Kinetics and products of the reactions of oh radicals with 4,4-dimethyl-1-pentene and 3,3-dimethylbutanal at 296 ± 2 k

Article abstract of DOI:10.1021/jp101893g

Using a relative rate method, rate constants have been measured for the reactions of OH radicals with 4,4-dimethyl-1-pentene [(CH₃) ₃CCH₂CH=CH₂] and its major reaction product, 3,3-dimethylbutanal [(CH₃)₃CCH₂CHO], at 296 ± 2 K and atmospheric pressure of air. The rate constants obtained were 2.41 × 10^-11 and 2.73 × 10^-11 cm³ molecule^-1 s^-1, respectively, with estimated uncertainties of ±10%. The products identified and quantified by gas chromatography with mass spectrometry and/or flame ionization detection from the 4,4-dimethyl-1-pentene reaction were acrolein [CH₂=CHCHO], 3,3-dimethylbutanal, and a molecular weight 112 carbonyl attributed to 4,4-dimethyl-2-pentenal [(CH₃)₃CCH=CHCHO], with formation yields of 2.7 ± 0.5%, 59 ± 6%, and 3.4 ± 0.6%, respectively. Using direct air sampling atmospheric pressure ionization mass spectrometry, additional products of molecular weight 146, 177, and 193 were observed, and on the basis of expected reaction schemes these are attributed to the dihydroxycarbonyl HOCH₂C(CH₃)₂CH ₂C(O)CH₂OH, the hydroxynitrates (CH₃) ₃CCH₂CH(OH)CH₂ONO₂ and/or (CH ₃)₃CCH₂CH(ONO₂)CH₂OH, and the dihydroxynitrate O₂NOCH₂C(CH₃) ₂CH₂CH(OH)CH₂OH, respectively. The hydroxynitrates were also tentatively identified by gas chromatography, with a summed yield of ～15%. Acrolein and 4,4-dimethyl-2-pentenal arise from H-atom abstraction from the three equivalent CH₃ groups and the 3-position CH₂ group, and the sum of their formation yields (6.1 ± 0.8%) is expected to be very close to the fraction of the overall reaction proceeding by H-atom abstraction.

Full text of DOI:10.1021/jp101893g

5810
J. Phys. Chem. A 2010, 114, 5810–5816
Kinetics and Products of the Reactions of OH Radicals with 4,4-Dimethyl-1-pentene and
3,3-Dimethylbutanal at 296 ( 2 K
Sara M. Aschmann, Janet Arey,† and Roger Atkinson*,†,‡
Air Pollution Research Center and Departments of EnVironmental Sciences and Chemistry, UniVersity of
California RiVerside, California 92521
ReceiVed: March 2, 2010; ReVised Manuscript ReceiVed: April 2, 2010
Using a relative rate method, rate constants have been measured for the reactions of OH radicals with 4,4-
dimethyl-1-pentene [(CH3)3CCH2CHdCH2] and its major reaction product, 3,3-dimethylbutanal
[(CH3)3CCH2CHO], at 296 ( 2 K and atmospheric pressure of air. The rate constants obtained were 2.41 ×
10-11 and 2.73 × 10-11 cm3 molecule-1 s-1, respectively, with estimated uncertainties of (10%. The products
identified and quantified by gas chromatography with mass spectrometry and/or flame ionization detection
from the 4,4-dimethyl-1-pentene reaction were acrolein [CH2dCHCHO], 3,3-dimethylbutanal, and a molecular
weight 112 carbonyl attributed to 4,4-dimethyl-2-pentenal [(CH3)3CCHdCHCHO], with formation yields of
2.7 ( 0.5%, 59 ( 6%, and 3.4 ( 0.6%, respectively. Using direct air sampling atmospheric pressure ionization
mass spectrometry, additional products of molecular weight 146, 177, and 193 were observed, and on the basis of
expected reaction schemes these are attributed to the dihydroxycarbonyl HOCH2C(CH3)2CH2C(O)CH2OH, the
hydroxynitrates (CH3)3CCH2CH(OH)CH2ONO2 and/or (CH3)3CCH2CH(ONO2)CH2OH, and the dihydroxynitrate
O2NOCH2C(CH3)2CH2CH(OH)CH2OH, respectively. The hydroxynitrates were also tentatively identified by gas
chromatography, with a summed yield of ∼15%. Acrolein and 4,4-dimethyl-2-pentenal arise from H-atom abstraction
from the three equivalent CH3 groups and the 3-position CH2 group, and the sum of their formation yields (6.1 (
0.8%) is expected to be very close to the fraction of the overall reaction proceeding by H-atom abstraction.
Introduction
SCHEME 1: Expected Reactions of the Alkyl Radical
Formed after H-atom Abstraction from the 3-Position
CH2 Group in 4,4-Dimethyl-1-pentene. Observed
Products are Shown in Boxes. Although Acetone was also
Observed, it was Formed at Least in Part as a
Second-Generation Product from OH +
Alkenes are the major class of volatile organic compounds
(VOCs) emitted from vegetation,1 and are also emitted from
anthropogenic sources, including in vehicle exhaust.2 In urban
areas, alkenes (primarily from anthropogenic sources) comprise
∼10% of nonmethane VOCs.3 In the atmosphere, reaction with
OH radicals is an important, and sometimes dominant, loss
process.3–5 Reactions of alkenes with OH radicals proceed by
OH radical addition to the carbon atoms of the CdC double
bond(s) and by H-atom abstraction from the C-H bonds of the
alkyl substituent groups (but not from vinylic C-H bonds).3–5
Although rate constants for the overall reaction of OH radicals
with a large number of alkenes have been reported,5 few data
are available concerning the rate constants for H-atom abstrac-
tion (or, equivalently, for the fractions of the overall OH radical
reactions proceeding by H-atom abstraction)6–11 and those are
mainly for cyclic dienes6,9–11 and trienes.10
3,3-Dimethylbutanal (see Scheme 5)
In this work, we have investigated the kinetics and products
of the reaction of OH radicals with 4,4-dimethyl-1-pentene
[(CH3)3CCH2CHdCH2], chosen because the products antici-
pated after H-atom abstraction are amenable to quantitative
analysis and are not formed from the OH radical addition
pathway (see Schemes 1-4). The products attributed to the
H-atom abstraction pathway allow the fraction of the overall
reaction proceeding by H-atom abstraction to be determined,
and this fraction is compared to that predicted by an oft-used
structure-reactivity estimation method.12 Since 3,3-dimeth-
ylbutanal [(CH3)3CCH2CHO] was the major product observed
from the OH radical-initiated reaction of 4,4-dimethyl-1-pentene,
its rate constant for reaction with OH radicals was also measured
and its products briefly investigated.
Experimental Methods
* Author to whom correspondence should be addressed. E-mail:
ratkins@mail.ucr.edu. Telephone: (951) 827-4191.
† Department of Environmental Sciences.
Experiments were carried out in two ∼7000 L Teflon
chambers, each equipped with two parallel banks of blacklamps
for irradiation, at 296 ( 2 K and 735 Torr total pressure of dry
‡ Department of Chemistry.
10.1021/jp101893g  2010 American Chemical Society
Published on Web 04/16/2010

Reactions of Alkenes with OH Radicals
J. Phys. Chem. A, Vol. 114, No. 18, 2010 5811
SCHEME 2: Expected Reactions of the Alkyl Radical
Formed after H-Atom Abstraction from the Three
Equivalent CH3 Groups in 4,4-Dimethyl-1-pentene.
Observed Product is Shown in Box. While Acetone was
also Observed, it was Formed at Least in Part as a
Second-Generation Product from OH +
SCHEME 4: Expected Reactions of the Hydroxyalkyl
Peroxy Radical Formed after OH Radical Addition at the
2-Position in 4,4-Dimethyl-1-pentene. Observed Products
are Shown in Boxes
3,3-Dimethylbutanal (see Scheme 5)
SCHEME 5: Expected Reactions after H-Atom
Abstraction from the CHO Group in 3,3-Dimethylbutanal
(The Expected Major Initial Reaction Pathway12)
SCHEME 3: Expected Reactions of the Hydroxyalkyl
Peroxy Radical Formed after OH Radical Addition at the
1-Position in 4,4-Dimethyl-1-pentene. Observed Products
are Shown in Boxes
and NO was included in the reactant mixtures to suppress the
formation of O3 and hence of NO3 radicals.
Kinetic Studies. Rate constants for the reactions of OH
radicals with 4,4-dimethyl-1-pentene and 3,3-dimethylbutanal
were measured by a relative rate method in which the
concentrations of the organic and a reference compound (whose
OH radical reaction rate constant is reliably known) were
measured in the presence of OH radicals:15
OH + organic f products
(1)
(2)
OH + reference compound f products
purified air. Both chambers are equipped with Teflon-coated
fans to ensure rapid mixing of reactants during their introduction
into the chamber, and one of the chambers is interfaced to a
PE SCIEX API III MS/MS direct air sampling atmospheric
pressure ionization mass spectrometer (API-MS).13,14 Hydroxyl
radicals were generated by the photolysis of methyl nitrite
(CH3ONO) in the presence of O2 at wavelengths >300 nm,13,14
Providing that the organic and the reference compound reacted
only with OH radicals then,15
[organic]t
[reference compound]t
0
k1
k2
0
ln
)
ln
(I)
(
)
(
)
[organic]t
[reference compound]t

5812 J. Phys. Chem. A, Vol. 114, No. 18, 2010
Aschmann et al.
where [organic]t and [reference compound]t are the concentra-
then temperature programmed, using an Agilent 6890N GC
interfaced to an Agilent 5975 Inert XL Mass Selective Detector
operated with methane as the reagent gas in positive ion mode
(PCI-GC/MS) or in negative ion mode (NCI-GC/MS).
0
0
tions of the organic and reference compound, respectively, at
time t0, [organic]t and [reference compound]t are the corre-
sponding concentrations at time t, and k1 and k2 are the rate
constants for reactions 1 and 2, respectively. The initial reactant
concentrations (in molecules cm-3) were: CH3ONO and NO,
∼2.4 × 1014 each; and that of organic and reference compounds
∼2.4 × 1013 each. Methacrolein and, for experiments with 4,4-
dimethyl-1-pentene, 1-octene were chosen as the reference
compounds. Irradiations were carried out at 20% of the
maximum light intensity for up to 15 min, resulting in up to 39
and 56% of the initially present 4,4-dimethyl-1-pentene and 3,3-
dimethylbutanal, respectively, being consumed by reaction.
A sample was also collected from an irradiated CH3ONO/
NO/4,4-dimethyl-1-pentene/air mixture through a 16.7 L min-1,
2.5 µm Teflon-coated aluminum cyclone (URG-2000-30EH,
UCR, Chapel Hill, NC) onto a 5-channel, 400 mm length
denuder (URG-2000-30B5, URG, Chapel Hill, NC) coated with
finely ground XAD-4 resin.17 Prior to sampling, the XAD-coated
denuder was further coated with ∼100 mg of PFBHA in 5 mL
of methanol and then dried with a nitrogen gas flow.18 After
sampling, the denuder was stored overnight and then extracted
three times with 50 mL aliquots of CH2Cl2 (95% of the products
were in the first extract). The extracts were rotoevaporated to a
volume of e10 mL and then analyzed by PCI-GC/MS and NCI-
GC/MS as described above. Note that each carbonyl group
derivatized to an oxime added 195 mass units to the compound’s
molecular weight (MW), and methane-PCI gave characteristic
protonated molecules ([M + H]+) and smaller adduct ions at
[M + 29]+ and [M + 41]+.18,19
The concentrations of the organics and reference compounds
were measured during the experiments by gas chromatography
with flame ionization detection (GC-FID). For the analyses of
3,3-dimethylbutanal, methacrolein, 1-octene, and products (see
below), gas samples of 100 cm3 volume were collected from
the chamber onto Tenax-TA adsorbent, with subsequent thermal
desorption at ∼250 °C onto a 30 m DB-1701 megabore column,
initially held at -40 °C and then temperature programmed to
250 °C at 8 °C min-1. Although 4,4-dimethyl-1-pentene could
also be analyzed using this procedure, 4,4-dimethyl-1-pentene
was incompletely collected onto the Tenax solid adsorbent and
hence gas samples were collected from the chamber into a 100
cm3 volume all-glass gastight syringe 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. Replicate analyses of 4,4-dimethyl-1-
pentene, 3,3-dimethylbutanal, methacrolein, and 1-octene showed
that the measurement uncertainties were typically <3%.
Product Studies Using Gas Chromatography. Irradiations
of CH3ONO/NO/4,4-dimethyl-1-pentene/air and CH3ONO/NO/
3,3-dimethylbutanal/air mixtures were carried out with initial
concentrations (molecules cm-3) of CH3ONO and NO, ∼2.4
× 1013, ∼4.8 × 1013 or ∼2.4 × 1014 each (with the initial
CH3ONO and NO concentrations being equal in all experi-
ments); and 4,4-dimethyl-1-pentene or 3,3-dimethylbutanal,
∼2.4 × 1013. Analyses were carried out by GC-FID and
combined gas chromatography-mass spectrometry (GC-MS).
The GC-MS analyses were carried out using a DB-5 column
(see below for details); the GC-FID analyses of 4,4-dimethyl-
1-pentene and 3,3-dimethylbutanal and their products, collected
onto Tenax solid adsorbent, were carried out using 30 m DB-
1701 and DB-5 megabore columns with the temperature
program noted above. The use of DB-5 columns for both the
GC-MS and GC-FID analyses, together with trans-2-hexenal
and trans-2-heptenal as additional retention time markers added
to the chamber after the reaction, facilitated the identification
of products from 4,4-dimethyl-1-pentene by providing an
excellent linear correlation of retention times from the GC-MS
and GC-FID analyses.
GC-FID response factors for 4,4-dimethyl-1-pentene, 3,3-
dimethylbutanal, and acrolein were measured by introducing
measured amounts of the chemicals into the chamber and then
conducting several replicate analyses. The FID response factors
for (CH3)3CCHdCHCHO and the peaks that were assigned to
hydroxynitrates were calculated using the effective carbon
number (ECN)20 of the products (5.9 for (CH3)3CCHdCHCHO),
the measured GC-FID response factor for 3,3-dimethylbutanal,
and the ECN of 3,3-dimethylbutanal (5.0).20 For the hydrox-
ynitrates, the ECNs were calculated using ECN(-CH(ONO2)-)
) ECN(-CH2ONO2) ) 0.2 derived from measured GC-FID
response factors for 3-methyl-2-butyl nitrate, 3-methyl-2-pentyl
nitrate, 2-hexanone, and 3-methyl-2-pentanone.
Analyses by API-MS. Three CH3ONO/NO/4,4-dimethyl-1-
pentene/air irradiations were carried out, during which the
chamber contents were sampled through a 25 mm diameter by
75 cm length Pyrex tube at ∼20 L min-1 directly into the API-
MS source. The operation of the API-MS in the MS (scanning)
and MS/MS [with collision activated dissociation (CAD)] modes
has been described previously.13,14 Both positive and negative
ion modes were used in this work. In positive ion mode,
protonated water hydrates (H3O+(H2O)n) generated by the corona
discharge in the chamber diluent air were responsible for the
formation of protonated molecules ([M + H]+), water adduct
ions [M + H + H2O]+, and protonated homo- and het-
erodimers,13 whereas in negative ion mode O2 , NO2- and NO3
-
-
ions were responsible for formation of adduct ions.14
The initial concentrations (molecules cm-3) were: CH3ONO,
NO, and 4,4-dimethyl-1-pentene, ∼2.4 × 1013 each. The reactant
mixtures were irradiated for 3-8.3 min at 20% of the maximum
light intensity.
Chemicals. The chemicals used, and their stated purities,
were: acrolein (90%), 3,3-dimethylbutanal (95%), 4,4-dimethyl-
1-pentene (99%), 2,2-dimethylpropanal [trimethylacetaldehyde]
(97%), trans-2-hexenal (98%), trans-2-heptenal (97%), and
methacrolein (95%), Aldrich; 1-octene (99.9%), ChemSampCo;
1-octene (99.0%), TCI America; acetone (HPLC grade), Fisher
Scientific; and NO (99.0%), Matheson Gas Company. Methyl
nitrite was prepared as described by Taylor et al.21 and stored
at 77 K under vacuum.
For the GC-MS analyses, samples from the chamber were
collected onto Tenax solid adsorbent (100 or 500 cm3 volume),
onto a 65 µm polydimethylsiloxane/divinylbenzene (PDS/DVB)
solid phase MicroExtraction (SPME) fiber exposed to the
chamber contents for 10-60 min, or onto a PDS/DVB SPME
fiber, precoated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxyl
amine (PFBHA) for on-fiber derivatization of carbonyl com-
pounds,16 and exposed to the chamber contents for 5 min. The
Tenax solid adsorbent or SPME fibers were then thermally
desorbed (injection port temperature at 250 °C) onto a 60 m
DB-5MS capillary column (250 µm i.d., 0.25 µm phase) initially
held at -50 °C (Tenax samples) or 40 °C (SPME samples) and

Reactions of Alkenes with OH Radicals
J. Phys. Chem. A, Vol. 114, No. 18, 2010 5813
good agreement with that of 0.698 derived from the recom-
mended 296 K rate constant for methacrolein5,22 and the rate
constant at 295 ( 1 K for 1-octene recently measured in our
labratory.23
Products of OH + 4,4-Dimethyl-1-pentene. Analyses by
Gas Chromatography. GC-FID analyses of irradiated CH3ONO/
NO/4,4-dimethyl-1-pentene/air and CH3ONO/NO/3,3-dimeth-
ylbutanal/air mixtures showed that many of the GC peaks
observed in the CH3ONO/NO/4,4-dimethyl-1-pentene/air ir-
radiations were due to secondary formation from OH + 3,3-
dimethylbutanal. In addition to GC-FID analyses of authentic
standards of acrolein, acetone, 4,4-dimethyl-1-pentene, 3,3-
dimethylbutanal, and 2,2-dimethylpropanal introduced into the
chamber, GC-FID analysis of an irradiated CH3ONO/NO/2-
methylpropane/air mixture was also carried out. The OH radical-
initiated reaction of 2-methylpropane leads to formation of
(CH3)3C• radicals and hence to acetone, tert-butyl nitrite, and
tert-butyl nitrate,4,5 and this analysis aided in the peak assign-
ments. As expected (see Scheme 5), the OH + 3,3-dimethylbu-
tanal reaction led to the formation of acetone, tert-butyl nitrite,
and tert-butyl nitrate (and methyl nitrate from photooxidation
of CH3ONO), but with no observable amount of 2,2-dimeth-
ylpropanal.24
Figure 1. Plots of eq I for the reactions of OH radicals with 4,4-
dimethyl-1-pentene and 3,3-dimethylbutanal, with methacrolein as the
reference compound. GC-FID analyses of 3,3-dimethylbutanal and
methacrolein by thermal desorption of samples collected onto Tenax
solid adsorbent onto a 30 m DB-1701 column; GC-FID analyses of
4,4-dimethyl-1-pentene used the syringe/gas sampling loop procedure
and a 30 m DB-5 column.
Products identified as arising uniquely from the OH + 4,4-
dimethyl-1-pentene reaction were 3,3-dimethylbutanal, acrolein,
and a MW 112 product attributed to (CH3)3CCHdCHCHO. 3,3-
Dimethylbutanal was identified by retention time and mass
spectral matching of samples collected onto Tenax solid
adsorbent, PFBHA-coated SPME fibers, and the PFBHA-coated
XAD-denuder with those of an authentic standard similarly
sampled. Acrolein was identified by retention time and mass
spectral matching of a sample collected onto the PFBHA-coated
XAD-denuder and from retention time matching in the GC-
FID analyses, and the MW 112 carbonyl attributed to
(CH3)3CCHdCHCHO was observed from GC-MS analyses of
samples collected onto Tenax solid adsorbent and the PFBHA-
coated XAD-denuder (see footnotes to Table 2 for details).
Unfortunately, attempts to have (CH3)3CCHdCHCHO and its
possible precursor (from OH radical- or O3-initiated reactions)
(CH3)3CCHdCHCHdCH2 synthesized by commercial labora-
tories were unsuccessful.
Figure 2. Plots of eq I for the reactions of OH radicals with 4,4-
dimethyl-1-pentene and methacrolein, with 1-octene as the reference
compound. GC-FID analyses of methacrolein and 1-octene by thermal
desorption of samples collected onto Tenax solid adsorbent onto a 30 m
DB-1701 column; GC-FID analyses of 4,4-dimethyl-1-pentene used
the syringe/gas sampling loop procedure and a 30 m DB-5 column.
Acrolein, 3,3-dimethylbutanal and (CH3)3CCHdCHCHO
react with OH radicals, hence their measured concentrations
were corrected for reaction with OH radicals as described
previously,25 using rate constants (in units of 10-11 cm3
molecule-1 s-1) of: 4,4-dimethyl-1-pentene, 2.28 (relative to
methacrolein); acrolein, 1.99;26 3,3-dimethylbutanal, 2.73; and
(CH3)3CCHdCHCHO, 3.9 (estimated12 and similar to the rate
constant for OH + crotonaldehyde26). The multiplicative factors
to account for secondary reactions increase with k(OH +
product)/k(OH + 4,4-dimethyl-1-pentene) and with the extent
of reaction25 and were e1.28 for acrolein, e1.55 for 3,3-
dimethylbutanal, and e1.59 for (CH3)3CCHdCHCHO, with
(10% uncertainties in the rate constant ratios k(OH + product)/
k(OH + 4,4-dimethyl-1-pentene) leading to <5% uncertainties
in the maximum values of the multiplicative correction factors.
Figure 3 shows a plot of the amounts of 3,3-dimethylbutanal
formed, corrected for reaction with OH radicals, against the
amounts of 4,4-dimethyl-1-pentene reacted, and Figure 4
shows analogous plots for formation of acrolein and
(CH3)3CCHdCHCHO. Data for 3,3-dimethylbutanal in Figure
3 are from two independent sets of experiments with analyses
of 3,3-dimethylbutanal using a DB-1701 GC column and a DB-5
GC column, with separate calibrations of 4,4-dimethyl-1-pentene
Results
Rate Constants for OH + 4,4-Dimethyl-1-pentene and 3,3-
Dimethylbutanal. The experimental data from CH3ONO/NO/
air irradiations of 4,4-dimethyl-1-pentene + methacrolein +1-
octene and 3,3-dimethylbutanal + methacrolein mixtures are
plotted in accordance with eq I in Figures 1 and 2. The rate
constant ratios k1/k2 obtained from least-squares analyses of these
data are listed in Table 1 and are placed on an absolute basis
using rate constants of k2(methacrolein) ) 2.89 × 10-11 cm3
molecule-1 at 296 K5,22 and k2(1-octene) ) 4.14 × 10-11 cm3
molecule-1 at 295 ( 1 K.23 Since both methacrolein and
1-octene were used as reference compounds for 4,4-dimethyl-
1-pentene and were present in the same reactant mixtures, a
rate constant ratio k(OH + methacrolein)/k(OH + 1-octene) was
also obtained. Our measured rate constant ratio k(OH +
methacrolein)/k(OH + 1-octene) ) 0.777 ( 0.011 (where the
indicated error is two least-squares standard deviations) is in

5814 J. Phys. Chem. A, Vol. 114, No. 18, 2010
Aschmann et al.
TABLE 1: Rate Constant Ratios k1/k2 and Rate Constants k1 at 296 ( 2 K and Atmospheric Pressure of Air
organic
reference compound
k1/k2
1011 × k1 (cm3 molecule-1 s-1
)
a
b
4,4-dimethyl-1-pentene
4,4-dimethyl-1-pentene
3,3-dimethylbutanal
methacrolein
methacrolein
1-octene
methacrolein
1-octene
0.788 ( 0.041
0.614 ( 0.031
0.943 ( 0.068
0.777 ( 0.011
2.28 ( 0.12
2.54 ( 0.13
2.73 ( 0.20
3.22 ( 0.05
a Indicated errors are two least-squares standard deviations. b Placed on an absolute basis using k2(OH + methacrolein) ) 2.89 × 10-11 cm3
molecule-1 at 296 K5,22 and k2(OH + 1-octene) ) 4.14 × 10-11 cm3 molecule-1 at 295 ( 1 K.23 Indicated errors are two least-squares standard
deviations and do not take into account the uncertainties in the rate constants k2 (see text).
TABLE 2: Observed Products of the Reaction of OH Radicals with 4,4-Dimethyl-1-pentene in the Presence of NO
analysis by
product
Tenax GC-FIDa
GC-MS
API-MS
3,3-dimethylbutanal (MW 100)
(CH3)3CCH2CHO
acrolein (MW 56)
CH2dCHCHO
4,4-dimethyl-2-pentenal (MW 112)
(CH3)3CCHdCHCHO
59 ( 6b
confirmed with standard
confirmed with standard
aldehyde of MW 112e
observedc
2.7 ( 0.5d
3.4 ( 0.6d
∼15g
observedf
observedi
MW 177 hydroxynitrates
PCI and NCI consistent with hydroxynitrate of MW 177h
(CH3)3CCH2CH(OH)CH2ONO2 and/or
(CH3)3CCH2CH(ONO2)CH2OH
MW 146 dihydroxycarbonyl
HOCH2C(CH3)2CH2C(O)CH2OH
MW 193 dihydroxynitrate
O2NOCH2C(CH3)2CH2CH(OH)CH2OH
observedj
observedk
a Indicated errors are two least-squares standard deviations (from plots such as those shown in Figures 3 and 4) combined with estimated
overall uncertainties in the GC-FID response factors of (5% for 4,4-dimethyl-1-pentene, 3,3-dimethylbutanal, and acrolein and (15% for the
response factor of (CH3)3CCHdCHCHO relative to that measured for 3,3-dimethylbutanal. b Formation yield of 58 ( 6% from experiments
with GC-FID analysis of 3,3-dimethylbutanal using a DB-1701 column, and 60 ( 6% from experiments with GC-FID analysis of
3,3-dimethylbutanal using a DB-5 column. c Observed in positive ion mode from ion peaks at 101 u [100 + H]+, 119 u [100 + H2O + H]+,
159 u [100 + 58 + H]+, 201 u [100 + 100 + H]+, 213 u [100 + 112 + H]+ and 278 u [100 + 177 + H]+, where the MW 58 species is
attributed to acetone formed from 4,4-dimethyl-1-pentene (minor) and 3,3-dimethylbutanal. d Data from experiments with GC-FID analyses of
acrolein and (CH3)3CCHdCHCHO using
a DB-5 column. Formation yield of 2.9 ( 0.5% derived from GC peak attributed to
(CH3)3CCHdCHCHO in experiments with GC-FID analyses using a DB-1701 column (see text). e [M + H]+ of 113 u observed in Tenax
GC-MS PCI analysis. [M + H]+ of oxime derivative observed at 308 u in GC-MS analysis of denuder extract and small 239 u fragment
consistent with a derivatized aldehyde of MW 112. f Observed in positive ion mode from ion peaks at 171 u [112 + 58 + H]+, 213 u [100 +
112 + H]+ and 225 u [112 + 112 + H]+, where the MW 58 species is attributed to acetone formed from 4,4-dimethyl-1-pentene (minor) and
3,3-dimethylbutanal. g Two peaks tentatively identified as 1,2-hydroxynitrates were observed in Tenax samples and in denuder samples
analyzed with cool on-column injection (but not seen in SPME samples or denuder extracts analyzed with splitless injection). Quantification
based on estimated ECNs (see text) results in formation yields of the two 1,2-hydroxynitrates of 4.4% and 10.9%, which should strictly be
considered as lower limits because of potential losses during the GC analyses. h No molecular ions observed in PCI analysis; both peaks gave a
115 u fragment ion attributed to [M + H - HNO3]+ and a small 160 u fragment ion attributed to [M + H - H2O]+. In NCI mode, [NO2]- at
46 u was a strong fragment ion and ion peaks at 129 u and 131 u were attributed27 to [M - NO2 - 2H]- and [M - NO2]-, respectively, of
MW 177 1,2-hydroxynitrates. i Observed in positive ion mode from ion peak at 278 u [100 + 177 + H]+, and in negative ion mode from ion
peaks at 209 u [177 + O2]-, 223 u [177 + NO2]-, 239 u [177 + NO3]- (see also footnote k below) and 400 u [177 + 177 + NO2]-.
j Observed in positive ion mode from ion peak at 129 u [146 + H - H2O]+, and in negative ion mode from ion peak at 192 u [146 + NO2]-.
k Observed in negative ion mode from ion peak at 225 u [193 + O2]-. Note that the 239 u ion peak could be [177 + NO3]- and/or [193 +
NO2]-.
and 3,3-dimethylbutanal for each set of experiments. Least
squares analyses of the two sets of data result in formation yields
of 3,3-dimethylbutanal of 58 ( 6% (DB-1701 column) and 60
( 6% (DB-5 column), respectively, where the indicated errors
are the two least-squares standard deviations combined with
estimated uncertainties in the GC-FID response factors for 4,4-
dimethyl-1-pentene and 3,3-dimethylbutanal of (5% each. The
agreement is excellent, with an average 3,3-dimethylbutanal
formation yield of 59 ( 6%. The data shown in Figure 4 for
acrolein and (CH3)3CCHdCHCHO are from experiments with
analyses using the DB-5 column. On this GC column, acrolein
was significantly better resolved from acetone (but still not
completely baseline resolved) than on the DB-1701 column.
As noted above, the MW 112 product attributed to
(CH3)3CCHdCHCHO identified by GC-MS analyses on a DB-5
column could be confidently assigned on the DB-5 column used
for the GC-FID analyses from the excellent linear correlation
of retention times from the GC-MS and GC-FID analyses. A
GC peak on the GC-FID analyses using the DB-1701 column
was assigned, with less confidence, to (CH3)3CCHdCHCHO,
resulting in a formation yield of 2.9 ( 0.5%, essentially identical
to that of 3.4 ( 0.6% for the data presented in Figure 4 from
the use of the DB-5 column.
Two other products were observed from the Tenax analyses
of the OH + 4,4-dimethyl-1-pentene reaction and they exhibited
intense 46 u fragment ions in their NCI-GC/MS analyses, and
hence are likely to be nitrates.27 In the denuder extracts, these
peaks were only seen when cool on-column injection was
utilized, suggesting thermal decomposition and/or irreversible
adsorption in the hot injector when using the spitless mode.
This behavior is consistent with that reported for 1,2-
hydroxynitrates by Muthuramu et al.28 On the basis of the
PCI and NCI mass spectra (see footnotes g and h in Table 2
for details), we attribute these to the MW 177 hydroxynitrates
(CH3)3CCH2CH(OH)CH2ONO2 and (CH3)3CCH2CH(ONO2)-
CH2OH (see Schemes 3 and 4). Because the estimated rate

Reactions of Alkenes with OH Radicals
J. Phys. Chem. A, Vol. 114, No. 18, 2010 5815
same as the hydroxynitrate yields measured from C14-C17
alkenes (14.0 ( 0.9%) by Matsunaga and Ziemann.31
Analyses by API-MS. API-MS analyses of irradiated CH3ONO/
NO/4,4-dimethyl-1-pentene/air mixtures were carried out in both
positive and negative ion modes. In positive ion mode, the API-
MS spectrum was dominated by ion peaks at 119, 129, and
201 u, with less intense peaks at 101, 159, 171, 213, 225, and
278 u. API-MS/MS product ion spectra showed these ion peaks
to be due to products of MW 58, 100, 112, 146, and 177 (see
Table 2). In negative ion mode, the API-MS was dominated by
peaks at 223, 239, and 400 u, with less intense ion peaks at
192, 209, and 225 u, with API-MS/MS product ion spectra
showing these to be due to products of MW 146, 177, and 193 u
(Table 2).
Discussion
Our rate constant for 3,3-dimethylbutanal (2.73 × 10-11
molecule-1 s-1) is a factor of 1.3 higher than that of 2.14 ×
10-11 molecule-1 s-1 measured by D’Anna et al.32 at 298 ( 2
K relative to 1-butene, whereas our rate constant for the reaction
of OH radicals with 4,4-dimethyl-1-pentene (2.41 × 10-11
molecule-1 s-1; average of the two values relative to methac-
rolein and 1-octene) is the first reported for this alkene. Our
measured rate constant for 4,4-dimethyl-1-pentene is consistent
with literature data for reactions of OH radicals with 1-alkenes5,23
and with the value of 2.8 × 10-11 molecule-1 s-1 calculated
using the structure-reactivity estimation method of Kwok and
Atkinson.12 The observation that the rate constant for OH +
4,4-dimethyl-1-pentene is slightly lower than predicted and
lower than the corresponding rate constants for propene and
the C4-C7 1-alkenes5,23 suggests that this may be due to steric
effects, with the (CH3)3C group screening the CdC bond to
some extent (see, for example, ref 33 for steric effects in the
reactions of O3 with alkenes).
Figure 3. Plot of the amounts of 3,3-dimethylbutanal formed, corrected
for reaction with OH radicals (see text), against the amounts of 4,4-
dimethyl-1-pentene reacted. GC-FID analyses of 3,3-dimethylbutanal
by thermal desorption of samples collected onto Tenax solid adsorbent
using: O, a 30 m DB-1701 column; b, a 30 m DB-5 column. In all
cases, GC-FID analyses of 4,4-dimethyl-1-pentene used the syringe/
gas sampling loop procedure and a 30 m DB-5 column. The solid line
is a least-squares fit to the combined data set.
3,3-Dimethylbutanal was the major product observed and
quantified from OH + 4,4-dimethyl-1-pentene, with a formation
yield of 59 ( 6%. As noted above, many of the product peaks
observed in the GC-FID and GC-MS analyses of the OH +
4,4-dimethyl-1-pentene reaction were second-generation prod-
ucts from OH + 3,3-dimethylbutanal (for example, acetone, tert-
butyl nitrite and tert-butyl nitrate; see Scheme 5). The first-
generation products observed from OH + 4,4-dimethyl-1-
pentene were, in addition to 3,3-dimethylbutanal, acrolein (2.7
( 0.5%), a MW 112 carbonyl attributed to 4,4-dimethyl-2-
pentenal [(CH3)3CCHdCHCHO] (3.4 ( 0.6%); and products
of MW 146, 177, and 193. On the basis of the expected reaction
schemes shown in Schemes 3 and 4 for OH radical addition to
the terminal and internal carbon atoms of the CdC bond,
respectively, the products of MW 146, 177, and 193 can be
attributed to the dihydroxycarbonyl HOCH2C(CH3)2CH2C(O)-
CH2OH, the hydroxynitrates (CH3)3CCH2CH(OH)CH2ONO2
and/or (CH3)3CCH2CH(ONO2)CH2OH, and the dihydroxynitrate
O2NOCH2C(CH3)2CH2CH(OH)CH2OH, respectively. The MW
146 dihydroxycarbonyl and MW 193 dihydroxynitrate are
expected to be formed only after OH radical addition at the
terminal (1-position) carbon atom and subsequent isomer-
ization of the resulting intermediate hydroxyalkoxy radical
(CH3)3CCH2CH(O•)CH2OH. For both (CH3)3CCH2CH(O•)-
CH2OH and (CH3)3CCH2CH(OH)CH2O•, reaction with O2 to
form 1,2-hydroxycarbonyls is expected to be minor or
negligible compared to decomposition and/or isomerization,34
and the O2 reactions have therefore been omitted from
Schemes 3 and 4. The products we observe are therefore in
accord with expectations34 of the products formed after initial
OH radical addition to 4,4-dimethyl-1-pentene.
Figure 4. Plots of the amounts of acrolein, 4,4-dimethyl-2-pentenal,
and the two 1,2-hydroxynitrates (attributed to (CH3)3CCH2CH(ONO2)-
CH2OH and (CH3)3CCH2CH(OH)CH2ONO2) formed against the amounts
of 4,4-dimethyl-1-pentene reacted. The measured concentrations of
acrolein and 4,4-dimethyl-2-pentenal have been corrected for reaction
with OH radicals (see text). GC-FID analyses of acrolein, 4,4-dimethyl-
2-pentenal, and the 1,2-hydroxynitrates by thermal desorption of
samples collected onto Tenax solid adsorbent using a 30 m DB-5
column; GC-FID analyses of 4,4-dimethyl-1-pentene used the syringe/
gas sampling loop procedure and a 30 m DB-5 column.
constants for reactions of OH radicals with (CH3)3CCH2CH-
(OH)CH2ONO2 and (CH3)3CCH2CH(ONO2)CH2OH are 5.9
× 10-12 and 2.6 × 10-12 cm3 molecule-1 s-1, respectively,29
corrections for secondary reactions were minor (<8%) and
were neglected. Assuming that these two GC-FID peaks were
the C7-hydroxynitrates, then their formation yields were 4.4
and 10.9%, and since OH radical addition to the terminal
carbon is expected to dominate,5 these yields would be for
(CH3)3CCH2CH(OH)CH2ONO2 and (CH3)3CCH2CH(ONO2)-
CH2OH, respectively. The sum of the hydroxynitrate forma-
tion yields of ∼15% is a factor of ∼2 higher than expected
based on the yield data of O’Brien et al.30 for hydroxynitrate
formation from a series of C2-C6 alkenes and is about the

5816 J. Phys. Chem. A, Vol. 114, No. 18, 2010
Aschmann et al.
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The OH radical can also react by H-atom abstraction from
the 3-position CH2 group and from the three CH3 groups, and
reaction schemes based on our knowledge of the subsequent
reactions are shown in Schemes 1 and 2, respectively. In Scheme
•
2, it is expected that decomposition of the OCH2C(CH3)2-
CH2CHdCH2 radical will dominate over its reaction with O2,
by a factor of ∼6 at 298 K and atmospheric pressure of air.34
Hence, the products expected after H-atom abstraction from the
three CH3 groups are mainly formaldehyde, acrolein, and
acetone (Scheme 2). H-atom abstraction from the 3-position CH2
group is predicted (Scheme 1) to lead to acrolein + acetone +
methyl radical (with the methyl radical reacting to form HCHO,
CH3ONO, and CH3ONO2 under our conditions22) and/or 4,4-
dimethyl-2-pentenal. Acetone is also a second-generation prod-
uct of OH + 3,3-dimethylbutanal (Scheme 5), and hence its
formation yield from OH + 4,4-dimethyl-1-pentene could not
be quantified. However, our acrolein and 4,4-dimethyl-2-
pentenal formation yields of 2.7 ( 0.5% and 3.4 ( 0.6%,
respectively, indicate that H-atom abstraction accounts
for 6.1
( 0.8%, neglecting any minor formation of
CH2dCHCH2C(CH3)2CHO (see Scheme 2). H-atom abstraction
from the 3-position CH2 group accounts for g3.4 ( 0.6% of
the overall reaction, based on the yield of 4,4-dimethyl-2-
pentenal. The Kwok and Atkinson estimation method12 predicts
that at 298 K H-atom abstraction from the three equivalent CH3
groups and from the 3-position CH2 group account for 1.8 and
4.1% of the overall reaction, respectively, with partial rate
constants of 5.0 × 10-13 and 1.15 × 10-12 cm3 molecule-1 s-1
,
respectively.12 The predicted12 total partial rate constant for
H-atom abstraction of 1.65 × 10-12 cm3 molecule-1 s-1 is in
excellent agreement with our value of (1.47 ( 0.25) × 10-12
cm3 molecule-1 s-1 derived from the measured overall rate
constant [(2.41 ( 0.25) × 10-11 cm3 molecule-1 s-1] and the
fraction of the reaction proceeding by H-atom abstraction. This
agreement suggests that this structure-reactivity estimation
method12 can be used to calculate the importance of H-atom
abstraction in the reactions of OH radicals with other acyclic
alkenes for which data are not presently available.
Acknowledgment. The authors gratefully thank the National
Science Foundation (Grant No. ATM-0234586) for supporting
this research. Although this research has been supported by this
agency, it has not been reviewed by the agency and no official
endorsement should be inferred. Dr. Douglas Lane (Environment
Canada) is thanked for supplying the ground XAD used to coat
the denuder.
(33) McGillen, M. R.; Carey, T. J.; Archibald, A. T.; Wenger, J. C.;
Shallcross, D. E.; Percival, C. J. Phys. Chem. Chem. Phys. 2008, 10, 1757.
(34) Atkinson, R. Atmos. EnViron. 2007, 41, 8468.
References and Notes
(1) Guenther, A.; Hewitt, C. N.; Erickson, D.; Fall, R.; Geron, C.;
Graedel, T.; Harley, P.; Klinger, L.; Lerdau, M.; McKay, W. A.; Pierce,
JP101893G