Gas-phase chemistry of dihydromyrcenol with ozone and OH radical: Rate constants and products

Article abstract of DOI:10.1002/kin.20174

A bimolecular rate constant, k_{OH+dihydromyrcenol}, of (38±9) × 10^-12 cm³ molecule^-1 s^-1 was measured using the relative rate technique for the reaction of the hydroxyl radical (OH) with 2,6-dimethyl-7-octen-2-ol (dlhydromyrcenol,) at 297± 3 K and 1 atm total pressure. Additionally, an upper limit of the bimolecular rate constant, & k_{O3+dihydromyrcenol}, of approximately 2 × 10^-18 cm³ molecule^-1 s^-1 was determined by monitoring the decrease in ozone (O ₃) concentration in an excess of dihydromyrcenol. To more clearly define part of dihydromyrcenol's indoor environment degradation mechanism, the products of the dihydromyrcenol + OH and dihydromyrcenol + O³ reactions were also investigated. The positively identified dihydromyrcenol/OH and dihydromyrcenol/O₃ reaction products were acetone, 2-methylpropanal (O=CHCH(CH₃)₂), 2-methylbutanal (O=CHCH(CH₃)CH₂CH₃), ethanedial (glyoxal, HC(=O)C(=O)H), 2-oxopropanal (methylglyoxal, CH₃C(=O)C(=O)H). The use of derivatizing agents O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) clearly indicated that several other reaction products were formed. The elucidation of these other reaction products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible dihydromyrcenol/OH and dihydromyrcenol/O₃ reaction mechanisms based on previously published volatile organic compound/OH and volatile organic compound/O₃ gas-phase reaction mechanisms.

Full text of DOI:10.1002/kin.20174

Gas-Phase Chemistry of
Dihydromyrcenol with
Ozone and OH Radical:
Rate Constants and
Products
CRYSTAL D. FORESTER, JASON E. HAM, J. R. WELLS
Exposure Assessment Branch, Health Effects Laboratory Division, National Institute for Occupational Safety
and Health,1095 Willowdale Road, Morgantown, WV 26505
Received 1 July 2005; revised 23 November 2005; accepted 7 December 2005
DOI 10.1002/kin.20174
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT:
A
bimolecular rate constant, kOH + dihydromyrcenol
,
of (38 9) × 10−12 cm3
molecule−1 s−1 was measured using the relative rate technique for the reaction of the hydroxyl
radical (OH) with 2,6-dimethyl-7-octen-2-ol (dihydromyrcenol,) at 297 3 K and 1 atm total
pressure. Additionally, an upper limit of the bimolecular rate constant, kO + dihydromyrcenol,
3
of approximately 2 × 10−18 cm3 molecule−1 s−1 was determined by monitoring the decrease
in ozone (O3) concentration in an excess of dihydromyrcenol. To more clearly define part
of dihydromyrcenol’s indoor environment degradation mechanism, the products of the
dihydromyrcenol + OH and dihydromyrcenol + O3 reactions were also investigated. The
positively identified dihydromyrcenol/OH and dihydromyrcenol/O3 reaction products were
acetone, 2-methylpropanal (O CHCH(CH3)2), 2-methylbutanal (O CHCH(CH3)CH2CH3),
ethanedial (glyoxal, HC( O)C( O)H), 2-oxopropanal (methylglyoxal, CH3C( O)C( O)H).
The use of derivatizing agents O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and
N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) clearly indicated that several other reaction
products were formed. The elucidation of these other reaction products was facilitated by mass
spectrometry of the derivatized reaction products coupled with plausible dihydromyrcenol/OH
and dihydromyrcenol/O3 reaction mechanisms based on previously published volatile organic
ꢀ
C
compound/OH and volatile organic compound/O3 gas-phase reaction mechanisms.
Wiley Periodicals, Inc.∗ Int J Chem Kinet 38: 451–463, 2006
2006
INTRODUCTION
Correspondence to: J. R. Wells; e-mail: ozw0@cdc.gov.
Thefindingsandconclusionsinthisreportarethoseoftheauthors
and do not necessarily represent the views of the National Institute
Work-related asthma (WRA) has increasingly become
one of the most commonly diagnosed respiratory dis-
eases in the United States [1]. The reason for the
increases in work-related asthma has been partially
for Occupational Safety and Health.
2006 Wiley Periodicals, Inc. ∗This article is US Government
c
ꢀ
work and, as such, is in public domain of the United States of
America.

452
FORESTER, HAM, AND WELLS
attributed to the presence of volatile organic com-
pounds (VOCs) [2–5]. VOCs in the indoor environment
come from a number of sources, but a significant por-
tion results from the use of cleaning products and air
fresheners [6]. These products act not only as primary
emission sources for VOCs, but also contribute to the
formation of secondary pollutants due to reaction with
other indoor constituents [6].
Experimental evidence has implicated that several
initiator species such as ozone (O3), hydroxyl radical
(OH), and nitrate radical (NO3) are present indoors
[7,8]. These indoor reactants can convert VOCs that
are present (i.e., cleaning products, air fresheners) into
other oxygenated organic compounds such as aldehy-
des, ketones, and dicarbonyls. In a recent paper by
Jarvis et al., chemicals with these carbonyl substruc-
tures were associated with the potential to cause work-
related asthma [9].
Structure 1 2,6-dimethyl-7-octen-2-ol
concentration in an excess of dihydromyrcenol [22].
The products of the OH + dihydromyrcenol and
O3 + dihydromyrcenol reactions are also reported.
Neither the OH rate constant, O3 rate constant, nor the
respective reaction mechanisms for dihydromyrcenol
have been reported previously.
EXPERIMENTAL METHODS
Apparatus and Materials
Other oxygenated organic compounds, such as
ethers, alcohols, and esters, are also becoming more
prevalent in the indoor environment as they are substi-
tuted for other chemicals in consumer products. While
several hydroxyl radical (OH) + oxygenated organic
and ozone (O3) + oxygenated organic bimolecular rate
constants are well known, details pertaining to these
reaction mechanisms are limited [10,11]. A few recent
studies of the products from OH + oxygenated organic
reactions have illustrated the complexity of their gas-
phase reaction mechanisms [12–20]. These investiga-
tions are needed to support OH and O3 reaction mech-
anism patterns based on chemical structure–reactivity
relationships [21].
Before attributing a particular VOC or group of
VOCs to the effect of WRA, identification and quan-
tification of organic compounds present in the indoor
environment is necessary to determine occupant expo-
sures. Additionally, the mechanisms from which these
are emitted or formed must also be understood to pro-
vide a basis for chemical selection based on structure.
With this knowledge, new compounds may be syn-
thesized that incorporate environmentally and techni-
cally beneficial molecular structures. The information
gainedfromthetypeofresearchpresentedherecanlead
to more beneficial use of these and similar compounds
in the future. One such compound of interest is 2,6-
dimethyl-7-octen-2-ol (dihydromyrcenol, Structure 1)
a volatile organic alcohol that is a significant compo-
nent of indoor cleaner emissions and a component of
the lavender scent [6].
Experiments to measure the gas-phase rate con-
stant of the OH + 2,6-dimethyl-7-octen-2-ol (dihy-
dromyrcenol, Structure 1) reaction were conducted
with a previously described apparatus [16,22–24]. A
brief description is provided here. Reactants were in-
troduced, and samples were withdrawn through a 6.4-
mm Swagelok fitting attached to a 60–100 L Teflon
film chamber. Compressed air from the NIOSH facility
was passed through anhydrous CaSO4 (Drierite, Xenia,
OH) and molecular sieves (Drierite, Xenia, OH) to re-
move both moisture and organic contaminants. This
dry compressed air was added as a diluent to the reac-
tion chambers and measured with a 0–100 L min−1
mass flow controller (MKS, Andover, MA). Analy-
sis of this treated compressed air by gas chromatog-
raphy/mass spectrometry revealed that if contaminants
were present they would be below the part per trillion
range. The filler system was equipped with a syringe
injection port facilitating the introduction of both liq-
uid and gaseous reactants into the chambers by a flow-
ing air stream. All reactant mixtures and calibration
standards were generated by this system. Irradiations
were carried out in a light-tight chamber using 5-mil
FEP Teflon-film chambers (60–100 L), which were
surrounded by the following mix of lamps: six Philips
TL40W/03, one GE F40BL, two QPANEL (Cleveland,
OH) UV351, and seven QPANEL UV340. This lamp
mixture approximates solar radiation from 300 to
450 nm.
All reaction kinetic samples were quantitatively
monitored using an Agilent (Palo Alto, CA) 6890 gas
chromatograph with a 5973 mass selective detector
(GC/MS) and Agilent ChemStation software. Gas sam-
ples were cryogenically collected employing an Entech
(Simi Valley, CA) 7100 sampling system utilizing the
following trap and temperature parameters: 50 mL of
In the work presented here, the rate constant
of the OH radical with dihydromyrcenol was mea-
sured by the relative rate method. An upper limit for
the dihydromyrcenol/O3 rate constant was also de-
termined by monitoring the decrease in the ozone

GAS-PHASE CHEMISTRY OF DIHYDROMYRCENOL WITH OZONE AND OH RADICAL
453
chamber contents were collected onto Trap 1 (packed
with Tenax TA) at −150◦C. After sample collection,
Trap 1 was heated to 40◦C and the sample trans-
ferred under a flow of ultra high purity helium (UHP
He) onto Trap 2 (packed with Tenax TA) cooled to
−30◦C. Trap 2 was then heated to 180◦C and the sam-
ple 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 sam-
ple onto an Rtx-VRX (Restek, Bellefonte, PA) GC
column (0.25 mm i.d., 30-m long, 1.4 ꢀm 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 GC temperature program used was
initial temperature of 45◦C held for 8 min after sample
injection then increased 10◦C/min to 220◦C and held
for4min. TheAgilent5973massselectivedetectorwas
tuned using perfluorotributylamine (FC-43). Full-scan
electron impact (EI) ionization spectra were collected
from m/z 35 to 220. Preliminary compound identifi-
cations from the Agilent 6890/5973 GC/MS data sets
were made by searching the NIST 98 Mass Spectral
Library.
Experiments to measure the reaction of ozone with
2,6-dimethyl-7-octen-2-olwereconductedusingasim-
ilar chamber as described above, but the ozone con-
centration was measured using a UV photometric
ozone analyzer (Thermo Environmental model 49-C
Franklin, MA). An additional port was added to the
Teflon chamber to facilitate the injection of ozone.
Reaction product identification experiments were
performed utilizing direct sampling and chemical
derivatization methods. Direct gas-phase sampling
and Agilent 6890/5973 GC/MS analysis of the re-
action chamber contents was performed as described
above. The gas-phase carbonyl reaction products were
derivatized using O-(2,3,4,5,6-pentafluorobenzyl)-
hydroxylamine (PFBHA), while the alcohol and
carboxylic acid reaction products were deriva-
tized using PFBHA with N,O-bis(trimethylsilyl)-
trifluoroacetamide (BSTFA). Derivatized reaction
products were analyzed using a Varian (Palo Alto,
CA) 3800/Saturn 2000 GC/MS system operated in
EI mode [25]. The combination of PFBHA/BSTFA
derivatizations was used previously to identify the re-
action products and propose reaction mechanisms for
the ozonolysis of ꢁ-pinene and ꢀ3-carene [25,26]. Ex-
perimental methods for reaction product identification
were similar to methods used for kinetic experiments
except the reference compound was excluded from the
reaction mixture.
long, 1 ꢀm film thickness) column and the following
GC oven parameters: 60◦C for 1 min then 10◦C/min to
280◦Candheldfor10min. Sampleswereinjectedinthe
splitless mode, and the GC injector was returned to split
mode 1 min after sample injection, with the following
injector temperature parameters: 60◦C for 1 min then
180◦C/min to 250◦C and held to the end of the chro-
matographic run [25]. The Saturn 2000 ion trap mass
spectrometer was tuned using perfluorotributylamine
(FC-43). Full-scan EI spectra were collected from m/z
40 to 650.
OH radicals, one of the primary oxidizing radi-
cals in the indoor environment [27–30], were gener-
ated from the photolysis of methyl nitrite (CH3ONO)
in the presence of nitric oxide (NO) in air [22]
(Eqs. (1)–(3)).
CH3ONO + hꢂ → CH3O + NO
CH3O + O2 → CH2O + HO2
HO2 + NO → OH + NO2
(1)
(2)
(3)
CH3ONO was prepared in gram quantities using the
method of Taylor et al. [31] and stored in a lecture bottle
atroomtemperature. TheCH3ONOpurity(>95%)was
verified by GC/MS.
Ozone was produced by photolyzing air with a mer-
cury pen lamp in a separate Teflon chamber. Aliquots
of this O3/air mixture were added to the Teflon reaction
chamber using a gas-tight syringe.
All compounds were used as received and had the
following purities: from Sigma-Aldrich (Milwaukee,
WI): cyclohexane (99.9%), hexane (99%), 2,5-
dimethylfuran (99%), 2,6-dimethyl-7-octen-2-ol
(99%), decane(99+%), 2-butanol(99.5%), acetonitrile
(99.93%), N,O-bis(trimethylsilyl)trifluoroacetamide
(BSTFA) (99+%), O-(2,3,4,5,6-pentafluorobenzyl)-
hydroxylamine hydrochloride (PFBHA) (98+%); from
Fisher Scientific: methanol (99%); from Spectrum
Analytical (New Brunswick, NJ): methylene chloride
(99.5%). Nitric oxide (99+% pure) was obtained as
a 4942 ppm mixture in nitrogen 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 1 atm pressure.
Experimental Procedures
Compound separation was achieved by a J&W
Scientific (Folsom, CA) DB-5MS (0.32 mm i.d., 30-m
The experimental procedures for determining the
dihydromyrcenol + OH reaction kinetics were similar

454
FORESTER, HAM, AND WELLS
to those described previously [17,18].
For the dihydromyrcenol/OH kinetic experiments,
the typical concentrations of the pertinent species
in the Teflon chamber were 0.3–0.6 ppm (0.7–
1.4 × 1013 molecule cm−3) dihydromyrcenol, 0.5–
0.7 ppm (1.2–1.6 × 1013 molecule cm−3) reference,
10 ppm (23 × 1013 molecule cm−3) CH3ONO, and 0–
0.6 ppm (0–1.4 × 1013 molecule cm−3) NO in air. Di-
hydromyrcenol was diluted in a 25:75 (v:v) solution
with methanol. Methanol was chosen because both the
methanol/OH and methanol/O3 rate constants and re-
action products were not expected to affect the exper-
imental results. These mixtures were allowed to reach
equilibrium before initial species concentration ([X]0)
samples were collected. Typically, four photolysis in-
tervals of 10–20 s each were used on the reaction mix-
ture for a combined total photolysis time of approx-
imately 40–60 s. The total ion chromatogram (TIC)
from the Agilent 5973 mass selective detector was used
to determine dihydromyrcenol and reference concen-
trations.
The experimental methods and parameters for ob-
servation of dihydromyrcenol/OH reaction products
were similar to those for reaction rate experiments
except that the reference compound was excluded
from the reaction mixture. The reaction mixtures
were irradiated for 10–20 s intervals followed by
sample collection. When possible, commercially avail-
able samples of the identified products were analyzed
to verify matching ion spectra and chromatographic
retention times.
kOH + Dihydromyrcenol
Dihydromyrcenol + OH
−→ Products (4)
kRef
Reference + OH −→ Products (5)
The rate equations for reactions (4) and (5) are
combined and integrated, resulting in the following
equation:
ꢀ
ꢁ
[Dihydromyrcenol]0
[Dihydromyrcenol]t
ln
ꢀ
ꢁ
kOH + Dihydromyrcenol
[Ref]0
[Ref]t
=
× ln
(6)
kRef
If reaction with OH is the only removal mech-
anism for dihydromyrcenol and reference, a plot
of ln([dihydromyrcenol]0/[dihydromyrcenol]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 kOH + dihydromyrcenol (Fig. 1). The OH rate con-
stant experiments for dihydromyrcenol employed the
use of three reference compounds: decane, 2-butanol,
and 2,5-dimethylfuran. The use of three different ref-
erence compounds with different OH rate constants
more definitively assured the accuracy of the dihy-
dromyrcenol/OH rate constant and demonstrates that
other reactions are not removing dihydromyrcenol.
Typically five experimental runs were conducted on
each dihydromyrcenol/reference pair.
The experimental procedures for the determination
of the dihydromyrcenol + O3 reaction kinetics were
Figure 1 2,6-dimethyl-7-octen-2-ol (dihydromyrcenol) relative rate plot with decane (ꢀ), 2,5-dimethyl furan (ꢀ), and 2-butanol
(ꢁ) as reference compounds. The OH + dihydromyrcenol rate constant, kdihydromyrcenol + OH, measured is (37.7 0.8) × 10−12
cm3 molecule−1 s−1
.

GAS-PHASE CHEMISTRY OF DIHYDROMYRCENOL WITH OZONE AND OH RADICAL
455
similar to those described previously [32].
analyzed as described above. No chromatographic in-
terferences were observed. Two experiments were con-
ducted to determine the stability of the reference and
dihydromyrcenolcoexistinginthesamechamber. First,
the reference and dihydromyrcenol were injected into
the chamber, analyzed, photolyzed for 6 min and rean-
alyzed. In a separate experiment, methyl nitrite, NO,
reference, and dihydromyrcenol were injected into the
chamber, analyzed, left several hours, and then reana-
lyzed. None of these preliminary experiments yielded
chromatographic peak interferences or observable re-
actions occurring without photoinitiation. At the end
of each run, the Teflon chamber was cleaned by flush-
ing the chamber six times with air (<0.1 ppm total
hydrocarbon). Measurements of an air-filled chamber
showed no cross contamination between runs. All mea-
surements were at least duplicated. A relative stan-
dard deviation (the data set standard deviation divided
by the data set average) of approximately 2.5% was
achieved with the described sampling method utiliz-
ingtheEntech7100/Agilent6890/5973cryogenicsam-
pling GC/MS system.
kO3
+ Dihydromyrcenol
Dihydromyrcenol + O3
−→ Products (7)
Ozone was injected into the reaction chamber as it was
being filled with air and dihydromyrcenol. The range
of dihydromyrcenol concentrations was 0.3–1.2 ppm
(0.7–2.9 × 1013 molecule cm−3) with an initial O3 con-
centration of 0.1 ppm (0.2 × 1013 molecule cm−3). Cy-
clohexane (410 ppm) was also injected into the reaction
chamber to scavenge any OH formed as a byproduct
of the dihydromyrcenol/O3 reaction [33]. The cham-
ber was transferred within 5 s to the Thermo Electron
UV photometric ozone analyzer model 49C, and ozone
concentration measurements integrated over 10 s time
intervals were collected up to a total of 1100 s. In these
series of experiments, no decrease in O3 concentration
was observed over the reactant concentration range.
In order to determine an upper limit estimate for the
dihydromyrcenol/O3 reaction rate constant, both dihy-
dromyrcenol and O3 concentrations were increased to
2.8 ppm and 0.3 ppm, respectively. As before, O3 was
injected into the reaction chamber as it was being filled
with air and dihydromyrcenol.
RESULTS
Derivatization of the carbonyl reaction products was
initiated by flowing of 15 to 25 L of chamber contents at
3.8 L min−1 through 3 mL of acetonitrile in an impinger
with no effort to prevent acetonitrile evaporation during
sample collection. The sample was removed from the
impinger, and 200 ꢀL of 0.02 M PFBHA was added
to derivatize aldehyde, ketone, and dicarbonyl reaction
products to oximes [25] over a 24–48 h time period in
the dark. The reacted solutions were gently blown to
dryness with UHP N2, reconstituted with 100 ꢀL of
methanol, and 1 ꢀL of the reconstituted solution was
injected onto the Varian 3800/Saturn 2000 GC/MS sys-
tem. The derivatization of hydroxy groups (either al-
cohol or carboxylic acid) was achieved by subsequent
addition of 20 ꢀL of commercially available BSTFA to
the PFBHA oximes reconstituted with 100 ꢀL of hex-
ane: methylene chloride (1:1). These PFBHA/BSTFA
solutions were heated to approximately 60◦C for 45
min to complete the silylation, and then 1 ꢀL of the
solution was injected into the Varian 3800/Saturn 2000
GC/MS system [25]. Additionally, chemical ioniza-
tion (CI) mass spectra using acetonitrile were also col-
lected on the derivatized reaction products allowing
the determination of the PFBHA or PFBHA/BSTFA-
derivatized compound’s molecular weight.
Dihydromyrcenol/OH Reaction Rate
Constant
The OH rate constant for 2,6-dimethyl-7-octen-2-ol
(dihydromyrcenol, Structure 1) was obtained using
the relative rate method described above. The plot
of a modified version of Eq. (6) is shown in Fig. 1.
The ln([Ref]0/[Ref]t) term is divided by the respec-
tive reference rate constant (decane (11.0 0.28) ×
10−12 cm3 molecule−1 s−1), 2,5-dimethyl furan (132
33) × 10−12 cm3 molecule−1
s
−1), and 2-butanol
(8.65 2.1) × 10−12 cm3 molecule−1
s
−1) [10,11,18,
34]andmultipliedby10−12cm3 molecule−1 s−1, result-
ing in a unitless number. This yields a slope that
is equal to the OH radical/dihydromyrcenol rate
constant, kOH + dihydromyrcenol, divided by 10−12 cm3
molecule−1 −1. This modification allows for a di-
s
rect comparison of the three reference compound/
dihydromyrcenol data sets. The slope of the line
shown in Fig. 1 yields an OH radical bimolec-
ular rate constant, kOH + dihydromyrcenol, of (37.7
0.8) × 10−12 cm3 molecule−1
s
−1. The data points at
the origin are experimental points because preirradi-
ation, t = 0, data showed no detectable loss of dihy-
dromyrcenol or reference. The error in the rate con-
stant stated above is the 95% confidence level from
the random uncertainty in the slope. Incorporating
the uncertainties associated with the reference rate
To determine possible chromatographic interfer-
ences from reference/OH reaction products, both di-
hydromyrcenol and the reference compounds were
reactedwiththeOHradicalinseparateexperimentsand

456
FORESTER, HAM, AND WELLS
constants ( 25% for decane, 2,5-dimethylfuran, and
2-butanol) used to derive the dihydromyrcenol/OH rate
constant yields a final value for kOH + dihydromyrcenol, of
(38 9) × 10−12 cm3 molecule−1 s−1 [10,11,34].
based on previously published volatile organic com-
pound/OH gas-phase reaction mechanisms [10,12–19].
These other reaction products are listed in Table I.
The results for both the positively identified reaction
products and the other observed reaction products are
described below.
The
dihydromyrcenol/OH
rate
constant,
kOH + dihydromyrcenol, has not been previously reported.
The observed rate constant can be compared to a
k(calc)OH + dihydromyrcenol = 37 × 10−12 cm3 molecule−1
Derivatization of nonsymmetric carbonyls using
PFBHA or PFBHA/BSTFA typically resulted in mul-
tiple chromatographic peaks due to geometric iso-
mers of the oximes. 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. Typically,
the PFBHA-derivatized oximes’ (generic structure:
F5C6CH2ON C(R1)(R2)) mass spectra included an
ion at m/z 181 ([CH2C6F5]+ fragment) with a large
relative intensity (>40%) and a [PFBHA oxime +
181]+ ion (due to reactions in the ion trap mass spec-
trometer) [25]. In most cases, the m/z 181 ion rela-
tive intensity for the chromatographic peaks due to di-
hydromyrcenol/OH and dihydromyrcenol/O3 reaction
product oximes was either the largest or one of the
largest in the mass spectrum and was used to gener-
ate selected ion chromatograms [25]. The mass spectra
of compounds that were additionally derivatized with
BSTFA contained m/z 73 ions from the [Si(CH3)3]+
fragments [25]. The product data are described below.
The following chronological chromatographic
retention time results and mass spectra data
were observed utilizing PFBHA or PFBHA/BSTFA
derivatization and the Varian 3800/Saturn 2000
GC/MS system. The reaction products reported
here had chromatographic peak areas propor-
tional to initial dihydromyrcenol concentration and
were observed only after OH initiation of dihy-
dromyrcenol/methanol/methyl nitrite/NO/air mixtures
or addition of O3 to dihydromyrcenol/methanol/air.
Derivatization experiments performed in the absence
of dihydromyrcenol but in the presence of all other
chemicals in the reaction chamber (methanol/methyl
nitrite/NO/air) did not result in any of the data reported
below except for small amounts (as noted by chro-
matographic peak areas) of acetone, 2-methylpropanal,
2-methylbutanal, 2-oxopropanal, and ethanedial. Ace-
tone was also observed in prephotoinitiated dihy-
dromyrcenol/OH or preozonated dihydromyrcenol/O3
derivatization samples. However, the acetone, 2-
oxopropanol, and ethanedial oxime peak areas in-
creased significantly, from 30% to 65%, with dihy-
dromyrcenol/OH reaction initiation, indicating that
acetone, 2-oxopropanal, and ethanedial are likely prod-
ucts of the dihydromyrcenol + OH reaction. Increases
in both 2-oxopropanal and ethanedial peak areas with
dihydromyrcenol/O3 reaction initiation indicated that
s
−1, calculated using the Environmental Protection
Agency’s rate constant calculation software, AOPWIN
v1.91 [35].
Dihydromyrcenol/O3 Reaction Rate
Constant
The experimental conditions, described above, to de-
termine the dihydromyrcenol/O3 reaction rate con-
stant yielded a first-order decrease in ozone concen-
tration over a 15 min monitoring period. Analysis
of the data resulted in an upper limit determination
of the dihydromyrcenol/O3 rate constant of 2 × 10−18
cm3 molecule−1 s−1. The dihydromyrcenol/O3 rate
constant, kO + dihydromyrcenol, has not been previ-
3
ously reported. The estimated rate constant can
be compared with a k(calc)O + dihydromyrcenol, = 1.2
3
× 10−17 cm3 molecule−1 s−1, calculated using the En-
vironmental Protection Agency’s rate constant calcu-
lation software, AOPWIN v1.91 [35].
Dihydromyrcenol/OH and
Dihydromyrcenol/O3 Reaction Products
The reaction products observed from the initial
dihydromyrcenol/OH hydrogen abstraction or OH ad-
dition to the carbon–carbon double bond are con-
sistent with previously observed OH radical reac-
tion mechanisms for oxygenated organic species
[12–19]. Typically, the oxygenated organic parent
compound reacts with OH to subsequently gener-
ate other oxygenated organic products. For dihy-
dromyrcenol, the dihydromyrcenol/OH reaction prod-
ucts observed and positively identified, using the
pure compound for verification by either direct
gas-phase detection or derivatization or both, were
acetone, 2-methylpropanal (O CHCH(CH3)2), 2-
methylbutanal (O=CHCH(CH3)CH2CH3), ethanedial
(glyoxal, HC( O)C( O)H), 2-oxopropanal (methyl-
glyoxal, CH3C( O)C( O)H), and are listed in
Table I. Many other dihydromyrcenol/OH reaction
products were observed utilizing the derivatization
methods described above. Elucidation of these other
reaction products was facilitated by mass spectrome-
try of the derivatized reaction products coupled with
plausible dihydromyrcenol/OH reaction mechanisms

GAS-PHASE CHEMISTRY OF DIHYDROMYRCENOL WITH OZONE AND OH RADICAL
457
Table I Molecular Structure of Some Dihydromyrcenol/OH Reaction Products
Retention Time
(min)
Molecular Weight
(amu)
CI Ions
Observed
EI Ions Observed
(Relative Intensity %)
Name
Structure
11.4
Acetone
58
72
86
60
254 181(100), 223(20),
253(14)
12.5
12.6
2-Methylpropanal
2-Methylbutanal
Glycolaldehyde
268 181(100), 250(60)
13.6
282 55(27), 181(100),
264(80)
14.1
14.4
256 57(8–32), 69(15),
97(15), 127(15),
161(10), 181(100),
195(17), 206(10),
225(15), 226(22)
15.9
16.3
2-Methylbut-3-enal
4-Methylhex-5-enal
84
280 59(100), 181(97),
239(60), 282(24)
112
308 55(16), 95(12),
126(20), 181(100),
239(10)
17.4
3-Methylpent-4-enal
98
294 59(35), 130(42.5),
181(100), 294(12.5)
18.1
18.2
19.5
19.7
2,6-Dimethylhept-5-enal
140
336 41(28), 59(57–65),
72(62), 112(5–25),
154(15–22), 181(48–100),
253(30–100), 266(27–42),
336(5–17), 429(32)
21.0
21.2
Ethanedial
58
72
449 181(100), 448(28)
21.5
2-Oxopropanal
463 181(100), 265(34),
462(7.5)
theyarelikelyreactionproducts. Acetoneisalsoaprod-
uct of the dihydromyrcenol/O3 reaction, but its peak
area increased by only a few percent. Aside from the
positively identified reaction products (Table I) with
the synthesis of the derivatized oxime compound, di-
hydromyrcenol/OH and dihydomyrcenol/O3 reaction
product identification was derived from mass spectra
data and previously published VOC/OH and VOC/O3
reaction mechanisms [12–19,25].
Acetone
Acetone was identified using the Agilent 6890/5973
GC/MS system and PFBHA derivatization method

458
FORESTER, HAM, AND WELLS
described above. The acetone oxime (PFBHA
C(CH3)2) was observed at approximately 11.4 min em-
ploying the Varian 3800/Saturn 2000 GC/ion trap mass
spectrometer system described above. Acetone oxime
was synthesized to confirm this chromatographic as-
signment. Acetone oxime was observed in prephotoly-
sis samples, but the peak area increased upon initiation
of dihydromyrcenol/OH reaction indicating acetone
as a dihydromyrcenol/OH reaction product. Acetone
was also identified as a dihydromyrcenol/O3 reaction
product.
Oxime at Retention Time 15.9 min
The oxime observed with a chromatographic peak at
a retention time of 15.9 min had ions of m/z (rela-
tive intensity) 59 (100%), 181 (97%), 239 (60%), and
282 (24%). Using acetonitrile for chemical ionization,
an M + 1 ion of m/z of 280 was observed. Using the
derivatized molecular weight of 279 for the oxime sug-
gests a carbonyl compound with a molecular weight of
84. The single chromatographic peak suggests an alde-
hyde structure or a symmetric ketone structure. A pro-
posed dihydromyrcenol/OH reaction product assign-
ment of 2-methylbut-3-enal (see Table I) was made
based upon the observed data.
2-Methylpropanal (O CHCH(CH3)2)
The chromatographic peaks for the oxime observed at
12.5 and 12.6 min were observed as a reaction product
of dihydromyrcenol/OH and had ions at m/z (relative
intensity) 181 (100%) and 250 (60%). Using acetoni-
trile for chemical ionization, an M + 1 ion of m/z of
268 was observed for the PFBHA-derivatized sample.
The 2-methylpropanal oxime was synthesized to con-
firm this chromatographic assignment. This carbonyl
compound was also observed as a dihydromyrcenol/O3
reaction product.
Oxime at Retention Time 16.3 min
The oxime observed with a chromatographic peak at a
retention time of 16.3 min had ions of m/z (relative in-
tensity) 55 (16%), 95 (12%), 126 (20%), 181 (100%),
and 239 (10%) as seen in Fig. 2A. This reaction prod-
uct chromatographic peak was the largest one observed
for the dihydromyrcenol/OH reaction. Using acetoni-
trile for chemical ionization, an M + 1 ion of m/z of
308 was observed as seen in Fig. 2B. Using the deriva-
tized molecular weight of 307 for the oxime suggests
a carbonyl compound with a molecular weight of 112.
The single chromatographic peak suggests an aldehyde
structure or a symmetric ketone structure. A proposed
dihydromyrcenol/OH reaction product assignment of
4-methylhex-5-enal (see Table I) was made based upon
the observed data.
2-Methylbutanal (O CHCH(CH3)CH2CH3)
The single chromatographic peak for the oxime ob-
served at 13.6 min was observed as a reaction prod-
uct of dihydromyrcenol/OH and had ions at m/z (rela-
tive intensity) 55 (27%), 181 (100%), and 264 (80%).
Using acetonitrile for chemical ionization, an M + 1
ion of m/z of 282 was observed for the PFBHA-
derivatized sample. The 2-methylbutanal oxime was
synthesized to confirm this chromatographic assign-
ment. This carbonyl compound was also observed as a
dihydromyrcenol/O3 reaction product.
Oxime at Retention Time 17.4 min
The oxime observed with a chromatographic peak at
a retention time of 17.4 min had ions of m/z (relative
intensity) 59 (35%), 130 (42.5%), 181 (100%), and 294
(12.5%). Using acetonitrile for chemical ionization, an
M + 1 ion of m/z of 294 was observed. Using the
derivatized molecular weight of 293 for the oxime sug-
gests a carbonyl compound with a molecular weight of
98. The single chromatographic peak suggests an alde-
hyde structure or a symmetric ketone structure. A pro-
posed dihydromyrcenol/OH reaction product assign-
ment of 3-methylpent-4-enal (see Table I) was made
based upon the observed data.
Oxime at Retention Time 14.1 and 14.4 min
The oxime observed with a chromatographic peak at a
retentiontimeof14.1and14.4minhadionsofm/z (rel-
ative intensity) 57 (8–32%), 69 (15%), 97 (15%), 127
(15%), 161 (10%), 181 (100%), 195 (17%), 206 (10%),
225 (15%), 226 (22%). Using acetonitrile for chemical
ionization, an M + 1 ion of m/z of 256 was observed.
Using the derivatized molecular weight of 255 for the
oxime suggests a carbonyl compound with a molecular
weight of 60. A proposed dihydromyrcenol/OH reac-
tion product assignment of glycolaldehyde (see Table I)
was made based upon the observed data.
Oxime at Retention Time 18.1, 18.2, 19.5,
19.7 min
The oxime observed with chromatographic peaks at
retention times of 18.1, 18.2, 19.5, and 19.7 min was
observed as a reaction product of dihydromyrcenol/OH

GAS-PHASE CHEMISTRY OF DIHYDROMYRCENOL WITH OZONE AND OH RADICAL
459
Figure 2 PFBHA-derivatized product of dihydromyrcenol (16.3 min) (A) electron ionization spectrum (B) acetonitrile chemical
ionization spectrum.
and had ions of m/z (relative intensity) 41 (28%), 59
(57–65%), 72 (62%), 112 (5–25%), 154 (15–22%), 181
(48–100%), 253 (30–100%), 266 (27–42%), 336 (5–
17%), and 429 (32%). Using acetonitrile for chemi-
cal ionization, an M + 1 ion of m/z of 336 was ob-
served for the PFBHA-derivatized sample. Using the
derivatized molecular weight of 335 for the oxime sug-
gests a carbonyl compound with a molecular weight of
140. A proposed dihydromyrcenol/OH reaction prod-
uctassignmentof2,6-dimethylhept-5-enal(seeTableI)
was made based upon the observed data. This carbonyl
compound was also observed as a dihydromyrcenol/O3
reaction product.
of 58. Using acetonitrile for chemical ionization, an
M + 1 ion of m/z of 449 was observed for the PFBHA-
derivatized sample. The PFBHA-glyoxal oxime was
synthesized to confirm this chromatographic assign-
ment. This carbonyl compound was also observed as a
dihydromyrcenol/O3 reaction product.
2-Oxopropanal (Methylglyoxal,
CH3C( O)C( O)H)
The single peak for the oxime observed at 21.5 min was
observed as a reaction product of dihydromyrcenol/OH
and had ions at m/z (relative intensity) 181 (100%),
265 (34%), and 462 (8%). The m/z 462 ion is the re-
sult of a double PFBHA derivatization indicating a re-
action product with a molecular weight of 72. Using
acetonitrile for chemical ionization, an M + 1 ion of
m/z of 463 was observed for the PFBHA-derivatized
sample The PFBHA-methylglyoxal oxime was syn-
thesized to confirm this chromatographic assignment,
and the second chromatographic peak for PFBHA-
methylglyoxal overlaps with the 21.2 min peak of
Ethanedial (Glyoxal, HC( O)C( O)H)
The chromatographic peaks for the oxime observed at
21.0 and 21.2 min were observed as reaction products
of dihydromyrcenol/OH and had ions at m/z (relative
intensity) 181 (100%) and 448 (28%). The m/z 448
ion is the result of a double PFBHA derivatization in-
dicating a reaction product with a molecular weight

460
FORESTER, HAM, AND WELLS
PFBHA-glyoxal. This carbonyl compound was also
observed as a dihydromyrcenol/O3 reaction product.
zation experiments from the dihydromyrcenol/OH re-
action. 4-Methylhex-5-enal is likely formed from OH
hydrogen abstraction from site III. After hydrogen ab-
straction an oxygen molecule adds to the radical form-
ing a ꢃ-hydroxyalkyl radical. Subsequent reaction with
oxygen and NO yields the ꢃ-hydroxyalkoxy radical
DISCUSSION
OH reacts with dihydromyrcenol by H-atom abstrac-
tion or OH addition to the carbon–carbon double bond
[7]. The “reactive structure” of dihydromyrcenol can
be drawn as shown in Structure 1. Dihydromyrcenol
is a molecule with seven distinct sites for possible hy-
drogen abstraction and one site for OH addition. How-
ever, the sites labeled I, II, and III identified in Struc-
ture 1 contribute approximately 71%, 4%, and 12%,
respectively, to the calculated dihydromyrcenol/OH
rate constant of 37 × 10−12 cm3 molecule−1 s−1 [21].
This calculated value is in very good agreement
with the measured value reported here ((38 9) ×
10−12 cm3 molecule−1 s−1), considering that OH rate
constants calculated using structure reactivity are typ-
ically within a factor of two of the measured OH rate
constant [21].
For the dihydromyrcenol/OH reaction, the experi-
mental parameters were set to minimize other side re-
actions and highlight the first OH hydrogen abstraction
and OH addition step. The dihydromyrcenol concen-
tration was kept low, and the photolysis times were as
short as possible. Additionally, nitric oxide (NO) was
added to facilitate the generation of OH and to mini-
mize O3 and NO3 formation, preventing other possible
radical reactions. The possible mechanistic steps lead-
ing to product formation are described below. Depend-
ing on the nature of the radicals formed in reactions
(4) and (7), some reaction products may be formed by
multiple pathways.
•
(RO ) which results in 4-methylhex-5-enal and NO2.
H2C CHCH(CH3)CH2CH2CH2C(CH3)2OH + OH
r
→ H2C CHCH(CH3)CH2CH2CH C(CH3)2OH
+ H2O (8)
r
H2C CHCH(CH3)CH2CH2CH C(CH3)2OH + O2
r
→ H2C CHCH(CH3)CH2CH2CH(OO) C(CH3)2OH
(9)
r
H2C CHCH(CH3)CH2CH2CH(OO) C(CH3)2OH
+ NO → NO2 + H2C CHCH(CH3)CH2CH2CH(O)
r
C(CH3)2OH (10)
r
H2C CHCH(CH3)CH2CH2CH(O) C(CH3)2OH
→ beta scission → H2C CHCH(CH3)CH2CH2
r
CH O + C(CH3)2OH (11)
Recently, PFBHA-coated solid-phase microextrac-
tion (SPME) fibers were used for the passive sampling
of the gas-phase reaction products of Z-3-hexen-1-
ol/OH, 2-methyl-3-buten-2-ol/OH, and a series of
diols/OH. These reactions yielded products that
supported OH addition to carbon–carbon dou-
ble bond and hydrogen abstraction from the
C H bonds of HC(OH) groups [37,38]. How-
ever, if OH added to the dihydromyrcenol carbon–
carbon double bond (site I) an expected reaction
product would be 6-hydroxy-2,6-dimethylheptanal
(O CHCH(CH3)CH2CH2CH2C(CH3)2OH). Unfortu-
nately, it was not observed in the experimental system
described above. Long chained alcohol carbonyls have
been observed previously from our laboratory using the
described derivatization techniques highlighting that if
6-hydroxy-2,6-dimethylheptanal were formed as a re-
action product it would most likely have been detected
[20]. This expected reaction product could adhere to
the Teflon surface or be incorporated into ultrafine
particulate matter and thus escape described detection
methods. Another possible reason for not observing 6-
hydroxy-2,6-dimethylheptanalisthatoncetheOHadds
to the double bond there are possible fragmentation
pathways or isomerization leading to other observed
products. It is likely that both glycolaldehyde (Table I)
and 2,6-dimethylhept-5-enal (Table I) are the result
Acetone
The dihydromyrcenol/OH reaction mechanism has
several potential pathways leading to acetone
(O C(CH3)2) formation. OH can react with dihy-
dromyrcenol by hydrogen abstraction from site III and
subsequent addition of oxygen resulting in the disso-
•
ciation of the HOC( )(CH3)2 radical leaving the per-
•
oxyradical H2C CHCH(CH3)CH2CH2CH(OO) . The
•
HOC( )(CH3)2 radical can then react with oxygen-
yielding acetone and HO2. Acetone product formation
from the dihydromyrcenol/O3 reaction is less straight-
forward and may involve isomerization [36].
4-Methylhex-5-enal
The chromatographic peak proposed to be 4-
methylhex-5-enal, based on mass spectral data, was the
largest product peak observed in the PFBHA derivati-

GAS-PHASE CHEMISTRY OF DIHYDROMYRCENOL WITH OZONE AND OH RADICAL
461
of OH addition to the carbon–carbon double bond.
A possible mechanism for the formation of these two
products is shown in Fig. 3. However, the formation of
4-methylhex-5-enal and acetone as the major hydrogen
abstraction products, based on chromatographic peak
area, was consistent with structure–reactivity calcula-
tions [35]. Several of the possible reaction products
could be formed by isomerization of the alcoholic hy-
drogen due to the length of the carbon chain backbone
of dihydromyrcenol [36].
constant using Environmental Protection Agency’s
rate constant calculation software, AOPWIN v1.91,
is 1.2 × 10−17 cm3 molecule−1 s−1 [35]. This value
is approximately six times faster than the experi-
mentally estimated upper limit value of 2 × 10−18
cm3 molecule−1 s−1. The difference between the esti-
mated and calculated dihydromyrcenol/O3 rate con-
stant (∼600%) is significant when compared to
the difference between the measured and calcu-
lated dihydromyrcenol/OH rate constant of ∼3%.
This suggests that the interaction between dihy-
dromyrcenol and O3 is not accurately described by
AOPWIN v1.91.
Ozone reacts with dihydromyrcenol by addition
to the carbon–carbon double bond (site I, Struc-
ture 1). The calculated dihydromyrcenol/O3 rate
Figure 3 Proposed mechanism for OH addition to dihydromyrcenol carbon–carbon double bond.

462
FORESTER, HAM, AND WELLS
The reaction products observed from dihydro-
myrcenol/O3 reaction are due solely to ozone react-
ing with dihydromyrcenol and not due to secondary
OH formed from dihydromyrcenol/O3 reacting with
dihydromyrcenol. The primary dihydromyrcenol/OH
reaction product—the carbonyl product observed
as the 16.3 min oxime and tentatively identified
as 4-methylhex-5-enal—was not observed in the
dihydromyrcenol/O3 experiments.
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CONCLUSIONS
In order to investigate the detailed gas-phase chem-
istry of 2,6-dimethyl-7-octen-2-ol (dihydromyrcenol,
Structure 1), the hydroxyl (OH) radical reaction rate
constant, ozone reaction rate constant, and respec-
tive reaction mechanisms were determined. The OH
radical can either abstract hydrogen or add to the
carbon–carbon double bond of dihydromyrcenol. A bi-
molecular rate constant, kOH + dihydromyrcenol, of (38 9)
× 10−12 cm3 molecule−1 s−1 was measured using the
relative rate technique. Ozone is expected to add to
the carbon–carbon double bond of dihydromyrcenol.
An upper limit of 2 × 10−18 cm3 molecule−1 s−1 was
determined for kO + dihydromyrcenol
.
3
The identification of the dihydromyrcenol/OH and
dihydromyrcenol/O3 reaction products was facilitated
by the use of derivatizing agents PFBHA and BSTFA.
While many dihydromyrcenol/OH reaction products
were simply proposed based on observed experimen-
tal data and previously published VOC/OH reaction
mechanisms, several reaction products such as acetone,
2-methylpropanal, 2-methylbutanal, ethanedial, and 2-
oxopropanal were positively identified. The structures
of the identified reaction products indicate that the
carbon–carbon double bond and the secondary hy-
drogen of a carbon next to a carbon with an alco-
hol group play important roles in the formation of
dihydromyrcenol/OH and dihydromyrcenol/O3 reac-
tion products.
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