β-ionone reactions with the nitrate radical: Rate constant and gas-phase products

Article abstract of DOI:10.1002/kin.20438

The bimolecular rate constant of script k sign_{NO3+ β-ionone} (9.4 ± 2.4 × 10^-12 cm³ molecule^-1 s^-1 was measured using the relative rate technique for the reaction of the nitrate radical (NO₃

Full text of DOI:10.1002/kin.20438

β-Ionone Reactions with
the Nitrate Radical: Rate
Constant and Gas-Phase
Products
JOEL C. HARRISON, JASON E. HAM
Exposure Assessment Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health,
1095 Willowdale Road, Morgantown, WV 26505
Received 6 March 2009; revised 15 April 2009, 5 May 2009; accepted 5 May 2009
DOI 10.1002/kin.20438
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The bimolecular rate constant of kNO ·+β-ionone (9.4
2.4 × 10−12 cm3
3
molecule−1 s−1 was measured using the relative rate technique for the reaction of the
nitrate radical (NO3•) with 4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-buten-2-one (β-ionone)
at (297
3) K and 1 atmosphere total pressure. In addition, the products of β-
ionone + NO3• reaction were also investigated. The identified reaction products were
glyoxal (HC( O)C( O)H), and methylglyoxal (CH3C( O)C( O)H). Derivatizing agents
O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine and N,O-bis(trimethylsilyl)trifluoroacetamide
were used to propose the other major reaction products: 3-oxobutane-1,2-diyl nitrate,
2,6,6-trimethylcyclohex-1-ene-carbaldehyde, 2-oxo-1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethyl
nitrate, pentane-2,4-dione, 3-oxo-1-(2,6,6-trimethylcyclohex-1-en-1-yl)butane-1,2-diyl dini-
trate, 3,3-dimethylcyclohexane-1,2-dione, and 4-oxopent-2-enal. The elucidation of these
products was facilitated by mass spectrometry of the derivatized reaction products cou-
pled with plausible β-ionone + NO3• reaction mechanisms based on previously published
volatile organic compound + NO3• gas-phase mechanisms. The additional gas-phase prod-
ucts 5-acetyl-2-ethylidene-3-methylcyclopentyl nitrate, 1-(1-hydroxy-7,7-dimethyl-2,3,4,5,6,7-
hexahydro-1H-inden-2-yl)ethanone, 1-(1-hydroxy-3a,7-dimethyl-2,3,3a,4,5,6,-hexahydro-1H-
inden-2-yl)ethanone, and 5-acetyl-2-ethylidene-3-methylcyclopentanone are proposed to be
the result of cyclization through a reaction intermediate. ꢀ 2009 Wiley Periodicals, Inc.∗ Int J
C
Chem Kinet 41: 629–641, 2009
related, resulting in ∼24.5 million missed workdays
INTRODUCTION
annually. Unfortunately, the direct causes of many of
these work-related asthma (WRA) cases are still un-
known; although research efforts have indicated that
volatile organic compounds (VOCs) and their oxida-
tion products may be partially responsible [2]. Sev-
eral studies have been conducted on VOCs and their
specific health effects in the indoor environment; how-
ever, the presence of individual VOCs does not com-
pletely explain elevated irritation levels [3,4]. These
Recently, the American Academy of Allergy, Asthma
& Immunology [1] issued a report that states that ap-
proximately 15% (1 in 7) cases of asthma are work
Correspondence to: Jason E.∗Ham; e-mail: bvo2@cdc.gov.
c
ꢀ
2009 Wiley Periodicals, Inc. This article is a U.S. Government
work and, as such, is in the public domain of the United States of
America.

630
HARRISON AND HAM
levels may be attributed to the formation of yet un-
known products generated from oxidation reactions
between unsaturated VOCs and indoor oxidants such
as OH• (hydroxyl radical), O3 (ozone), and NO3• (ni-
trate radical).
of the products of this reaction are also reported. Nei-
ther the NO3• rate constant nor the respective reaction
mechanisms for β-ionone + NO3• have been reported
previously.
The chemistry of the NO3• with a number of VOCs
and semi-VOCs, which can be found both outdoors
and indoors, has been well studied [5–9]. In the in-
door environment, these types of reactions can occur
quickly compared with a typical air exchange of 0.6
h−1 [10]. For example, NO3• (given the estimated in-
door [NO3•] = 1 part per trillion [ppt]) [11,12] re-
actions with terpinolene and α-terpinene (found in
pine oil cleaners) can occur 14 and 27 times faster,
respectively, than they can be removed by a typi-
cal building air exchange [10]. Consequently, these
types of reactions generate a number of indoor oxy-
genated products such as aldehydes, ketones, dicar-
bonyls (e.g., glyoxal, methylglyoxal, 4-oxopentanal),
and organic nitrates, (e.g., alkyl nitrates, peroxyacyl
nitrates [PANs], hydroxynitrates, dinitrates) [5,13,14].
Furthermore, NO3• reactions have been shown to ir-
reversibly damage amino acids by transformation into
β-nitrate esters, β-carbonyl compounds, and/or aro-
matic nitro compounds [15]. All of these oxygenated
organic compounds have the potential to induce a res-
piratory response, including WRA. Organic nitrates
may also have the potential to be carcinogenic. For ex-
ample, polyaromatic hydrocarbons (PAHs) react with
nitrate radicals to form nitro-PAHS. Several nitro-
PAHS (e.g., 6-nitrobenzo[a]pyrene, 3-nitroperylene,
1-nitropyrene, and 6-nitrochysene) have been identi-
fied as mutagens and carcinogens [16].
EXPERIMENTAL
Apparatus and Materials
Experiments to measure the gas-phase rate constant
of the NO3• + 4-(2,6,6-trimethyl-1-cyclohexen-1-
yl)-3-buten-2-one (β-ionone, Structure 1) reaction
were conducted with a previously described appa-
ratus [18,19]. A brief description is provided here.
Reactants were introduced and samples were with-
®
drawn through a 6.4-mm Teflon Swagelok fitting at-
tached to a 65-L Teflon film chamber. Compressed
®
air from the National Institute for Occupational Safety
and Health (NIOSH) facility was passed through an-
hydrous CaSO4 (Drierite, Xenia, OH) and molecular
sieves (Drierite, Xenia, OH) to remove both mois-
ture and organic contaminants. This dry compressed
air was added as a diluent to the reaction chambers
and measured with a 0–100 L min−1 mass flow con-
troller (MKS Instruments, Inc., Andover, MA). Anal-
ysis of this treated compressed air by gas chromatog-
raphy/mass spectrometry revealed that if contaminants
were present, they would be below the ppt range. The
treated compressed air was also analyzed for nitric
oxide (NO) with a Thermo Electron Model 42i NO-
NO2-NOx analyzer (Thermo Fisher Scientific, Inc.,
Waltham, MA). The filler system was equipped with
a syringe injection port facilitating the introduction of
both liquid and gaseous reactants into the chambers
with the flowing airstream. All reactant mixtures and
calibration standards were generated by this system.
All reaction kinetic samples were quantitatively
monitored using an Agilent 6890 gas chromatograph
(Agilent Technologies, Palo Alto, CA) with a 5973
mass selective detector (GC/MS) and Agilent Chem-
Station software. Gas samples were cryogenically col-
lected employing an Entech 7100 sampling system
(Entech Instruments, Inc., Simi Valley, CA) utilizing
the following trap and temperature parameters: 50 mL
of chamber contents were collected onto trap 1 (packed
silanized glass beads) at −150◦C. After sample collec-
tion, trap 1 was heated to 230◦C and the sample trans-
ferred under a flow of ultrahigh purity helium (UHP
He) onto trap 2 (packed silanized glass beads) cooled to
−30◦C. trap 2 was then heated to 180◦C and the sam-
ple was transferred under a UHP He flow onto trap 3, a
silanized 0.53-mm i.d. tube cooled to −160◦C, which
was subsequently heated to 220◦C to inject the sample
Room deodorization introduces a number of VOCs
and semi-VOCs into the indoor environment. One
such compound used in deodorizing consumer prod-
ucts is β-ionone (4-(2,6,6-trimethyl-1-cyclohexen-1-
yl)-3-buten-2-one) (Structure 1), a component of the
violet scent. Worldwide usage has been estimated at
100 to 1000 metric tons annually [17]. In the work pre-
sented here, the β-ionone + NO3• rate constant has
been measured using the relative rate method. Some
Structure 1
International Journal of Chemical Kinetics DOI 10.1002/kin

β-IONONE REACTIONS WITH THE NITRATE RADICAL
631
onto an Rtx-VRX (Restek, Bellefonte, PA) GC column
(0.25-mm i.d., 30-m length, and 1.4-μm film thick-
ness). These series of cryogenic trap manipulations re-
duced the background water level, ensured consistency
of replicate samples, and improved the chromatograph
peak shapes. The reduction in background water levels
is accomplished by gentle heating (40◦C) with helium
flow on each of the traps along with careful timing
of cooling the next trap for collection. The GC tem-
perature program used was as follows: initial tempera-
ture of 45◦C held for 8 min after sample injection and
then increased 10◦C/min to 220◦C and held for 4 min.
The Agilent 5973 mass selective detector was tuned
using perfluorotributylamine (FC-43). Full-scan elec-
tron impact (EI) ionization spectra were collected from
m/z = 35 to m/z = 650. Preliminary compound identi-
fications from the Agilent 6890/5973 GC/MS data sets
were made by searching the NIST 98 Mass Spectral
Library.
The identification of reaction products was made
using O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine
(PFBHA) to derivatize carbonyl products,
whereas alcohols were derivatized using N,O-
bis(trimethylsilyl)trifluoroacetamide (BSTFA) [20].
Possible carboxylic acids, ketal-alcohols, and
aldehydic-alcohols were derivatized using a combina-
tion of PFBHA and BSTFA. Experimental methods
for reaction product identification were similar to
methods used for kinetic experiments, except that the
reference compound was excluded from the reaction
mixture. An additional port was added to the Teflon
chamber to facilitate the injection of N2O5 (described
in the following text).
Derivatized reaction products were analyzed using a
Varian (Varian, Inc., Palo Alto, CA) 3800/Saturn 2000
GC/MS system operated in both the EI and CI modes
[20]. Compound separation was achieved by a J&W
Scientific (Folsom, CA) DB-5MS (0.32-mm i.d., 30-m
length, 1-μm film thickness) column and the following
GC oven parameters: 60◦C for 1 min, then 20◦C/min
to 170◦C, then 3◦C/min to 280◦C, and held for 5 min.
Samples were injected in the splitless mode, and
the GC injector was returned to split mode 1 min after
sample injection, with the following injector temper-
ature parameters: 60◦C for 1 min, then 180◦C/min to
250◦C, and held to the end of the chromatographic
run [20]. The Saturn 2000 ion trap mass spectrometer
was tuned using FC-43. Full-scan EI ionization spectra
were collected from m/z = 40 to m/z = 650. Acetoni-
trile was the chemical ionization (CI) reagent used for
all CI spectra. When possible, commercially available
samples of the identified products were derivatized and
subsequently analyzed to verify matching ion spectra
and chromatographic retention times.
Nitrate radicals were generated by the thermal de-
composition of N2O5 using a similar method as de-
scribed by Atkinson et al. [21,22] N2O5 (solid) kept at
−85◦C was heated and allowed to transfer to an evac-
uated 2-L collection bottle until pressure was between
0.1 and 0.6 Torr. The collection bottle was then pressur-
ized with UHP nitrogen up to 1000 Torr and connected
®
to the reaction chamber via a Teflon shutoff valve.
The valve to the collection bottle and the chamber
shutoff valve were opened and the system was allowed
to equilibrate for 10 s. For kinetics and product ex-
periments, approximately 10 min elapsed before any
samples were collected after the introduction of N2O5.
All compounds were used as received and
had the following purities: from Sigma-Aldrich
(Milwaukee, WI): limonene (99%), 2-carene (97%),
4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-buten-2-one
(β-ionone) (95%), acetonitrile (99.93%), BSTFA
(99%), PFBHA (98+%), and methanol (99%).
Nitrogen dioxide as a 5% mixture in nitrogen
and UHP oxygen were obtained from Butler Gases
(Morrisville, PA). Helium (UHP grade), the carrier gas,
was supplied by Amerigas (Sabraton, WV) and used
as received. Experiments were carried out at 297
3 K and at a pressure of 1 atm.
Methods
The experimental procedures for determining the β-
ionone + NO3• reaction kinetics were similar to those
described previously [18,19].
·
kNO
+β-ionone
3
β-Ionone + NO3 • ———−→ Products
(1)
(2)
kRef
Reference + NO3 • —−→ Products
The rate equations for reactions (1) and (2) are
combined and integrated, resulting in the following
equation:
ꢀ
ꢁ
ꢀ
ꢁ
·
[β-ionone]0
[β-ionone]t
kNO
[Ref]0
[Ref]t
3
ln
=
+β-ionone ln
kRef
(3)
If the reaction with NO3• is the only removal
mechanism for β-ionone and the reference com-
pound, a plot of ln([β-ionone]0/[β-ionone]t ) versus
ln([Ref]0/[Ref]t ) yields a straight line with an intercept
of zero. Multiplying the slope of this linear plot by kRef
yields kNO ·+β-ionone (Fig. 1). The NO3• rate constant
3
experiments for β-ionone employed the use of two
reference compounds: 2-carene and limonene. The use
of two different reference compounds with different
NO3• rate constants strengthens the accuracy of the β-
ionone/NO3• rate constant measurement and demon-
strates that other reactions are not removing β-ionone.
International Journal of Chemical Kinetics DOI 10.1002/kin

632
HARRISON AND HAM
Figure 1 4-(2,6,6-Trimethyl-1-cyclohexen-1-yl)-3-buten-2-one (β-ionone) relative rate plot with 2-carene (ꢀ) and limonene
(ꢁ) as reference compounds. The NO3• + β-ionone rate constant, kNO ·+β-ionone, was measured to be (9.4 0.2) × 10−12 cm3
3
molecule−1 s−1
.
For the β-ionone/NO3• kinetic experiments, the
typical concentrations of the pertinent species in the
65-L Teflon chamber were 1.1–5.6 ppm (2.7–
13.8 × 1013 molecule cm−3) β-ionone, 0.3–0.8 ppm
(0.7–2.0 × 1013 molecule cm−3) reference, 0.1–0.6
Torr of N2O5 (3.9–23 ppm, which corresponds to an
NO3• concentration of 0.3–1.5 ppm at 298 K) [23], and
6 ppb (1.4 × 1011 molecule cm−3) NO as background
in NIOSH air. The gas-phase mixtures were allowed
to reach equilibrium before initial species concentra-
tion ([X]0) samples were collected. The total ion chro-
matogram (TIC) from the Agilent 5973 mass selective
detector was used to determine β-ionone and reference
concentrations.
RESULTS
β-Ionone/NO3• Reaction Rate Constant
The NO3• rate constant for (β-ionone, Structure 1)
was obtained using the relative rate method described
above. The plot of a modified version of Eq. (3) is
shown in Fig. 1. The ln([Ref]0/[Ref]t ) term is divided
by the respective reference rate constant (2-carene (19
−12
5) × 10
cm3 molecule−1 s−1 and limonene (12.2
3.1) × 10−12 cm3 molecule−1
s
−1) [24] and mul-
s
−1, resulting in a
tiplied by 10−12 cm3 molecule−1
unitless number. This yields a slope that is equal to
the NO3•/β-ionone rate constant, kNO ·+β-ionone, di-
3
vided by 10−12 cm3 molecule−1 −1. This modifica-
s
Derivatization of the carbonyl reaction products was
initiated by flowing 15–25 L of chamber contents at
3.8 L min−1 through an impinger containing 4 mL of
acetonitrile and 250 μL of PFBHA (0.02 M) in ace-
tonitrile to derivatize the carbonyl reaction products
to oximes [20], with no effort to prevent acetonitrile
evaporation during sample collection. The sample was
removed from the impinger and allowed to react for
a 24- to 48-h time period in the dark. The reacted so-
lutions were gently blown to dryness with UHP N2,
reconstituted with 100 μL of methanol, and then 1 μL
of the reconstituted solution was injected onto the Var-
ian 3800/Saturn 2000 GC/MS system. The derivati-
zation of hydroxy groups (alcohols) was achieved by
subsequent reconstitution of the dried PFBHA oximes
by the addition of 150 μL of commercially available
BSTFA. These PFBHA/BSTFA solutions were heated
to approximately 60◦C for 45 min to complete the sily-
lation, and then 1 μL of the solution was injected into
the Varian 3800/Saturn 2000 GC/MS system [18].
tion allows for a direct comparison of the two ref-
erence compound/β-ionone data sets. The slope of
the line shown in Fig. 1 yields an NO3• bimolecu-
lar rate constant, kNO ·+β-ionone, of (9.4 0.2) × 10−12
3
cm3 molecule−1 s−1. The use of 2-carene and limonene
as references resulted in NO3 · +β-ionone bimolecu-
lar rate constants of (9.3 0.3) × 10−12 and (9.4
0.2) × 10−12 cm3 × 10−12 molecule−1
s
−1, respec-
tively. The data points at the origin are experimen-
tal points before NO3• addition, t = 0; data showed
no detectable loss of β-ionone or reference. The er-
ror in the rate constant is the 95% confidence interval
from the random uncertainty in the slope. Incorporat-
ing the uncertainties associated with the reference rate
constants ( 25% for 2-carene and limonene) used to
derive the β-ionone/NO3• rate constant yields a final
value of (9.4
2.4) × 10−12 cm3 molecule−1 s−1
for kNO ·+β-ionone [24]. The β-ionone/NO3• rate
3
constant, kNO ·+β-ionone, has not been previously
3
International Journal of Chemical Kinetics DOI 10.1002/kin

β-IONONE REACTIONS WITH THE NITRATE RADICAL
633
reported. The ratios kNO ·+β-ionone/kNO ·+2-carene and
significantly by a factor of 5 and 16, respectively, with
β-ionone + NO3•, indicating that methylglyoxal and
glyoxal are likely products of the reaction.
3
3
kNO ·+β-ionone/kNO ·+limonene incorporating the uncer-
3
3
tainties are 0.5 0.1 and 0.8 0.2, respectively.
β-Ionone/NO3• Reaction Products
Oxime at Retention Time 11.4 min
The products observed from the β-ionone/NO3• re-
action (hydrogen abstraction or NO3• addition to
carbon-carbon double bonds) are listed in Table I. The
β-ionone/NO3• reaction products observed and posi-
tively identified using the pure compound for verifica-
tion by derivatization were glyoxal (HC( O)C( O)H)
and methylglyoxal (CH3C( O)C( O)H). Structures
and ions (expected CI vs. observed CI) used to identify
these compounds are listed in Table I. Elucidation of
the other major proposed reaction products (also listed
in Table I) were facilitated by mass spectrometry of
the derivatized reaction product coupled with plausi-
ble β-ionone/NO3• reaction mechanisms on the basis
of previously published VOC/NO3• gas-phase reaction
as described in the following text [5,12,24–27].
The oxime observed with a chromatographic retention
time of 11.4 min had ions of m/z (relative intensity):
59 (7%), 161 (6%), 181 (100%), 195 (5%), and 280
(5%–10%). Using acetonitrile for CI, an M + 1 ion of
m/z = 298 was observed for the PFBHA-derivatized
sample. A proposed reaction product assignment of
3-oxobutane-1,2-diyl dinitrate was based on observed
data.
Oxime at Retention Time 20.8 min
The oxime observed with a chromatographic reten-
tion time of 20.8 min had ions of m/z (relative inten-
sity): 91 (30%), 105 (22%), 117 (25%), 132 (40%),
148 (100%), 181 (85%), and 330 (30%). In the CI
spectra, an M + 1 ion of m/z = 346 was observed
for the PFBHA-derivatized sample. A proposed re-
action product assignment of 5-acetyl-2-ethylidene-3-
methylcyclopentyl nitrate was based on observed data.
The derivatization of nonsymmetric carbonyls us-
ing PFBHA or PFBHA/BSTFA typically resulted in
multiple chromatographic peaks due to stereoisomers
of the oximes. The identification of multiple peaks of
the same oxime compound is relatively simple since
the mass spectra for each chromatographic peak of
a particular oxime are almost identical [20]. In most
cases, the m/z = 181 ion relative intensity for the chro-
matographic peaks due to β-ionone + NO3• reaction
product oximes was the base peak in the mass spectrum
and was used to generate selected ion chromatograms.
The mass spectra of compounds that were addition-
ally derivatized with BSTFA contained m/z = 73 ions
from the [Si(CH3)3]+ fragments. A chromatogram dis-
playing the latter portion of the GC run (29–44 min) in
which organic nitrate products were observed is shown
in Fig. 2. The product data are described in the follow-
ing text.
The following chronological chromatographic re-
tention time results and mass spectra data were
observed utilizing PFBHA or PFBHA/BSTFA deriva-
tization and the Varian 3800/Saturn 2000 GC/MS
system. The reaction products’ chromatographic peak
areas were a function of the initial β-ionone concen-
tration and were observed only after NO3• initiation
of β-ionone/methanol/air mixtures. Derivatization ex-
periments performed in the absence of β-ionone, but
in the presence of all other chemicals in the reaction
chamber (NO3•/air/methanol), did not result in any
of the data reported in the subsequent text except for
small amounts (as noted by chromatographic peak ar-
eas) of methylglyoxal and glyoxal. However, the gly-
oxal and methylglyoxal oxime peak areas increased
Oxime at Retention Time 21.1 min
The oxime with a chromatographic retention time
of 21.1 min had ions of m/z (relative intensity): 96
(30%), 150 (58%), 166 (100%), 181 (90%) 347 (17%),
and 429 (44%). In the CI spectra, an M + 1 ion of
m/z = 348 was observed for the PFBHA-derivatized
sample. A proposed reaction product assignment
of 2,6,6-trimethylcyclohex-1-ene-1-carbaldehyde was
based on observed data.
Glyoxal (HC( O)C( O)H)
The chromatographic peaks for the oxime observed
at 24.1 and 24.4 min have been described previously
[18,28].
Methylglyoxal (CH3C( O)C( O)H)
The chromatographic peaks for the oxime observed at
23.4, 24.5, and 25.5 min have been described previ-
ously [18,28].
Oxime at Retention Times 25.7, 26.6,
and 27.2 min
The oxime observed at retention times of 25.7, 26.6,
and 27.2 min had ions of m/z (relative intensity): 122
(35%), 150 (100%) 181 (55%), and 331 (35%). In the
International Journal of Chemical Kinetics DOI 10.1002/kin

634
HARRISON AND HAM
Table I Proposed β-Ionone/NO3• Reaction Products
Retention
Molecular
CI Ions
CI Ions
Time (min)
Name
3-Oxobutane-1,2-diyl dinitrate
Weight (amu)
Structure
Expected Observed
11.4
194
390
409
298
346
20.8
21.1
(2E)-5-Acetyl-2-ethylidene-3-methylcyclopentyl nitrate
213
2,6,6-Trimethylcyclohex-1-ene-1-carbaldehydea
152
348
348
24.1
24.4
23.4
24.5
25.5
25.7
26.6
27.2
Glyoxala
58
72
449
463
449
463
Methylglyoxala
2-Oxo-1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethyl nitrate
Pentane-2,4-dione
227
423
360
29.3
29.6
100
208
491
404
491
404
1-(1-Hydroxy-7,7-dimethyl-2,3,4,5,6,7-hexahydro-1H-
inden-2-yl)ethanonea
31.2
29.9
1-(1-Hydroxy-3a,7-dimethyl-2,3,3a,4,5,6-hexahydro-1H-
208
255
404
451
404
386
inden-2-yl)ethanonea
32.8
31.5
3-Oxo-1-(2,6,6-trimethylcyclohex-1-en-1-yl)butyl nitrate
31.9
33.3
3-Oxo-1-(2,6,6-trimethylcyclohex-1-en-1-yl)butane-1,2-
diyl dinitrate
316
512
418
35.5
3,3-Dimethylcyclohexane-1,2-dione
140
98
531
489
531
39.6
41.1
41.5
43.1
42.4
(2E)-4-Oxopent-2-enala
No CI ions
(2E)-5-Acetyl-2-ethylidene-3-methylcyclopentanone
166
557
557
a Products previously observed with β-ionone/OH and β-ionone/O3 (Forester et al. [28]).
International Journal of Chemical Kinetics DOI 10.1002/kin

β-IONONE REACTIONS WITH THE NITRATE RADICAL
635
Figure 2 Overlaid chromatograms of β-ionone only, NO3• only, and β-ionone + NO3• in the reaction chamber. Data were
offset for clarity. * indicates β-ionone/NO2 product.
CI spectra, an M + 1 ion of m/z = 360 was observed.
A proposed reaction product assignment of 2-oxo-
1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethyl nitrate was
based on observed data.
PFBHA/BSTFA derivation of the oximes at 29.6
and 31.2 min showed a chromatographic peak shift in
retention time to 30.9 min. Using acetonitrile for CI,
an M + 1 ion of m/z = 476 was observed. These data
were used to further verify the proposed assignment of
the 29.6- and 31.2-min peaks.
Oxime at Retention Times of 29.3 min
The oxime observed at a retention time of 29.3 min had
ions of m/z (relative intensity): 181 (100%) and 460
(5%). In the CI spectra, an M + 1 ion of m/z = 491 was
observed. A proposed reaction product assignment of
pentane-2,4-dione was based on observed data.
Oxime at Retention Times 29.9
and 32.8 min
The oxime observed at retention times of 29.9
and 32.8 min had ions of m/z (relative inten-
sity): 43 (15–20%), 109 (30%), 122 (20%), 138
(30%), 150 (25%), 181 (75%), 206 (100%), and
372 (45%). In the CI spectra, an M + 1 ion
of m/z = 404 was observed. A proposed reaction
product assignment of 1-(1-hydroxy-3a,7-dimethyl-
2,3,3a,4,5,6-hexahydro-1H-inden-2-yl)ethanone was
based on observed data.
Oxime at Retention Times 29.6
and 31.2 min
The oxime observed at retention times of 29.6 and
31.2 min had ions of m/z (relative intensity): 41
(18%), 108 (26%), 121 (26%), 124 (24%), 181
(75%), 194 (40%), 221 (60%), and 304 (100%). In
the CI spectra, an M + 1 ion of m/z = 404 was
observed. A proposed reaction product assignment
of 1-(1-hydroxy-7,7-dimethyl-2,3,4,5,6,7-hexahydro-
1H-inden-2-yl)ethanone was based on observed data.
PFBHA/BSTFA derivation of the oximes at 29.9
and 32.8 min showed a chromatographic peak shift in
retention time to 29.1 min. Using acetonitrile for CI,
International Journal of Chemical Kinetics DOI 10.1002/kin

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HARRISON AND HAM
an M + 1 ion of m/z = 476 was observed. These data
were used to further verify the proposed assignment of
the 29.9- and 32.8-min peaks.
ethylidene-3-methylcyclopentanone was based on ob-
served data (Fig. 3).
DISCUSSION
Oxime at Retention Time 31.5 min
The oxime observed at a retention time of 31.5 min
had ions of m/z (relative intensity): 91 (20%), 172
(30%), 181 (45%), 204 (7.5%), 370 (100%), and 385
(35%). Using acetonitrile for CI, an M + 1 ion of
m/z = 386 was observed. A proposed reaction product
assignment of 3-oxo-1-(2,6,6-trimethylcyclohex-1-en-
1-yl)butyl nitrate was based on observed data.
The nitrate radical can react with β-ionone by
H-atom abstraction or NO3• addition to carbon-
carbon double bonds [5,9,12,26]. Structure 1 shows
the most likely sites for these nitrate radical reac-
tions. The measured kNO ·+β-ionone of (9.4
0.2) ×
3
10−12 cm3 molecule−1 s−1 and can be compared with
the previously measured kOH·+β-ionone of 118 × 10−12
cm3 molecule−1 s−1 [28] and its corresponding cal-
culated value of 238 × 10−12 cm3 molecule−1 s−1
,
Oxime at Retention Times 31.9
and 33.3 min
using AOPWIN estimation software [30]. Primary re-
actions are expected to occur at both double bonds of
β-ionone; however, the measured rate constant (kOH•)
suggests that only one of the double bonds interacts
with OH•. This observation of the reaction occurring
at a single double bond may be comparable to the NO3•
reactions reported here by the formation of specific re-
action products (e.g., 20.8 and 25.7 min), which likely
occurs at the double bond (labeled A, Structure 1).
For the β-ionone + NO3• reaction, the experimen-
tal parameters were set to minimize side reactions and
highlight the NO3• hydrogen abstraction and/or NO3•
addition. Furthermore, experiments were done with
β-ionone + NO2 (by-product of N2O5 thermal degra-
dation) to determine whether any of the observed oxi-
dation products were from these reactions. No reaction
products were observed that appeared to be due to
NO2 chemistry. It is possible that NO2 generated by
the decomposition of the N2O5 may react with alkyl
radicals, formed after reaction with NO3•, leading to
the formation of organic nitrates. However, no exper-
iments were done to unravel these mechanisms. Also,
no reaction products were observed with β-ionone +
NO3• in UHP N2 environment, which suggests that
O2 must be present to form the oxidized products de-
scribed in the following text. The possible mechanistic
steps leading to product formation also are described
in the following text.
The oxime observed at retention times of 31.9 and
33.3 min had ions of m/z (relative intensity): 91
(15%), 164 (20%), 181 (40%), 220 (100%), and 370
(7.5%). In the CI spectra, an M + 1 ion of m/z =
418 was observed. A proposed reaction product as-
signment of 3-oxo-1-(2,6,6-trimethylcyclohex-1-en-1-
yl)butane-1,2-diyl dinitrate was based on observed
data.
Oxime at Retention Times 35.5 min
The oxime observed at a retention time of 35.5 min had
ions of m/z (relative intensity): 181 (100%), 304 (50%),
336 (90%), and 533 (6%). Using acetonitrile for CI, an
M + 1 ion of m/z = 531 was observed. A proposed re-
action product assignment of 3,3-dimetylcyclohexane-
1,2-dione was based on observed data.
Oxime at Retention Times 39.6, 41.1,
41.5, and 43.1 min
The oxime observed at retention times of 39.6, 41.1,
41.5, and 43.1 min had ions of m/z (relative intensity):
181 (100%) and 488 (40%). No CI ions were observed.
This was unexpected, since Arey et al. [29] observed
this product using methane as the CI reagent, which
gave the molecular ion of m/z = 489. A proposed
reaction product assignment of 4-oxopent-2-enal was
based on observed data.
Equivalent or Structurally Analogous
Products Observed from β-ionone/OH• or
β-ionone/O3 Reactions
Oxime at Retention Time 42.4 min
Several oxidation products were generated from
β-ionone/NO3• reactions that were similar to those
previously observed from β-ionone/OH• or β-
ionone/O3 gas-phase reactions [28]. The mechanisms
for the formation of these specific products were
detailed by Forester et al. [28] and only a brief
The oxime observed at a retention time of 42.4 min
had ions of m/z (relative intensity): 181 (50%), 359
(100%), 360 (20%), and 375 (15%). Using acetonitrile
for CI, an M + 1 ion of m/z = 557 was observed. A pro-
posed reaction product assignment of (2E)-5-acetyl-2-
International Journal of Chemical Kinetics DOI 10.1002/kin

β-IONONE REACTIONS WITH THE NITRATE RADICAL
637
Figure 3 PFBHA-derivatized product of β-ionone + NO3• (42.4 min): (a) electron ionization spectrum and (b) acetonitrile
chemical ionization spectrum.
description will be provided here. The mechanism
for the formation of the product observed at 20.8
min, (2E)-5-acetyl-2-ethylidene-3-methylcyclopentyl
nitrate, is analogous to the similar product, 1-[(3E)-3-
ethylidene-2-hydroxy-4-methylcyclopentyl]ethanone
observed from β-ionone/OH•, except NO3• adds to
the carbon-carbon double bond (labeled A, Structure 1)
with subsequent annelation of the alkyl radical leading
to the five-membered ring. The proposed identity
of this product was further supported by the ob-
servation of an ion at m/z = 346 using CI, which
is due to the loss of one nitrate molecule (m/z =
62) and a hydrogen atom to give HNO3. This loss
has been commonly observed in mass spectra of
alkyl and arylalkyl nitrates [31,32]. The product
observed at 21.1 min (2,6,6-trimethylcyclohex-1-
ene-1-carbaldehyde) was previously observed from
reactions of β-ionone/OH• and β-ionone/O3. The
mechanism is similar to OH• and O3 reactions by the
interaction of NO3• with the external carbon-carbon
double bond leading to the formation of methylglyoxal
and
2,6,6-trimethylcyclohex-1-ene-1-carbaldehyde
(see mechanism illustrated in Fig. 4). The two oximes
observed at 29.6 min (1-(1-hydroxy-7,7-dimethyl-
2,3,4,5,6,7-hexahydro-1H-inden-2-yl)ethanone) and
29.9 min (1-(1-hydroxy-3a,7-dimethyl-2,3,3a,4,5,6-
hexahydro-1H-inden-2-yl)ethanone) were also pre-
viously observed in β-ionone/OH• and β-ionone/O3
gas-phase reactions. These two are products are
formed by the initial addition of NO3• to the external
carbon-carbon double bond. NO3• may be lost,
leaving a radical that can subsequently react with O2
International Journal of Chemical Kinetics DOI 10.1002/kin

638
HARRISON AND HAM
Figure 4 Proposed reaction mechanism for NO3• addition to the external carbon-carbon double bond of β-ionone.
leading to the formation of a peroxy radical. Another
peroxy radical (RO2•) can react, resulting in the
formation of an alcoholic OH as previously reported
[33]. The adjacent radical can then undergo annelation
leading to the ringed oxidation products. These types
of cyclization are analogous to radical-mediated
ring-closure mechanisms discussed in published liter-
ature [34–36]. Although the formation of the oximes
(at 20.8, 29.6, and 29.9 min) generated by annelation
in the gas phase seems energetically unfavorable, this
may be overcome by an energy transfer via collisions
of radicals in the gas phase or through stabilization in
the impinger collection liquid.
The oxime product observed at 29.3 min (pentane-
2,4-dione) is possibly formed by the addition of NO3•
to the internal carbon-carbon double bond (labeled B,
Structure 1). The exact steps of this reaction product are
unclear and proposal of a potential mechanism would
International Journal of Chemical Kinetics DOI 10.1002/kin

β-IONONE REACTIONS WITH THE NITRATE RADICAL
639
be difficult to suggest on the basis of conventional
gas-phase VOC/NO3• mechanisms. However, the pro-
posed product assignment is reasonable given that this
product is structurally similar to 4-oxopentanal, which
has approximately the same retention time and same
observed derivatized mass (m/z = 491). It was con-
firmed that the 29.3-min product was not wrongly iden-
tified as 4-oxopentanal by using a “pure” synthesized
4-oxopentanal standard, which was observed at 29.7,
30.1, and 30.4 min [37]. Only one peak was observed
for pentane-2,4-dione; however, it is possible that the
other derivatized isomers may be incorporated under
this peak. Interestingly, the oxime observed at 29.3
min (pentane-2,4-dione) was not observed in previous
β-ionone OH•/O3 gas-phase experiments.
anisms. However, the identities of these products may
be inferred on the basis of fragmentation patterns and
partial CI data. The proposed identity of the oxime at
11.4 min (3-oxobutane-1,2-diyl dinitrate) was made by
the observation of an oxime ion at m/z = 298 using CI,
which can be due to the loss of two NO2 molecules
(m/z = 92) [31,32]. 3,3-Dimethylcyclohexane-1,2-
dione (35.5 min) is another proposed product and
was suggested by the oxime molecular ion observed
at m/z = 531. The proposed identity of the oxime
at 31.5 min (3-oxo-1-(2,6,6-trimethylcyclohex-1-en-1-
yl)butyl nitrate) was made by the observation of an ion
at m/z = 386 using CI. The proposed identity of this
product was based on the mass spectral fragmenta-
tion pattern and its expected reaction mechanism of
NO3• addition to the external carbon-carbon double
bond. The β-ionone/dinitrate product, 3-oxo-1-(2,6,6-
trimethylcyclohex-1-en-1-yl)butane-1,2-diyl dinitrate,
(31.9 min) was proposed by the observation of an ion
at m/z = 418 using CI, which is due to the loss of two
NO2 molecules (m/z = 92) [31,32].
The oxime observed at 39.6 min ((2E)-4-oxopent-
2-enal)) was also previously observed from β-
ionone/OH• and β-ionone/O3. This product is pos-
sibly formed by the addition of NO3• to the internal
carbon-carbon double bond (labeled B, Structure 1).
This leads to the dissociation and formation of the
carbonyl radical, CH3C( O)CH CHCH•. The car-
bonyl radical can then react with O2 to form the per-
oxy radical, CH3C( O)CH CHC(OO•)H. This can
further react with another RO2 molecule to form
CH3C( O)CH CHC( O)H. It is interesting that no
CI ions (using acetonitrile as CI reagent) were ob-
served for this product. Arey et al. [29] used PFBHA
derivatization and were able to observe the molecular
ion of m/z = 489 for this product using methane as the
CI reagent.
It is possible that additional reaction products, such
as hydroperoxides or nitrates without carbonyl groups,
were formed but were not observed using the PFBHA
and PFBHA/BSTFA derivatization techniques. In ad-
dition, oxidation products generated from these reac-
tions may have been lost to the surface of the reaction
chamber or inefficiency during impinger collection. In
future experiments, the Teflon chamber may be rinsed
to determine whether any oxidation products remain
adsorbed to the chamber surface.
Newly Observed β-ionone + NO3•
Indoor Nitrate Radical Chemistry
Reaction Products
Indoor environment concentrations of the nitrate rad-
ical (2 × 107 molecules cm−3) have been previously
estimated by Sarwar et al. [38]. Using the β-ionone +
NO3• rate constant reported here, a pseudo-first-order
rate constant (kꢁ) of 0.8 h−1 was determined. A compar-
ison of this value to a typical indoor air exchange rate
of 0.6 h−1 [39] suggests that nitrate radical chemistry
is the most likely removal mechanism for β-ionone.
When compared with the pseudo-first-order rate con-
stants of the hydroxyl radical (kOH•+β-Ionone = 118 ×
The oxime at 25.7 min (2-oxo-1-(2,6,6-
trimethylcyclohex-1-en-1yl)ethyl nitrate) (see Fig. 4)
is formed by the reaction of NO3• with the external
carbon-carbon double bond of β-ionone leading to the
formation of the carbon radical (I). This radical adds
oxygen, forming the peroxy radical, which undergoes
rearrangement with RO2 and then dissociates follow-
ing path A. The proposed identity of this product
was made by the observation of an ion at m/z = 360
using CI, which can be due to the loss of one nitrate
molecule (m/z = 62) and a hydrogen atom to make
HNO3 [31,32].
At present, the mechanistic pathways for the forma-
tion of the remaining proposed products that were ob-
served in the gas-phase reaction of β-ionone + NO3•
are speculative. The exact steps of these reaction prod-
ucts maybe surface dependent and the proposal of a
potential mechanism would be difficult to suggest on
the basis of conventional gas-phase VOC/NO3• mech-
10−12 cm3 molecule−1
s
−1, kꢁ = 0.05 h−1) and ozone
(kO +β-ionone = 0.19 × 10−16 cm3 molecule−1 s−1
,
3
kꢁ = 0.03 h−1) [28], it is evident that nitrate radical
chemistry could play a critical role in the transforma-
tion of β-ionone in the indoor environment.
Nitrate radical reactions can lead to a number of
organic nitrate products such as PANs, alkyl nitrates,
hydroxynitrates, and dinitrates [14]. Organic nitrates
have received considerable interest because of their
potential for deleterious health effects. However, only
International Journal of Chemical Kinetics DOI 10.1002/kin

640
HARRISON AND HAM
minimal toxicity data exist for alkyl and arylalkyl
nitrates, hydroxynitrates, and dinitrates. It is expected
that these compounds may have harmful health
effects and should be investigated. For example, nitro-
PAHS (e.g., 6-nitrobenzo[a]pyrene, 3-nitroperylene,
1-nitropyrene, and 6-nitrochysene) formed from
NO3•/PAH reactions have been identified as mutagens
and carcinogens [16]. Nevertheless, predictions
of organic nitrate hazards can be made using the
Hazassess program developed by Jarvis et al. [40,41].
Using this program, the calculated hazard index values
of 2,6,6-trimethylcyclohex-1-ene-1-carbaldehyde and
3-oxo-1-(2,6,6-trimetylcyclohexane-1-en-1-yl)butane-
1,2-diyl dinitrate were 0.66 and 0.57, respectively.
Hazard index values of more than 0.5 are labeled as
being “very probably hazardous” and are predicted to
induce occupational asthma. Although this program
should not be used to confirm the toxicity of these
compounds, it does give insight into potential factors
that can affect indoor air quality.
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