radicals. Using a relative rate method,
rate constants have been measured at 296 ( 2 K for the gas-
phase reactions of the OH radical with 1-butoxy-2-
propanol [CH CH CH CH OCH CH(OH)CH ], diethylene
Experimental Section
Experiments were carried out at 298 ( 2 K and 740 Torr total
pressure of air in a 5870 L evacuable, Teflon-coated chamber
containing an in situ multiple reflection optical system
interfaced to a Nicolet 7199 Fourier transform infrared (FT-
IR) absorption spectrometer and with irradiation provided
by a 24-kW xenon arc filtered through a 6 mm thick Pyrex
pane to remove wavelengths <300 nm; in a 7900 L Teflon
chamber with analysis by gas chromatography with flame
ionization detection (GC-FID) and combined gas chroma-
tography-mass spectrometry (GC-MS), with irradiation
provided by two parallel banks of blacklamps; and in a 7500
L Teflon chamber interfaced to a PE SCIEX API III MS/ MS
direct air sampling, atmospheric pressure ionization tandem
mass spectrometer (API-MS), again with irradiation provided
by two parallel banks of blacklamps. All three chambers are
fitted with Teflon-coated fans to ensure rapid mixing of
reactants during their introduction into the chamber. Hy-
droxyl radicals were generated in the presence of NO by the
photolysis of methyl nitrite [CH3ONO] or 2-propyl nitrite
[(CH3)2CHONO] in air at wavelengths >300 nm, where R and
RÄ are H or CH3. NO was added to the reactant mixtures to
suppress the formation of O3 and hence of NO3 radicals.
3
2
2
2
2
3
glycol ethyl ether [CH CH OCH CH OCH CH OH], and
3
2
2
2
2
2
diethylene glycol n-butyl ether [CH CH CH CH OCH CH OCH -
3
2
2
2
2
2
2
CH OH] of (in units of 10-11 cm3 molecule-1 s-1) 3.76 (
2
0.54, 5.72 ( 0.85, and 7.44 ( 0.94, respectively, where the
error limits include the estimated overall uncertainties
in the rate constants for the reference compounds. Products
of the OH radical-initiated reactions of these glycol
ethers have been investigated using gas chromatography
with flame ionization detection (GC-FID), combined gas
chromatography-mass spectrometry (GC-MS), in situ Fourier
transform infrared (FT-IR) spectroscopy, and in situ
atmospheric pressure ionization tandem mass spectrometry
(API-MS). The products identified and quantified account
for 102 ( 11% of the reaction products from 1-butoxy-2-
propanol, 87 ( 9% of those from diethylene glycol ethyl
ether, and 83 ( 12% of those from diethylene glycol n-butyl
ether. An empirical estimation method for calculating
reaction rates of alkoxy radicals under atmospheric conditions
appears to fairly well predict the products formed and
their yields. Detailed reaction schemes after the initial OH
radical reactions are formulated for each of these glycol
ethers, with the majority of the reactions involving H-atom
RRÄ CHONO + hν f RRÄ CHO˙ + NO
RRÄ CHO˙ + O2 f RC(O)RÄ + HO2
HO2 + NO f OH + NO2
Measurem ent of OH Radical Reaction Rate Constants.
Rate constants for the OH radical reactions were determined
using a relative rate method in which the relative disap-
pearance rates of the glycol ethers and a reference compound
(2-methylpropene in these experiments), whose OH radical
reaction rate constant is reliably known, were measured in
the presence of OH radicals (12). Providing that the glycol
ethers and 2-methylpropene reacted only with OH radicals,
then (12)
abstraction from the CH groups adjacent to the ether
2
linkage.
Introduction
Volatile organic compounds present in the atmosphere can
undergo photolysis and chemical reaction with OH radicals,
* Corresponding author phone: (909)787-4191; fax: (909)787-5004;
e-mail: ratkins@mail.ucr.edu.
ln([glycol ether]t / [glycol ether]t) )
† Present address: Universidad de Castilla-La Mancha, Facultad
de Ciencias, Campus Universitario s/ n, 13071, Ciudad Real, Spain.
‡ Interdepartmental Graduate Program in Environmental Toxicol-
ogy and Department of Environmental Sciences.
o
(k1/ k2) ln([2-methylpropene]t / [2-methylpropene]t) (I)
o
§
where [glycol ether]t and [2-methylpropene]t are the
Department of Chemistry.
o
o
# Air Pollution Research Center.
concentrations of the glycol ether and 2-methylpropene,
9
4 0 8 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 20, 2001
10.1021/es010831k CCC: $20.00
2001 Am erican Chem ical Society
Published on Web 09/19/2001

respectively, at time to, [glycol ether]t and [2-methylpropene]t
are the corresponding concentrations at time t, and k1 and
k2 are the rate constants for reactions 1 and 2, respectively.
1 min to the chamber contents. Following sampling, the fiber
in its needle housing was inserted into the heated (200 °C)
GC injection port on a Varian 2000 GC/ MS/ MS ion trap
system, and the analytes thermally desorbed onto a 30 m
DB-1701 fused silica capillary column. To obtain molecular
weight information, the ion trap was operated in the chemical
ionization (CI) mode with methane as the reagent gas.
Evacuable Cham ber Experim ents with FT-IR Analysis.
For the experiments carried out in the 5870 L evacuable,
Teflon-coated chamber (at <1% relative humidity), methyl
nitrite and 2-propyl nitrite were used as the OH radical source
(with the use of 2-propyl nitrite allowing HCHO formation
yields from the glycol ethers to be measured). The initial
concentrations were (in molecule cm-3 units) as follows:
methyl nitrite or 2-propyl nitrite (1.48-2.46) × 1014; NO,
2.46 × 1014; and 1-butoxy-2-propanol, 2.24 × 1014, diethylene
glycol ethyl ether, 2.39 × 1014, or diethylene glycol n-butyl
ether, 1.49 × 1014. Partial pressures of methyl nitrite, 2-propyl
nitrite, and nitric oxide in calibrated 5 and 2 L volume Pyrex
bulbs were measured by a capacitance manometer (MKS
Baratron, 100 Torr sensor), and the contents of the bulbs
were then flushed into the chamber with N2 gas. Vapors of
the glycol ethers were introduced into the chamber by heating
a measured amount of the liquid sample and flushing with
a stream of heated N2 gas. IR spectra of the reaction mixtures
were recorded prior to and during irradiation with 64 scans
(corresponding to 2.0-min averaging time) per spectrum, a
full-width-at-half-maximum resolution of 0.7 cm-1, and a
path length of 62.9 m (13, 18). The mixtures were irradiated
continuously for a 23-38 min period with FT-IR monitoring
every 2-4 min.
Teflon Cham ber with Analysis by API-MS. In the experi-
ments with API-MS analyses, 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, particularly in the positive ion mode, has been
described elsewhere (19). 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 (19).
Both positive and negative ion modes were used in these
experiments. In the positive ion mode, protonated water
hydrates (H3O+(H2O)n) were the reagent ions. Product ion
peaks were identified based on the observation of homo- or
heterodimer ions (for example, [(MP1)2 + H]+, [(MP2)2 + H]+,
and [MP1 + MP2 + H]+, where P1 and P2 are products or the
glycol ether reactant) in the API-MS/ MS “precursor ion”
spectra, and consistency of the API-MS/ MS “product ion”
spectrum of a homo- or heterodimer ion with the “precursor
ion” spectra of the various [MP + H]+ ion peaks (19). Water
cluster ion peaks of the product ions, [M + H + H2O]+, were
also occasionally observed.
OH + glycol ether f products
(1)
(2)
OH + 2-methylpropene f products
Hence plots of ln([glycol ether]t / [glycol ether]t) against ln-
o
([2-methylpropene]t / [2-methylpropene]t) should be straight
o
lines with slope k1/ k2 and zero intercept.
Experiments were carried out in the 7900 L Teflon
chamber, using methyl nitrite photolysis as the OH radical
source. The initial reactant concentrations (in molecule cm-3
units) were as follows: CH3ONO, (2.3-2.4) × 1014; NO, (1.6-
1.7) × 1014; glycol ether, ∼2.4 × 1013; and 2-methylpropene,
∼2.4 × 1013. Irradiations were carried out at 20% of the
maximum light intensity for 1.5-12 min. The concentrations
of the glycol ethers and 2-methylpropene were measured
during the experiments by gas chromatography with flame
ionization detection (GC-FID). For the analysis of the glycol
ethers, 100 cm3 volume gas samples were collected from the
chamber onto Tenax-TA solid adsorbent, with subsequent
thermal desorption at ∼250 °C onto a 30 m DB-1701
megabore column held at 0 °C and then temperature
programmed to 200 °C at 8 °C min-1. For the analysis of
2-methylpropene, gas samples were collected from the
chamber in 100 cm3 all-glass, gastight syringes and transferred
via a 1 cm3 stainless steel loop and gas sampling valve onto
a 30 m DB-5 megabore column held at -25 °C and then
temperature programmed to 200 °C at 8 °C min-1. Based on
multiple analyses in the dark, the GC-FID measurement
uncertainties for the glycol ethers were in the range 1-5%
(and generally 1-3%). NO and initial NO2 concentrations
were measured using a Thermo Environmental Instruments,
Inc., Model 42 chemiluminescent NO-NOx analyzer.
Products of the OH Radical-Initiated Reactions. Teflon
Cham ber with Analyses by GC-FID and GC-MS. For the
experiments carried out in the 7900 L Teflon chamber (at
∼5% relative humidity), methyl nitrite photolysis was used
as the OH radical source, and the initial reactant concentra-
tions (in molecule cm-3 units) were as follows: CH3ONO,
(2.1-2.4) × 1014; NO, (2.0-2.2) × 1014; and glycol ether, (2.30-
2.65) × 1013 [note that 1 part-per-million (ppm) mixing
ratio ) 2.40 × 1013 molecule cm-3 at 298 K and 740 Torr total
pressure]. Irradiations were carried out at 20% of the
maximum light intensity for 3.5-12 min (1-butoxy-2-pro-
panol), 2-9 min (diethylene glycol ethyl ether), and 1.5-6
min (diethylene glycol n-butyl ether), resulting in up to 54-
69% reaction of the initially present glycol ether. The
concentrations of the glycol ethers and selected products
were measured during the experiments by GC-FID. Gas
samples of 100 cm3 volume were collected from the chamber
onto Tenax-TA solid adsorbent prior to and during the
reaction, with subsequent thermal desorption at ∼250 °C
onto a DB-1701 megabore column, initially held at -40, -20,
or 0 °C, depending on the products to be analyzed, and then
temperature programmed to 200 °C at 8 °C min-1. Typically,
2-3 analyses were carried out prior to the irradiations, with
an analysis after each of the three irradiation periods per
experiment. GC-FID response factors for the glycol ethers
and selected products were determined by introducing
measured amounts of the chemicals into the 7900 L chamber
and conducting several replicate GC-FID analyses (16).
Under the experimental conditions used, in the negative
ion mode analytes were detected as adducts ions formed
from reaction of the neutral analyte (M) and the reagent ion
-
NO2 (20).
NO2- + M f [NO2•M]-
The initial concentrations of CH3ONO, NO, and glycol ether
were ∼2.4 × 1013 molecule cm-3 each, and irradiations were
carried out for 1-3 min at 20% of the maximum light intensity.
Chem icals. The chemicals used and their stated purities
were as follows: acetaldehyde (99.5+%), butanal (99%),
2-butoxyethanol (99+%), n-butyl formate (97%), diethylene
glycol n-butyl ether (99+%), diethylene glycol ethyl ether
(99+%), ethyl formate (97%), propanal (99+%), propylene
Reaction products were also analyzed by combined gas
chromatography-mass spectrometry, using a Solid Phase
Microextraction (SPME) Fiber Assembly (Supelco) (17) with
a 100 µm poly(dimethylsiloxane) fiber. The fiber was inserted
through a port into the chamber, and the fiber exposed for
9
VOL. 35, NO. 20, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 0 8 1

and Atkinson (24) as revised by Bethel et al. (25) [using rate
constants for a series of diols to derive new values of the
group substituent factors F(-OH) and F(mCOH)] are also
given in Table 1. These calculated values agree with our
measured rate constants for the glycol ethers to within ∼25%,
which is considered to be very good agreement [estimated
room temperature OH radical reaction rate constants for
∼90% of ∼485 organic compounds agree with the measured
values to within a factor of 2 (24)]. Combining our measured
room temperature rate constants with the diurnally, season-
ally, and annually averaged global tropospheric OH radical
concentration of 1.0 × 106 molecule cm-3 (26, 27) leads to
calculated lifetimes of the glycol ethers studied here in the
range 0.15-0.3 day. Our previous measurements of rate
constants for reactions of NO3 radicals and O3 with 1-meth-
oxy-2-propanol and 2-butoxyethanol showed that the NO3
radical and O3 reactions were of minor or negligible
importance as tropospheric loss processes, respectively (12),
and it is anticipated that this is also the case for the glycol
ethers studied here.
FIGURE 1. Plots of eq I for the gas-phase reactions of the OH
radical with 1-butoxy-2-propanol, diethylene glycol ethyl ether, and
diethylene glycol n-butyl ether, with 2-methylpropene as the
reference compound. The data for diethylene glycol ethyl ether and
diethylene glycol n-butyl ether have been displaced vertically by
0.4 and 0.8 units, respectively, for clarity.
Products of the OH Radical-Initiated Reactions. Analyses
of irradiated CH3ONO (or (CH3)2CHONO)-NO-glycol ether-
air mixtures were carried out using GC-FID and GC-MS
analyses, in situ FTIR spectroscopy and in situ API-MS. In
the gas chromatographic analyses, products were identified
by comparison of the GC retention times with those of
authentic standards [the GC retention time of 2-hydroxyethyl
formate was obtained by conducting an irradiation of a CH3-
ONO-NO-2-butoxyethanol-air mixture (13)] and their
stuctures confirmed from the CI mass spectra. For each of
the glycol ethers studied, two products were observed in the
CI GC-MS analyses for which no authentic standards were
available; the CI GC-MS analyses enabled their molecular
weights to be determined, and they could be readily matched
to peaks observed in the GC-FID analyses. Based on the
potential reaction schemes discussed below, these products
were attributed to specific compounds, and the GC-FID
response factors were then calculated, relative to those
measured for the glycol ethers and selected products, from
their Effective Carbon Numbers [ECNs] (28).
TABLE 1. Rate Constant Ratios k1/k2 and Rate Constants k1
for the Gas-Phase Reactions of the OH Radical with Glycol
Ethers at 296 ( 2 K
1011 × k1
(cm3 molecule-1 s-1
)
a
organic
k1/k2
this worka,b estimatedc
1-butoxy-2-propanol 0.723 ( 0.074 3.76 ( 0.39
4.0
5.1
diethylene glycol
ethyl ether
1.10 ( 0.12
5.72 ( 0.63
diethylene glycol
n-butyl ether
1.43 ( 0.11
7.44 ( 0.58
5.6
a
Indicated uncertainties are two least-squares standard deviations.
Placed on an absolute basis by use of a rate constant k2 for the reaction
b
of OH radicals with 2-m ethylpropene at 296 K of 5.20 × 10-11 cm 3
m olecule-1 s-1 (1, 22, 23). The cited errors do not take into account the
uncertainties associated with the rate constant k2 (estim ated to be
∼(10%). c Calculated as described by Kwok and Atkinson (24) and Bethel
et al. (25).
The IR spectra of irradiated CH3ONO (or (CH3)2CHONO)-
NO-glycol ether-air mixtures showed strong, overlapped
absorptions in the ∼1750 cm-1 (CdO stretch) region and in
the ∼1150 cm-1 (C-O-C stretch) region, where absorption
bands of carbonyl products occurred together with those of
the parent glycol ether, rendering the quantitative analysis
of products difficult. The spectra of the reaction mixtures
were analyzed by desynthesis using calibrated reference
spectra and included subtraction of the absorption bands of
the unreacted CH3ONO or (CH3)2CHONO and of the known
photolysis products of their mixtures with NO; i.e., HNO3,
HONO, NO2, HC(O)OH, and CH3ONO2 and HCHO in the
case of CH3ONO, and (CH3)2CHONO2 and CH3C(O)CH3 in
the case of (CH3)2CHONO. For example, Figure 2A shows the
product spectrum from an irradiated (CH3)2CHONO-NO-
diethylene glycol ethyl ether-air mixture after 25 min of
irradiation, in which the absorptions by the remaining
diethylene glycol ethyl ether and by the products arising from
(CH3)2CHONO and NO have been subtracted. HCHO, ethyl
formate, and 2-hydroxyethyl formate were the major products
identified. The subtraction of the absorptions by the re-
maining diethylene glycol ethyl ether, leading to Figure 2A,
and the subtraction of the 2-hydroxyethyl formate and ethyl
formate absorptions, leading to Figure 2B and 2C, respec-
tively, were carried out iteratively. The absorption peaks
employed in the subtractive analysis were as follows:
diethylene glycol ethyl ether, 1131 cm-1; ethyl formate, 1160
cm-1 (Q-branch); 2-hydroxyethyl formate, 1080 cm-1; and
HCHO, 1746 cm-1 (Q-branch). The residual spectrum (Figure
glycol n-butyl ether (99+%, a mixture of both isomers shown
by GC-FID and comparison with the 1-butoxy-2-propanol
standard to contain ∼96.5% 1-butoxy-2-propanol), and
n-propyl nitrate (97%), Aldrich Chemical Co.; 1-butoxy-2-
propanol (96%), TCI America; and NO (g99.0%) and 2-
methylpropene (g99.0%), Matheson Gas Products. Methyl
nitrite was prepared as described by Taylor et al. (21), and
2-propyl nitrite was prepared in an analogous manner; both
were stored under vacuum at 77 K.
Results and Discussion
OH Radical Rate Constants. A series of CH3ONO-NO-glycol
ether-2-methylpropene-air irradiations were carried out,
and the data obtained from these irradiations are plotted in
accordance with eq I in Figure 1. Straight line plots are
observed, and the rate constant ratios k1/ k2 obtained from
least-squares analyses of the data are given in Table 1. These
rate constant ratios are placed on an absolute basis by use
of a rate constant k2 for the reaction of the OH radical with
2-methylpropene at 296 K of 5.20 × 10-11 cm3 molecule-1 s-1
(1, 22, 23). The resulting rate constants k1 are also given in
Table 1.
These are the first reported rate constants for the reactions
of the OH radical with these glycol ethers. The 298 K rate
constants calculated using the estimation method of Kwok
9
4 0 8 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 20, 2001

the OH radical source (because HCHO is the major photo-
oxidation product of methyl nitrite), and it was verified in a
separate irradiation of a (CH3)2CHONO-NO-air mixture that
acetone was the major product formed, with no measurable
formation of HCHO during a 30 min irradiation at the same
light intensity as used in the irradiations with the glycol ethers.
For the 1-butoxy-2-propanol and diethylene glycoln-butyl
ether reactions, only HCHO, organic nitrates, and, from the
diethylene glycol n-butyl ether reaction, 2-hydroxyethyl
formate could be quantified by FT-IR spectroscopy. For both
glycol ethers, after an initial subtraction of the absorptions
by the remaining glycol ether (using the absorption peaks of
1-butoxy-2-propanol and diethylene glycol n-butyl ether at
1129 and 1131 cm-1, respectively), the product yields
measured by GC-FID (Tables 2 and 3) were utilized in the
subtraction of absorptions due to n-butyl formate, propanal,
and acetaldehyde from the 1-butoxy-2-propanol reaction,
and of n-butyl formate and propanal from the diethylene
glycol n-butyl ether reaction. Thus, in the 1-butoxy-2-
propanol reaction, initial subtractions of the n-butyl formate,
propanal, and acetaldehyde absorptions, using appropriate
scaling factors of calibrated reference spectra based on GC-
FID yields (Table 2), facilitated the determination of the
HCHO and 1-butoxy-2-propanol concentrations by FT-IR,
and the final analysis was obtained by an iterative procedure.
For the diethylene glycol n-butyl ether reaction, subtraction
of the absorption bands of n-butyl formate and propanal
based on the yields determined by the GC-FID analysis (Table
3) aided in determining the 2-hydroxyethyl formate con-
centrations by FT-IR, and the initial subtractions of the
product absorptions based on GC-FID yields facilitated the
determination of the HCHO and diethylene glycol n-butyl
ether concentrations by FT-IR, again using an iterative
procedure to obtain the final analysis. Only minor adjust-
ments in the concentrations of the glycol ethers were needed
after the first cycle of the iterative subtraction procedure.
Organic nitrates were quantified from the 1-butoxy-2-
propanol and diethylene glycol n-butyl ether reactions as
described above for the diethylene glycol ethyl ether reaction.
The products identified by CI GC-MS, in situ FT-IR, and
in situ API-MS analyses and (in some cases) quantified by
GC-FID and/ or in situ FT-IR analyses are shown in Tables
2-4. In each case, the products observed and quantified
also react with OH radicals (1, 2), and these secondary
reactions of the products with the OH radical were taken
FIGURE 2. (A) Spectrum of products arising from diethylene glycol
ethyl ether during a (CH ) CHONO-NO-diethylene glycol ethyl
3 2
ether-air irradiation (see text). (B) After subtraction of absorptions
by 2-hydroxyethyl formate. (C) Residual spectrum after subtraction
of absorptions by HCHO and ethyl formate (see text). (D) Reference
spectrum of 2-hydroxyethyl formate (4.9 × 1013 molecule cm-3). (E)
Reference spectrum of ethyl formate (4.9 × 1013 molecule cm-3).
The spectra A, B, C, and D have been offset vertically by 1.0, 0.7,
0.4, and 0.2 absorbance units, respectively, for clarity.
2C) indicates the presence of small amounts of organic
nitrates (RONO2) [bands at 1298 and 1673 cm-1] and an
unidentified carbonyl product(s) (CdO stretch absorption
band(s) at ∼1750 cm-1) which most likely also contains a
C-O-C group(s) [broad band(s) at ∼1160 cm-1]. Estimates
of the organic nitrate, RONO2, yields were obtained from the
residual spectra from the integrated areas of the ∼1300 cm-1
and/ or ∼1670 cm-1 bands, using integrated absorption
coefficients of (1.3 ( 0.2) × 10-17 cm molecule-1 and (2.5 (
0.3) × 10-17 cm molecule-1, respectively (as elsewhere in this
article, the cited errors are two least-squares standard
deviations). These integrated coefficients, measured in this
laboratory, are the average values obtained from the cor-
responding absorption bands of methyl nitrate, ethyl nitrate,
1- and 2-propyl nitrate, and peroxyacetyl nitrate. For all three
glycol ethers, the HCHO yield was determined from experi-
ments which employed the photolysis of (CH3)2CHONO as
TABLE 2. Products Formed and Their Yields from the Reactions of OH Radicals with 1-Butoxy-2-propanol
molar yield
a
product
form aldehyde, HCHO
acetaldehyde, CH3CHO
propanal, CH3CH2CHO
GC-FID
FT-IR
API-MS
estimatedb
0.10 ( 0.03
0.01
0.56
0.24
<0.01
0.01
0.49 ( 0.06c
0.23 ( 0.03c
<0.02
<0.01
0.44 ( 0.04c
butanal, CH3CH2CH2CHO
2-hydroxyethyl form ate, HOCH2CH2OCHO
n-butyl form ate, CH3CH2CH2CH2OCHO
organic nitrates
observedd
0.51
0.08 ( 0.02
observedd,e (MW 193)f
observedd,e
MW 104 product, CH3CH(OH)CH2OCHOh
0.13 ( 0.03g
0.22 ( 0.04g
0.24
0.03
0.17
0.03
MW 118 producti
observede
j
MW 130 product, CH3CH2CH2CH2OCH2C(O)CH3
MW 146 productk
observedd
observedd
a
Indicated errors are two least-squares standard deviations com bined with the estim ated errors arising from the calibration and spectral
b
subtraction procedures (see text). Estim ated as discussed in the text (see also Supporting Inform ation). The cited yields are obtained from the
c
data presented in Table S2 (Supporting Inform ation), neglecting organic nitrate form ation from RO˙ + NO reactions. Indicated errors are two
2
least-squares standard deviations com bined with the estim ated uncertainties ((5%) in the m easured GC-FID response factors for the product and
1-butoxy-2-propanol. In positive ion m ode. In negative ion m ode, with NO2 as the reagent ion. f API-MS and API-MS/MS spectra indicate
d
e
-
g
presence of an organic nitrate(s) of m olecular weight 193, attributed to CH3CH2CH2CH(ONO2)OCH2CH(OH)CH3 and isom ers. Indicated errors are
two least-squares standard deviations com bined with an estim ated uncertainty in the calculated GC-FID response factor, relative to that for
h
1-butoxy-2-propanol, of (15%. Attributed to CH3CH(OH)CH2OCHO (or isom ers); GC identification based on CI GC-MS m olecular weight data.
i Could be CH3CH2CH(OH)CH2OCHO (see Table S2, Supporting Inform ation). j Attributed to CH3CH2CH2CH2OCH2C(O)CH3 (or isom ers); GC identification
based on CI GC-MS m olecular weight data. k Attributed to CH3C(O)CH2CH2OCH2CH(OH)CH3 (or isom ers).
9
VOL. 35, NO. 20, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 0 8 3

TABLE 3. Products Formed and Their Yields from the Reactions of OH Radicals with Diethylene Glycol Ethyl Ether
molar yield
a
product
form aldehyde, HCHO
GC-FID
FT-IR
API-MS
estimatedb
0.39 ( 0.03
0.37 ( 0.07
0.40 ( 0.08
0.06 ( 0.02
0.47
0.35
0.34
ethyl form ate, CH3CH2OCHO
0.39 ( 0.05c
0.37 ( 0.07e
observedd
2-hydroxyethyl form ate, HOCH2CH2OCHO
organic nitrates
observedd,f,g
MW 118 product, CH3CH2OCH2CH2OCHOh
MW 134 product, HOCH2CH2OCH2CH2OCHOi
0.26 ( 0.04e
0.17 ( 0.04e
observedd
0.40
0.05
a
Indicated errors are two least-squares standard deviations com bined with the estim ated errors arising from the calibration and spectral
b
subtraction procedures (see text). Estim ated as discussed in the text (see also Supporting Inform ation). The cited yields are obtained from the
c
data presented in Table S3 (Supporting Inform ation), neglecting organic nitrate form ation from RO˙ + NO reactions. Indicated errors are two
2
least-squares standard deviations com bined with the estim ated uncertainties ((5%) in the m easured GC-FID response factors for the product and
d
e
diethylene glycol ethyl ether. In positive ion m ode. Indicated errors are two least-squares standard deviations com bined with an estim ated
uncertainty in the calculated GC-FID response factor, relative to that for diethylene glycol ethyl ether, of (15%. f In negative ion m ode, with NO2
-
g
as the reagent ion. Product(s) of m olecular weight 90 observed, which could be HOCH2CH2OCHO, HOCH2OCH2CHO and/or CH3CH(OH)OCHO.
Attributed to CH3CH2OCH2CH2OCHO (or isom ers); GC identification based on CI GC-MS m olecular weight data. i Observed by GC-FID and m olecular
h
weight obtained from CI GC-MS analyses; assum ed to be HOCH2CH2OCH2CH2OCHO (note that this product could not be observed by API-MS
analyses because its m olecular weight is the sam e as that of diethylene glycol ethyl ether).
TABLE 4. Products Formed and Their Yields from the Reactions of OH Radicals with Diethylene Glycol n-Butyl Ether
molar yield
a
product
form aldehyde, HCHO
propanal, CH3CH2CHO
butanal, CH3CH2CH2CHO
GC-FID
FT-IR
API-MS
estimatedb
0.23 ( 0.03
0.37
0.17
<0.01
0.24
0.20 ( 0.02c
<0.01
2-hydroxyethyl form ate, HOCH2CH2OCHO
n-butyl form ate, CH3CH2CH2CH2OCHO
organic nitrates
0.30 ( 0.06d
0.23 ( 0.02c
0.43 ( 0.09
0.13 ( 0.04
observede,f,g
0.26
observede,f,h (MW 223)
observedf
MW 118 producti
0.11
0.17
0.37
0.02
MW 134 product, HOCH2CH2OCH2CH2OCHOj
MW 146 product, CH3CH2CH2CH2OCH2CH2OCHOk
MW 176 productl
0.16 ( 0.03d
0.16 ( 0.03d
observede
observede,f
a
Indicated errors are two least-squares standard deviations com bined with the estim ated errors arising from the calibration and spectral
b
subtraction procedures (see text). Estim ated as discussed in the text (see also Supporting Inform ation). The cited yields are obtained from the
c
data presented in Table S4 (Supporting Inform ation), neglecting organic nitrate form ation from RO˙ + NO reactions. Indicated errors are two
2
least-squares standard deviations com bined with the estim ated uncertainties ((5%) in the m easured GC-FID response factors for the product and
d
diethylene glycol n-butyl ether. Indicated errors are two least-squares standard deviations com bined with an estim ated uncertainty in the calculated
GC-FID response factor, relative to that for diethylene glycol n-butyl ether, of (15%. In positive ion m ode. f In negative ion m ode, with NO2- as
e
g
h
the reagent ion. MW 90 species could also include HOCH2OCH2CHO and/or HOCH2CH2OCHO. API-MS and API-MS/MS spectra indicate presence
of an organic nitrate(s) of m olecular weight 223, attributed to CH3CH2CH2CH(ONO2)OCH2CH2OCH2CH2OH and isom ers. i MW 118 species could
include CH3CH2CH(OH)CH2OCHO. j MW 134 species could include HOCH2CH2OCH2CH2OCHO; GC identification based on CI GC-MS m olecular
weight data. k MW 146 species could include CH3CH2CH2CH2OCH2CH2OCHO; GC identification based on CI GC-MS m olecular weight data. l MW
176 species could include CH3C(O)CH2CH2OCH2CH2OCH2CH2OH.
into account as described previously (29) using OH radical
reaction rate constants of (in units of 10-12 cm3 molecule-1
s-1) the following: glycol ethers, see Table 1; formaldehyde,
9.37 (1); acetaldehyde, 15.8 (1); propanal, 19.6 (1); butanal,
23.5 (1); methyl formate, 0.200 (30, 31); n-butyl formate, 3.33
(30, 31); ethyl formate, 0.955 (30, 31); 2-hydroxyethyl formate,
7.4 [estimated] (25); CH3CH(OH)CH2OCHO, 11.3 [estimated]
(25); CH3CH2CH2CH2OCH2C(O)CH3, 18.4 [estimated] (24);
CH3CH2OCH2CH2OCHO, 19.5 [estimated] (24); and HOCH2-
CH2OCH2CH2OCHO, 35.4 [estimated] (25). The multiplicative
correction factors F to take into account secondary reactions
increase with the rate constant ratio k(OH + product)/
k(OH + glycol ether) and with the extent of reaction (29).
The maximum value of the multiplicative correction factor
F occurred for formation of HOCH2CH2OCH2CH2OCHO from
diethylene glycol ethyl ether in the GC-FID analyses (F )
1.51), with most of the maximum values of the multiplicative
factors F being e 1.20 (all for the FT-IR analyses being e
1.09). Representative plots of the amounts of the products
formed, corrected for secondary reactions with the OH
radical, against the amounts of 1-butoxy-2-propanol reacted
are shown in Figure 3. The product formation yields obtained
from least-squares analyses of the data obtained are given
in Tables 2-4, together with upper limits to the formation
yields of butanal and 2-hydroxyethyl formate from 1-butoxy-
2-propanol and of butanal from diethylene glycol n-butyl
ether.
1-Butoxy-2-propanol, CH2CH2CH2CH2OCH2CH(OH)-
CH3. Based on the estimation method of Kwok and Atkinson
(24) [as revised by Bethel et al. (25)], the calculated percent-
ages of the overall OH radical reaction proceeding by H-atom
abstraction from the various C-H bonds in CH3CH2CH2-
CH2OCH2CH(OH)CH3 are (from left to right) 0.4%, 2.9%, 3.5%,
24.0%, 50.8%, 17.2%, and 0.9%, respectively, with H-atom
abstraction from the O-H group being calculated to account
for 0.3% of the overall reaction. Hence the three most reactive
sites are the two -CH2- groups adjacent to the ether -O-
atom and the -CH(OH)- group, with reaction at these sites
being estimated to account for >90% of the overall OH radical
reaction. H-atom abstraction from the C-H bond in the -CH-
(OH)- group results in formation of the R-hydroxy radical
CH3CH2CH2CH2OCH2C˙ (OH)CH3 which is expected to react
rapidly (and solely) with O2 (1) to form the molecular weight
130 product CH3CH2CH2CH2OCH2C(O)CH3. H-atom ab-
straction from the other C-H bonds leads to the formation
of alkyl-type radicals, which rapidly (and solely) add O2 to
form organic peroxy (RO˙ ) radicals (1, 2). In the presence of
2
NO, these organic peroxy radicals react to form an alkoxy
9
4 0 8 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 20, 2001

of propanal (23 ( 3%) and the failure to observe butanal
(<2% yield) show that decomposition of the CH3CH2CH2-
CH(O˙ )OCH2CH(OH)CH3 radical to CH3CH2C˙ H2 + CH3CH-
(OH)CH2OCHO [reaction 5] occurs, with the 1-propyl radical
reacting further to form propanal (plus a small amount of
1-propyl nitrate) (1). The molecular weight 146 product
observed in our API-MS analyses (but not in our GC-MS
analyses) could be CH3CH2CH2CH(OH)OCH2C(O)CH3 formed
in minor overall yield (Table 2) from the isomerization
reaction 7.
Analogous reactions occur for the CH3CH2CH2CH2OCH-
(O˙ )CH(OH)CH3 radical, with decomposition to CH3CH2CH2-
CH2OCHO + CH3C˙ HOH being predicted to dominate (by
factors of >105) over the other decomposition pathway,
isomerization, and reaction with O2. Because the CH3C˙ HOH
radical reacts rapidly with O2 to form acetaldehyde, the
expected products from the CH3CH2CH2CH2OCH(O˙ )CH-
(OH)CH3 radical are n-butyl formate and acetaldehyde in
equal yield. This agrees with our observations that the yields
of n-butyl formate (44 ( 4%) and acetaldehyde (49 ( 6%) are
very similar.
FIGURE 3. Plots of the amounts of acetaldehyde, propanal, n-butyl
formate, and the molecular weight 104 and 130 products (tentatively
identified as CH CH(OH)CH OCHO and CH CH CH CH OCH C(O)CH ,
3
2
3
2
2
2
2
3
Consideration of all of the initial H-atom abstraction
pathways and the subsequent reactions of the intermediate
alkoxy radicals allows prediction of the products expected
and their overall yields, and the results are shown in Table
S2 of the Supporting Information (note that organic nitrate
respectively) formed, corrected for secondary reactions, against
the amounts of 1-butoxy-2-propanol reacted with the OH radical.
The data for n-butyl formate, CH CH CH CH OCH C(O)CH , and
3
2
2
2
2
3
acetaldehyde have been displaced vertically by 1.0 × 1012, 1.0 ×
1012, and 2.0 × 1012 molecule cm-3, respectively, for clarity.
formation from the RO˙ + NO reactions is not included in
2
radical plus NO2 [reaction 3] or an organic nitrate [reaction
this table) and in Schemes S1 and S2, Supporting Information.
As shown by the comparison in Table 2, the predictions are
in generally good agreement with our experimental data.
Our product yield data and consideration of the potential
reaction schemes indicate that the products formed are as
follows: n-butyl formate + acetaldehyde (44 ( 4%, using the
n-butyl formate yield) from decomposition of the CH3CH2-
CH2CH2OCH(O˙ )CH(OH)CH3 radical; CH3CH2CH2CH2OCH2C-
(O)CH3 (22 ( 4%) from reaction of the CH3CH2CH2CH2-
OCH2C˙ (OH)CH3 radical with O2; and propanal + CH3-
CH(OH)CH2OCHO (23 ( 3%, using the propanal formation
yield) from decomposition of the CH3CH2CH2CH(O˙ )OCH2-
CH(OH)CH3 radical. These products therefore account for
89 ( 7% of the reaction products and pathways. In addition
the formation of acetaldehyde + CH3CH2CH(OH)CH2OCHO
from decomposition of the CH3CH2CH(O˙ )CH2OCH2CH(OH)-
CH3 radical and of acetaldehyde + other products from the
CH3CH(O˙ )CH2CH2OCH2CH(OH)CH3 radical accounts for
5 ( 7% (based on the acetaldehyde yield minus the n-butyl
formate yield). Combined with formation of organic nitrate(s)
from the first-generation C7H15O2(O˙ O) radicals + NO reac-
tions (8 ( 2%, which may include smaller nitrates formed
from reactions of second- and later-generation RO˙ 2 radicals),
we can account for 102 ( 11% of the reaction products and
pathways.
Diethylene Glycol Ethyl Ether, CH3CH2OCH2CH2OCH2-
CH2OH. The calculated percentages (24, 25) of the overall
OH radical reaction proceeding by H-atom abstraction from
the various C-H bonds in CH3CH2OCH2CH2OCH2CH2OH
are (from left to right) 0.3%, 15.3%, 18.9%, 18.9%, 39.9%, and
6.5%, respectively, with H-atom abstraction from the O-H
group being calculated to account for 0.3% of the overall
reaction. Hence the most reactive sites are the four -CH2-
groups adjacent to the two ether -O- atoms, and the -CH2-
OH group, with reaction at these sites being estimated to
account for >99% of the overall OH radical reaction.
Analogous to the 1-butoxy-2-propanol reaction, but with a
lower R-hydroxy radical yield, H-atom abstraction from the
C-H bond in the -CH2OH group is expected to result in
formation of the molecular weight 132 product CH3CH2OCH2-
CH2OCH2CHO. However, lacking an authentic standard it
was not possible to identify and quantify this compound. As
discussed above for 1-butoxy-2-propanol, the other alkyl
4] (1, 2), for example
CH3CH2CH2CH(OO˙ )OCH2CH(OH)CH3 + NO f
CH3CH2CH2CH(O˙ )OCH2CH(OH)CH3 + NO2 (3)
CH3CH2CH2CH(OO˙ )OCH2CH(OH)CH3 + NO f
CH3CH2CH2CH(ONO2)OCH2CH(OH)CH3 (4)
The molecular weight of these organic nitrates is 193,
consistent with our API-MS analyses.
The various alkoxy radicals then undergo decomposition,
isomerization, and reaction with O2 (15, 23, 32). For the CH3-
CH2CH2CH(O˙ )OCH2CH(OH)CH3 radical formed from reac-
tion 3, the reactions are
CH3CH2CH2CH(O˙ )OCH2CH(OH)CH3 f
CH3CH2C˙ H2 + CH3CH(OH)CH2OCHO (5)
CH3CH2CH2CH(O˙ )OCH2CH(OH)CH3 f
CH3CH2CH2CHO + CH3CH(OH)CH2O˙ (6)
CH3CH2CH2CH(O˙ )OCH2CH(OH)CH3 f
CH3CH2CH2CH(OH)OCH2C˙ (OH)CH3
VO2
CH3CH2CH2CH(OH)OCH2C(O)CH3 + HO2
(7)
CH3CH2CH2CH(O˙ )OCH2CH(OH)CH3 + O2 f
CH3CH2CH2C(O)OCH2CH(OH)CH3 + HO2 (8)
Estimations using the methods discussed in detail by
Atkinson (23, 32) and updated and revised by Aschmann
and Atkinson (15) [see also Supporting Information] indicate
that decomposition to CH3CH2C˙ H2 + CH3CH(OH)CH2OCHO
[reaction 5] dominates (2.9 × 107 s-1 at 298 K) over
decomposition to CH3CH2CH2CHO + CH3CH(OH)CH2O˙
[reaction 6] (1.5 × 104 s-1), isomerization [reaction 7] (6 ×
105 s-1) and reaction with O2 [reaction 8] (∼8 × 104 s-1). The
heats of formation of reactants and products were obtained
from the thermochemical compilations (33, 34) or calculated
using the NIST computer program (35). Significant formation
9
VOL. 35, NO. 20, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 0 8 5

predicted to be the dominant reaction pathways, the dashed
lines are predicted to be of minor or negligible importance,
and the products observed are shown in boxes (see Schemes
S3-S5, Supporting Information, for reactions of the other
three dominant alkoxy radicals predicted to be involved in
the diethylene glycol ethyl ether reaction). Table S3, Sup-
porting Information, shows the predicted products (15, 23,
32) from the various initial reaction pathways. As shown by
the comparison in Table 3, the predictions are in generally
good agreement with our experimental data. Our yield of
HOCH2CH2OCH2CH2OCHO (17 ( 4%) is close to the pre-
dicted formation yield of CH3C˙ HOCH2CH2OCH2CH2OH
radicals (15.3%) [Table S3, Supporting Information], sug-
gesting that the dominant reaction of the alkoxy radical CH3-
CH(O˙ )OCH2CH2OCH2CH2OH is by decomposition [reaction
14 in Scheme 1] (dominating over the isomerization reaction
15 in Scheme 1) and that the predicted decomposition rate
constant is too low by a factor of g 2 (note that an uncertainty
of 1 kcal mol-1 in the estimated Arrhenius activation energy
corresponds to an uncertainty in the rate constant at 298 K
of a factor of 5).
SCHEME 1
Our product yield data, together with consideration of
the potential reaction schemes, indicate that the products
formed are as follows: CH3CH2OCH2CH2OCHO + HCHO
from decomposition of the CH3CH2OCH2CH2OCH(O˙ )CH2-
OH radical (26 ( 4% based on the yield of CH3CH2OCH2-
CH2OCHO (using an estimated GC-FID response factor)
because HCHO is also formed as a coproduct to HOCH2-
CH2OCH2CH2OCHO [Scheme 1]); 2-hydroxyethyl formate
and ethyl formate from decompositions of the CH3CH2OCH2-
CH(O˙ )OCH2CH2OH and CH3CH2OCH(O˙ )CH2OCH2CH2OH
radicals (38 ( 5% based on the ethyl formate yield); and
HOCH2CH2OCH2CH2OCHO + HCHO (17 ( 4%) from de-
composition of the CH3CH(O˙ )OCH2CH2OCH2CH2OH radical
[as noted above, the magnitude of the formation yield of this
products suggests that the CH3CH(O˙ )OCH2CH2OCH2CH2OH
radical dominantly decomposes (reaction 14 in Scheme 1)].
Consistent with expectations, the measured HCHO yield
(39 ( 3%) is in excellent agreement with the sum of the CH3-
CH2OCH2CH2OCHO and HOCH2CH2OCH2CH2OCHO yields
(43 (6%). These products account for 81 (8% of the products
and reaction pathways. Combined with the organic nitrate
yield of 6 ( 2% (which may include nitrates formed from
second-generation RO˙ 2 radicals + NO reactions), 87 ( 9% of
the products and reaction pathways are then accounted for.
Assuming that the CH3CH(O˙ )OCH2CH2OCH2CH2OH radi-
cal dominantly decomposes (Scheme 1), then 2-hydroxyethyl
formate and ethyl formate arise from reactions of the CH3-
CH2OCH(O˙ )CH2OCH2CH2OH and CH3CH2OCH2CH(O˙ )OCH2-
CH2OH radicals (Table S3 and Schemes S3 and S4 in
Supporting Information). The close agreement of the mea-
sured ethyl formate (38 ( 5%) and 2-hydroxyethyl formate
(38 ( 6%) yields (and the agreement of their formation yields
with the sum of the predicted formation yields of the
precursor CH3CH2OC˙ HCH2OCH2CH2OH and CH3CH2OCH2-
C˙ HOCH2CH2OH radicals (38%) [Table S3, Supporting In-
formation]) suggests that the HOCH2CH2OCH2O˙ radical
formed from the CH3CH2OCH(O˙ )CH2OCH2CH2OH alkoxy
radical reacts with O2 [reaction 16] rather than undergoing
radicals formed after H-atom abstraction will add O2 and
then react with NO, forming organic nitrates of molecular
weight 195, or alkoxy radicals plus NO2. The alkoxy radicals
undergo decomposition, isomerization, or reaction with O2.
For the most important alkoxy radicals CH3CH(O˙ )OCH2CH2-
OCH2CH2OH, CH3CH2OCH(O˙ )CH2OCH2CH2OH, CH3CH2-
OCH2CH(O˙ )OCH2CH2OH, and CH3CH2OCH2CH2OCH(O˙ )-
CH2OH, decomposition to a formate ester plus alkyl radical
[reaction 10] and/ or isomerization from CH2 groups [reaction
13] are predicted to dominate over reaction with O2 [reaction
9] and decomposition to an aldehyde plus alkoxy radical
[reaction 11]. For example, for the CH3CH2OCH(O˙ )CH2OCH2-
CH2OH radical:
CH3CH2OCH(O˙ )CH2OCH2CH2OH + O2 f
CH3CH2OC(O)CH2OCH2CH2OH + HO2
(9)
(∼8 × 104 s-1
)
CH3CH2OCH(O˙ )CH2OCH2CH2OH f
CH3CH2OCHO + HOCH2CH2OC˙ H2 (10)
(7.1 × 107 s-1
)
CH3CH2OCH(O˙ )CH2OCH2CH2OH f
CH3CH2O˙ + HOCH2CH2OCH2CHO (11)
(1.2 × 104 s-1
)
CH3CH2OCH(O˙ )CH2OCH2CH2OH f
C˙ H2CH2OCH(OH)CH2OCH2CH2OH (12)
(7.7 × 103 s-1
)
CH3CH2OCH(O˙ )CH2OCH2CH2OH f
CH3CH2OCH(OH)CH2OC˙ HCH2OH (13)
(2.1 × 106 s-1
)
HOCH2CH2OCH2O˙ + O2 f HOCH2CH2OCHO + HO2
The rates in parentheses are those estimated (15, 23, 32)
for 298 K and (in the case of the O2 reaction) atmospheric
pressure of air. Obviously, the radicals formed in the
decomposition and isomerization reactions react further and
can result in fairly complex reaction schemes involving a
multitude of potential reaction pathways (noting that in many
cases one or two reaction pathways of a given alkoxy radical
are predicted to dominate and hence simplify the reaction
schemes). Scheme 1 shows the reactions for the CH3CH-
(O˙ )OCH2CH2OCH2CH2OH radical, where the solid lines are
(16)
isomerization [reaction 17] to form HOCH2OCH2CHO, as
predicted in Table S3, Supporting Information (see also
Scheme S3, Supporting Information).
HOCH2CH2OCH2O˙ f
HOCH2OCH2C˙ HOH
VO2
(17)
HOCH2OCH2CHO + HO2
9
4 0 8 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 20, 2001

Diethylene Glycol n-Butyl Ether, CH3CH2CH2CH2OCH2-
CH2OCH2CH2OH. The calculated percentages (25) of the
overall OH radical reaction proceeding by H-atom abstraction
from the various C-H bonds in CH3CH2CH2CH2OCH2CH2-
OCH2CH2OH are (from left to right) 0.3%, 2.1%, 2.5%, 17.4%,
17.4%, 17.4%, 36.7%, and 6.0%, respectively, with H-atom
abstraction from the O-H group being calculated to account
for 0.3% of the overall reaction. Hence the most reactive
sites are the four -CH2- groups adjacent to the two ether
-O- atoms, and the -CH2OH group, with reaction at these
sites being estimated to account for >90% of the overall OH
radical reaction. Table S4, Supporting Information, shows
the predicted products from the various initial reaction
pathways. As shown by the comparison in Table 4 (and
Schemes S6-S9, Supporting Information), the predictions
are in generally good agreement with our experimental data.
Our product formation yield data and consideration of the
potential reaction schemes suggest that the products are as
follows: CH3CH2CH2CH2OCH2CH2OCHO + HCHO (16 ( 3%)
from decomposition of the CH3CH2CH2CH2OCH2CH2OCH-
(O˙ )CH2OH radical; HOCH2CH2OCH2CH2OCHO + propanal
(20 ( 2% using the propanal yield, which is in reasonable
agreement with the yield of HOCH2CH2OCH2CH2OCHO
derived using an estimated GC-FID response factor) from
decomposition of the CH3CH2CH2CH(O˙ )OCH2CH2OCH2CH2-
OH radical; 2-hydroxyethyl formate + n-butyl formate from
decomposition of the CH3CH2CH2CH2OCH(O˙ )CH2OCH2CH2-
OH radical (with the HOCH2CH2OCH2O˙ radical leading to
formation of 2-hydroxyethyl formate [see the discussion
above concerning the diethylene glycol ethyl ether reaction]),
and 2-hydroxyethyl formate + products (including n-butyl
formate) from decomposition of the CH3CH2CH2CH2OCH2-
CH(O˙ )OCH2CH2OH radical (summing to 34 ( 10% based on
an average of the 2-hydroxyethyl formate yields measured
by GC-FID and FT-IR spectroscopy). These products account
for 70 ( 11% of the reaction pathways. Combined with
Acknowledgments
The authors gratefully thank the American Chemistry Council,
Glycol Ether Panels, for supporting this research, and P.M.
acknowledges support from the Spanish Ministerio de
Educacion y Cultura.
Supporting Information Available
A summary of the empirical estimation method used (15, 23,
32), Schemes S1-S9, and Tables S2-S4. This material is
available free of charge via the Internet at http:/ / pubs.acs.org.
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formation of organic nitrates from the RO˙ + NO reactions
2
(13 ( 4%, which may include nitrates formed from second-
generation RO˙ radicals + NO reactions), this accounts for
2
83 ( 12% of the reaction products and pathways. Reference
to Table S4, Supporting Information, indicates that products
arising from reactions of CH3CH2CH2CH2OCH2CH2OCH2-
C˙ HOH, CH3CH(O˙ )CH2CH2OCH2CH2OCH2CH2OH, and CH3-
CH2CH(O˙ )CH2OCH2CH2OCH2CH2OH radicals were not iden-
tified or quantified, although products of MW 118 and 176
were observed by API-MS (Table 4), and these may well
account for ∼10% of the reaction pathways.
(20) Arey, J.; Aschmann, S. M.; Kwok, E. S. C.; Atkinson, R. J. Phys.
Chem. A 2001, 105, 1020.
Atm ospheric Im plications. The product and mechanistic
data obtained here for the reactions of the OH radical with
1-butoxy-2-propanol, diethylene glycol ethyl ether, and
diethylene glycol n-butyl ether in the presence of NO account
for the majority of the reaction products and pathways and
can be used in detailed chemical mechanisms for the
atmospheric photooxidations of these glycol ethers. The
empirical estimation method of Atkinson (23, 32), as revised
by Aschmann and Atkinson (15), for calculating the reaction
rates of alkoxy radicals under atmospheric conditions appears
to fairly well predict the products formed, including their
yields. The detailed reaction mechanisms for these three
glycol ethers, and for 1-methoxy-2-propanol and 2-butoxy-
ethanol (13), are quite analogous, with the initial OH radical
reactions occurring mainly by H-atom abstraction from the
CH2 groups adjacent to the ether linkage and from the C-H
bonds of the carbon atom to which the hydroxyl group is
attached. Furthermore, the dominant reaction of the most
important initially formed alkoxy radicals, of structure RCH-
(O˙ )ORÄ , is decomposition to form a formate ester, RÄ OCHO,
plus an alkyl radical, with this decomposition of the RCH-
(O˙ )ORÄ alkoxy radical dominating over its reaction with O2
and, if feasible, isomerization.
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310.
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Cunnold, D. M.; Fraser, P. J.; Hartley, D. E.; Simmonds, P. G.
Science 1995, 269, 187.
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(29) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.;
Pitts, J. N., Jr. J. Phys. Chem. 1982, 86, 4563.
(30) Wallington, T. J.; Dagaut, P.; Liu, R.; Kurylo, M. J. Int. J. Chem.
Kinet. 1988, 20, 177.
(31) Le Calve´, S.; Le Bras, G.; Mellouki, A. J. Phys. Chem. A 1997, 101,
5489.
(32) Atkinson, R. Int. J. Chem. Kinet. 1997, 29, 99.
(33) Kerr, J. A.; Stocker, D. W. Strengths of Chemical Bonds. In
Handbook of Chemistry and Physics, 80th ed.; Lide, D. R., Ed.;
CRC Press: Boca Raton, FL, 1999-2000.
(34) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr,
J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data 2000, 29, 167.
9
VOL. 35, NO. 20, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 0 8 7

(35) NIST Structures and Properties, Version 2.0, Standard Reference
Database 25; Stein, S. E., Ed.; Chemical Kinetics and Thermo-
dynamics Division, National Institute of Standards and Tech-
nology; Gaithersburg, MD, January 1994.
Received for review April 6, 2001. Revised manuscript re-
ceived August 2, 2001. Accepted August 6, 2001.
ES010831K
9
4 0 8 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 20, 2001