Observation of the A ～ - X ～ electronic transition of C 6-C10 peroxy radicals

Article abstract of DOI:10.1016/j.cplett.2014.03.087

The A～-X～ electronic transitions of C₆-C₁₀ peroxy radicals were observed in the near infra red. Spectra have been obtained for straight chain peroxy radicals: hexyl, heptyl, octyl, nonyl, and decyl including one C₈ isomer, iso-octyl peroxy. The peroxy radicals were generated using hydrogen abstraction of the corresponding hydrocarbon precursors. Spectra was assigned through spectral/structural relationships established through prior studies of smaller peroxy radicals. The secondary peroxy isomer was determined to be the dominate carrier for the observed n-alkyl spectra. However, the iso-octyl peroxy radical was found to contain the tertiary and primary isomers in its spectrum.

Full text of DOI:10.1016/j.cplett.2014.03.087

Chemical Physics Letters 601 (2014) 149–154
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
e
e
Observation of the A—X electronic transition of C6–C10 peroxy radicals
⇑
Neal D. Kline, Terry A. Miller
Department of Chemistry and Biochemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, OH 43210, United States
a r t i c l e i n f o
a b s t r a c t
e
e
Article history:
The A—X electronic transitions of C6–C10 peroxy radicals were observed in the near infra red. Spectra
have been obtained for straight chain peroxy radicals: hexyl, heptyl, octyl, nonyl, and decyl including
one C8 isomer, iso-octyl peroxy. The peroxy radicals were generated using hydrogen abstraction of the
corresponding hydrocarbon precursors. Spectra was assigned through spectral/structural relationships
established through prior studies of smaller peroxy radicals. The secondary peroxy isomer was deter-
mined to be the dominate carrier for the observed n-alkyl spectra. However, the iso-octyl peroxy radical
was found to contain the tertiary and primary isomers in its spectrum.
Received 9 January 2014
In final form 29 March 2014
Available online 4 April 2014
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
number will increase by substitution of a methyl group as a side
chain. This change in the hydrocarbon chain will also affect the
For about a century the spark ignition engine has been a leading
choice for personal transportation around the world. As a result
combustion reactions have been some of the most heavily
investigated chemical processes, but are yet to be fully understood
because of the complexity. To comprehend the underlying
chemistry of combustion reactions, identification of reactive
intermediates that participate in these processes is necessary.
The formation of alkyl peroxy radicals (RO2) is a fundamental step
in the low-temperature combustion of hydrocarbons [1–5]. For the
mechanism of low-temperature combustion to proceed, a hydroxyl
radical (OH) abstracts a hydrogen atom from a hydrocarbon fuel
(RH) to form an alkyl radical (R). The alkyl radical rapidly under-
goes a 3-body reaction in air to form an alkyl peroxy radical
(RO2), which then participates in one of many reactions leading
to chain branching, chain propagation, or chain termination.
Larger hydrocarbons (PC6) make up a significant portion of the
gasoline mixtures used in spark ignition engines [6–8]. Figure 1
shows that the most prevalent hydrocarbons in regular unleaded
fuel contain five carbon atoms and that 73% of the blend is composed
of hydrocarbons larger than C5. In the premium blend the most pre-
valent hydrocarbons contain eight carbon atoms with hydrocarbons
larger than C5 making up 75% of the mixture [6]. The disparity in the
amount of larger hydrocarbons each fuel mixture correlates with the
difference in antiknock rating, or octane rating, of the fuels. The rela-
tionship between octane rating and structure of the carbonchain has
been well documented [9–12]. As the carbon chain increases in
length octane number systematically decreases; however, octane
structure of the RO2 radical that is formed as the combustion process
proceeds. The terminal oxygen atom of the peroxy moiety can inter-
nally abstract an alkyl hydrogen giving the QOOH species [13–15].
Formation of the QOOH species is the combustion pathway that
leads to chain branching and, ultimately, to engine knocking.
Miyoshi [16] investigated the unimolecular reaction of alkyl peroxy
radicals and found that the 1,3-hydrogen abstraction (using a four
membered transition state) is 160(5) kJ/mol high and unimportant.
The 1,4-hydrogen abstraction (five membered transition state) is
140(20) kJ/mol, while the 1,5- and 1,6-hydrogen abstractions (6
and 7 membered transition states, respectively) have even lower
barriers, with the 1,6-hydrogen abstraction having the lowest bar-
rier to surmount. The barrier for hydrogen atom abstraction then
increases slightly for the 1,7- and 1,8-shifts. By using branched
hydrocarbons in the gasoline, branched RO2 radicals are formed
which have larger barriers to surmount to form the QOOH species
than do straight chain peroxy radicals.
In addition to peroxy radicals being important combustion
intermediates, they are also relevant in atmospheric oxidation
cycles of hydrocarbons [17–19]. When hydrocarbons enter into
the atmosphere, they can encounter OH (or depending on the cir-
cumstances Cl) which will abstract a hydrogen atom forming an
alkyl radical. The alkyl radical reacts with O2 and a third body, typ-
ically N2, which collisionally stabilizes the peroxy radical. In a clean
atmosphere, RO2 will undergo self-reaction and cross-reactions
with another R0O2 or HO2 to form alcohols and other end products.
However, in polluted atmospheres organic peroxy radicals will
react with NO to produce NO2 which can be photolyzed by sunlight
to produce oxygen atoms that react with O2 to produce ozone (O3)
[20–22]. This specific set of reactions has been found to produce
⇑
Corresponding author. Fax: +1 (614) 292 1685.
E-mail address: tamiller@chemistry.ohio-state.edu (T.A. Miller).
http://dx.doi.org/10.1016/j.cplett.2014.03.087
0009-2614/Ó 2014 Elsevier B.V. All rights reserved.

150
N.D. Kline, T.A. Miller / Chemical Physics Letters 601 (2014) 149–154
and 250 cm3/min, respectively. Ringdown decays were detected
30
25
20
15
10
5
by an amplified InGaAs photodiode (Thorlabs, PDA400), and the sig-
nal is recorded via a 12-bit digitizing card (Measurement Comput-
ing). To obtain the CRDS spectra, 20–30 consecutive laser shots
were averaged at each dye-laser frequency point. Laser operation
and data acquisition were achieved by PC-based Labview software.
The scans were recorded with ꢀ1.0 cmꢁ1 laser step size and spectra
were calibrated using a wavemeter (HighFinesse, WS-7).
The chemistry required to create the peroxy radicals was
initiated by the 193 nm output from the photolysis excimer laser
(LPX120i, LambdaPhysik). The photolysis beam was focused by a
cylindrical and spherical lens to
a
rectangular shape
(13 ꢂ 0.5 cm2) and propagated perpendicular to the NIR radiation.
The excimer light passed once through the ringdown cell via UV-
grade quartz windows at the central part of the cavity. The laser
was typically fired 5 ms prior to the NIR probe light entering the
ringdown cavity. This is enough time to allow the peroxy radicals
to form but not to react further chemically or be pumped out of
the cell. The ringdown time (excimer on) determines the absorp-
tion within the cavity of the NIR light. To remove spectral artifacts
arising from water or precursor absorption, a background ring-
down trace (without excimer photolysis) was acquired immedi-
ately following the excimer-on trace and subtracted from it
leaving only spectral features of molecules produced from
photolysis.
0
2
4
6
8
10
12
Carbon Number
Regular Unleaded (87 Octane)
Premium Unleaded (92 Octane)
Figure 1. Graph (adapted from data in reference [6]) depicting the typical carbon
number distribution of regular unleaded and premium unleaded gasolines based on
volume percent.
large amounts of ozone in the troposphere where it is a major
pollutant for plants and animals, including humans.
Typically, the B—X electronic transition in the UV has been used
to spectroscopically study peroxy radicals [23,24]. While it is a
e
e
2.2. Production of radicals
strong transition and has a large cross-section (
r
ꢀ 10ꢁ18 cm2),
To produce the peroxy radicals, hydrogen atom abstraction of
the hydrocarbon precursors was used. Hydrogen atoms were
abstracted by chlorine atoms that were formed from the 193 nm
photolysis of oxalyl chloride, (COCl)2. Oxalyl chloride has a large
e
the B state is dissociative giving spectra that are broad, unstruc-
tured and cannot be used to characterize or differentiate among
organic peroxy radicals. However, over the past decade our group
has used cavity ringdown spectroscopy (CRDS) to exploit the much
absorption cross section at 193 nm (
r
= 3.8 ꢂ 10ꢁ18 cm2) and has
e
e
weaker A—X (
r
ꢀ 10ꢁ21 cm2) transition in the near IR (NIR) to pro-
been shown to be a clean source of chlorine atoms [28,29]. When
the chlorine atoms abstract a hydrogen atom, they can do it from
either a primary, secondary, or a tertiary site which will produce
a mixture of isomers. The newly formed alkyl radicals then add
O2 to give the peroxy radical. For this production method a stream
of N2, with a backing pressure ꢀ4.0 psi, was bubbled through the
liquid hydrocarbon precursors with 4.0–10.0 torr of this gas
mixture delivered to the ringdown cell. Typical partial pressures
in the cell for this method were [N2] ꢀ 30.0–33.0 torr,
[O2] ꢀ 20.0 torr, [(COCl)2] ꢀ 0.3–0.5 torr, and hydrocarbon precr-
sors = 0.2–0.4 torr.
duce spectra that can easily be used to distinguish between differ-
ent peroxy radicals and even isomers and conformers of the same
peroxy radical, for Cn (n = 1–5) [25]. Due to the significance of the
larger Cn (n = 6–10) hydrocarbons in gasoline and other commer-
cial products we have recently extended those studies to include
the C6–C10 peroxy radicals and present the findings in this Letter.
2. Experimental
2.1. Ringdown apparatus
The CRDS apparatus used to observe the NIR peroxy radical tran-
sitions has been described in detail previously [26,27]. The 532 nm
output of a 20 Hz pulsed Nd:YAG (Spectra Physics, Quanta-Ray
Pro 270) was used to pump a dye laser (Sirah, PrecisionScan) to give
50–90 mJ/pulse over the 645–581 nm range. The laser dyes used
were DCM, Rhodamine 101, and Rhodamine B (Exciton). Radiation
from the dye laser was focused into a 70 cm single-pass Raman cell
charged with 325–350 psi of H2. The desired second Stokes compo-
nent (7200–8900 cmꢁ1; 1–2 mJ/pulse) of the stimulated Raman
scattered radiation was isolated via a long pass filter (Newport,
1000 nm LP) and directed into the ringdown cell. The ringdown cell
is 55 cm in length and is terminated by two highly reflective mir-
3. Results and discussion
3.1. Spectral/structural relationships of peroxy radicals
Historically ab initio calculations were done to aid in the assign-
ment of peroxy radical spectra. Our group has successfully used the
G2 method of calculation to accurately predict origin frequencies
(within 100 cmꢁ1) and used DFT methods to calculate vibrational
e
e
frequencies for both the X and A states [30,31]. However, due to
the large size of the peroxy radicals in this Letter it was not feasible
to do the kind of calculations that have been done previously.
Therefore, to assign the spectra obtained we will use the spec-
tral/structural relationships that have been derived through the
rors (Los Gatos Research; 1.3 lm, P99.995%; 1.2 lm, P99.995%).
e
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The inner 20 cm portion of the cell is designed with sample gas
inlets on either end, a central vacuum port, and two rectangular
UV-grade quartz photolysis windows. The cell has a depth of
15.2 cm along the direction of the photolysis beam so that the
quartz windows are spatially removed from the reactant flow zone.
The ringdown mirrors and photolysis windows are protected from
corrosion and soot deposits by flowing nitrogen over them at 10
prior work done on the A—X transition of smaller peroxy radicals
[25]. Figure 2 shows the dependence of the origin frequency on
substitution and specific conformer of the peroxy radical. Origin
frequencies for the peroxy radicals appear in the 7300–
7800 cmꢁ1 range and highly depend on the substitution of the rad-
ical and its geometrical conformation. In addition to the origin
band, other characteristic transitions of peroxy radicals include

N.D. Kline, T.A. Miller / Chemical Physics Letters 601 (2014) 149–154
151
7900
7800
7700
7600
7500
7400
7300
7200
to the blue of the origin which is most likely the COO bend transi-
tion of the peroxy radical. The best experimentally determined fre-
quencies for the observed transitions are given in Table 1 and have
an estimated error of 10 cmꢁ1 due to there being several isomers/
conformers that may overlap under a single peak and a limited sig-
nal-to-noise ratio.
The origin frequencies for the straight chain peroxy radicals
show a slight upward trend for C6–C9 and then there is a decrease
for C10. It was somewhat surprising to see the decrease at C10, how-
ever, there may be additional chemical processes occurring as
there is evidence of HO2 (sharp structure from ꢃ7800–
8100 cmꢁ1) which is not present in any other spectra obtained.
Observing the spectrum at longer delay times gives some insight
to what is occurring. The upper right inset in Figure 3 shows the
7300–7800 cmꢁ1 region of the decyl peroxy spectrum at different
delay times. At the 5 ms delay time (black trace) there is some
unidentifiable structure ꢀ7385 cmꢁ1; as we increase the delay
time it becomes obvious that this is the origin of methyl peroxy
[32]. The appearance of methyl peroxy in this situation is quite
curious as now both it and HO2 appear in the decyl peroxy spec-
trum while not appearing in any others. As a check of the chemis-
try, a photolysis experiment was performed in the absence of the
oxalyl chloride to see if the methyl peroxy or HO2 formation could
be due to photolysis of the decane precursor itself. When oxalyl
chloride was not present none of the above spectra (decyl peroxy,
HO2, nor methyl peroxy) are observed. This means that the initial
hydrogen atom abstraction of the alkane precursor is required.
We also believe it suggests the formation of HO2 and methyl per-
oxy is related and could be part of additional chemistry that the
decyl peroxy radical is undergoing. The exact mechanism to pro-
duce HO2 and methyl peroxy from decyl peroxy is unclear but
there are a number of chemical pathways that are available to
decyl peroxy once it is formed including direct elimination of
HO2 to give an alkene (as stated in the introduction), and the ter-
minal oxygen atom of the peroxy moiety internally abstracting
an alkyl hydrogen to give the QOOH species which can also elimi-
nate HO2 to give an alkene. One could easily imagine methyl per-
oxy, and possibly larger alkyl peroxy, radicals being formed as
part of a reaction pathway including the QOOH species.
0
1
2
3
4
5
6
7
8
9
10
11
Carbon Number
Primary Peroxies (T1... Conformers)
Primary Peroxies (G1G2...Conformers)
Primary Peroxies (G1T2..G1'G2...Conformers)
Primary Peroxies (Current Study)
Secondary Peroxies (T1...Conformers)
Secondary Peroxies (G1...Conformers)
Secondary Peroxies (Current Study)
Tertiary Peroxy Radicals (t-butyl peroxy)
Tertiary Peroxy Radicals (Current Study)
Given that HO2 and methyl peroxy are formed, the effect their
presence can have on the spectrum should be addressed. The
HO2 signals are far enough from any decyl peroxy signal that they
probably have very little influence on the decyl peroxy spectrum
itself. However, methyl peroxy absorbs at lower energies than
decyl peroxy and its spectrum partially overlaps with decyl peroxy.
Larger alkyl peroxy radicals will still absorb to the red of decyl, but
closer in frequency so that their bands are even more overlapped.
This may be enough to give the appearance that the decyl peroxy
band center has been red shifted when in actuality there are other
peroxy radical absorbers present that are influencing the band.
This could explain why the decyl peroxy origin and OO stretch
bands do not seem to follow the upward trend in frequency of
the C6–C9 peroxy radicals. However, while the argument is sugges-
tive it is not definitive.
e
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Figure 2. Graph showing the dependence of A—X origin frequency on peroxy
radical species and its structure, based on data presented in reference [25] and
results from the present Letter.
an OO stretch band and sometimes a COO bend band that appear
ꢃ900 cmꢁ1 and 400–500 cmꢁ1 to the blue of the origin, respec-
tively. These relationships between substitution/conformer of the
peroxy radical and origin values and the typical shifts from the ori-
gin of COO bending and OO stretching will be applied to assign the
current spectra.
3.2. Peroxy radical spectra and assignments
Figure 3 shows the CRDS spectra resulting from the straight
chain hydrocarbon precursors that were studied (hexane–decane).
For each hydrocarbon precursor, oxygen dependent and time
dependent tests were performed to provide chemical evidence that
a peroxy radical was indeed the carrier of the observed spectrum.
In each trace there is a very intense band ꢃ7590 cmꢁ1 and another
almost as intense band about 900 cmꢁ1 to the blue. The very
intense bands ꢃ7590 cmꢁ1 can be assigned to the origin bands of
the corresponding alkyl peroxy radical. The aforementioned transi-
tions ꢀ900 cmꢁ1 to the blue of the origin bands in all the spectra
are the OO stretch bands of the peroxy radicals. In the hexyl peroxy
(red) trace there also appears to be another transition ꢀ450 cmꢁ1
Comparing with the values presented in Figure 2, the positions
e
e
of the A—X origin bands demonstrate that the predominant carri-
ers are the secondary peroxy radical isomers in the G1. . . conforma-
tion. This result is primarily an outcome of the chemistry used to
generate the peroxy radicals. Each straight chain hydrocarbon used
as a precursor has six primary sites from which a hydrogen atom
can be abstracted. However, in each precursor there are more sec-
ondary sites than primary sites with ratios (secondary:primary) of
1.33:1 for hexane, 1.67:1 for heptane, 2:1 for octane, 2.33:1 for
nonane, and 2.67:1 for decane giving more secondary sites to
abstract a hydrogen from. In addition the reported, relative
reactivities [26,33] of hydrogen atoms to abstraction, 2:9:18

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N.D. Kline, T.A. Miller / Chemical Physics Letters 601 (2014) 149–154
14
12
10
8
6
4
2
100
80
60
40
20
0
0
7400
7600
7800
8000
Wavenumber
CH3-(CH2)n-CH3
n=4
n=5
n=6
n=7
-20
n=8
7200
7400
7600
7800
8000
8200
8400
8600
8800
-1
Wavenumber (cm
)
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Figure 3. Main figure is A—X spectra of peroxy radicals from hexane-decane (straight chain) precursors. Red (top) trace is hexyl peroxy, blue trace is heptyl peroxy, green
trace is octyl peroxy, pink trace is nonyl peroxy, and black (bottom) trace is decyl peroxy. In the hexyl peroxy trace there is some interference that appears in the spectrum
from 8230–8489 cmꢁ1 caused by incomplete subtraction from precursor absorption in that region. The sharp line structure in the n = 8 likely arises from HO2. Upper right
inset is 7300–8000 cmꢁ1 region of decyl peroxy A—X spectrum at different time delays. Black (top) trace is 5
ls time delay, blue trace is 100
ls, pink trace is 500
l
s time
e
e
delay, and red trace is 1 ms delay. Structure near 7383 cmꢁ1 and 7488 cmꢁ1 are the origin and 1211 bands, respectively, of methyl peroxy [32]. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this Letter.)
Table 1
chain peroxy radicals studied presumably due to the fact that it has
primary, secondary, and tertiary sites for hydrogen atom abstrac-
tion. In the origin region the iso-octyl peroxy trace has two intense
Experimental origin frequencies, COO bend vibrations, and OO stretch bands of
hexyl–decyl peroxy radicals.
bands (ꢀ7500 and ꢀ7800 cmꢁ1) compared to the one intense band
of the straight chain peroxy radicals. The ꢀ7800 cmꢁ1 band is sig-
nificantly blue shifted from the other origin bands that have been
observed in this Letter. In Figure 2, there is a data point at
7757 cmꢁ1 representing the origin of t-butyl peroxy, a tertiary per-
oxy radical. We therefore assign the ꢀ7800 band of the iso-octyl
peroxy spectrum as the likely origin of the tertiary isomer.
Peroxy
radical
Origin
COO bend
(cmꢁ1
OO stretch
(cmꢁ1
(cmꢁ1
)
)
)
Hexyl
Heptyl
Octyl
Nonyl
Decyl
7584(10)
7587(10)
7591(10)
7592(10)
7576(10)
8053(10)
8511(10)
8515(10)
8520(10)
8526(10)
8523(10)
–
–
–
–
However, the band near 7500 cmꢁ1 has more than one possible
assignment. In Figure 2, the primary peroxy radicals in the G1G2. . .
(primary:secondary:tertiary), indicate that secondary hydrogens
are 4.5 times more reactive to abstraction than primary hydrogens.
Moreover, the origin frequencies (see Figure 2) of the primary per-
oxy radicals and secondary peroxy radicals (T1. . . conformation)
are not consistent with our experimental data. Origin positions
for primary peroxy radicals in the T1. . . and G1G2. . . conformation
are noticeably red shifted from our experimental data, while origin
values for primary and secondary peroxy radicals in the G1T2. . .
G01G2. . . and T1. . . conformation, respectively, are much more scat-
tered and do not correlate well with our data. (One exception is the
T1. . . conformer of 2-propyl peroxy which has an origin of
7692 cmꢁ1.) Taking into account the relative abundances and reac-
tivities of primary to secondary hydrogen atoms as well as origin
frequencies support assigning the carriers of the spectra in Figure 3
to the corresponding straight chain peroxy radicals, secondary iso-
mers in the G1. . . conformation.
conformation have origin frequencies from 7480 to 7592 cmꢁ1
,
with all but one origin falling into the 7480–7551 cmꢁ1 range.
One could easily argue that an origin value of 7503 cmꢁ1 falls well
within this range and make the assignment that this band belongs
to a primary isomer of iso-octyl peroxy in the G1G2. . . conforma-
tion. But if a rough calculation is made to predict band intensities
of the different isomers using the relative abundances of
primary:secondary:tertiary (p:s:t) abstraction sites (15:2:1) along
with relative reactivities of Cl atoms to p:s:t H atoms (2:9:18) we
obtain relative intensities for the p:s:t isomer bands of 30:18:18.
This predicts that the primary isomer origin will be 2/3 stronger
than either the secondary or tertiary origin bands while the transi-
tions we observe in our spectrum are the same intensity. This argu-
ment would be consistent with the band at 7503 cmꢁ1 belonging to
the secondary isomer even though it is red shifted nearly 60 cmꢁ1
from any other secondary isomer origin band. Moreover this latter
assignment would also leave unanswered the lack of any observa-
tion of a band attributable to the primarily isomer despite predic-
tions that it should be most abundant. Furthermore we searched
for spectra to the red of 7300 cmꢁ1 and could not see any structure
we could attribute to any peroxy radical, whereas assignment of
Figure 4 shows the spectrum when iso-octane is the precursor
and it is compared to the n-octyl peroxy spectrum. The same
chemical tests were performed on the iso-octane trace and it again
displayed behavior consistent with that of a peroxy radical. The
iso-octyl peroxy spectrum appears more complex than the straight

N.D. Kline, T.A. Miller / Chemical Physics Letters 601 (2014) 149–154
153
70
60
50
40
30
20
7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 9200
Wavenumber (cm-1
)
e
e
Figure 4. A—X spectra of iso-octyl peroxy blue (bottom) trace produced from iso-octane precursor (structure in upper right corner of Figure) and n-octyl peroxy red (top)
trace. Primary hydrogen abstraction sites on iso-octane are labeled with an ‘‘a’’, secondary sites with a ‘‘b’’, and tertiary sites with a ‘‘c’’. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this Letter.)
the 7503 cmꢁ1 band to a secondary isomer would lead to the
expectation of a primary isomer origin in this region. Regardless
of the assignment it is surprising that we do not observe origin
bands from each the p:s:t conformer.
60
50
40
30
20
10
A
A’
It is probably worth noting that the relative reactivities of Cl
atoms to p:s:t H atoms was determined through experimentation
on n-butane and iso-butane. Iso-octane is relatively complex com-
pared to either of those molecules and it is unknown whether the
relative reactivities of Cl atoms to p:s:t H atoms will hold quanti-
tatively for iso-octane in this situation. We therefore tentatively
assign the band near 7500 cmꢁ1 as the primary isomer based on
the spectral/structural relationships that we have derived, but
clearly note that we are puzzled by the absence of a third band
for iso-octane.
In addition to the spectral/structural relationships, one can also
utilize self-reaction rate information to determine to which isomer
a given band belongs. In our experiments we have the ability to
delay the time between the excimer photolysis pulse and the NIR
probe pulse. Figure 5 shows the results of these time delay exper-
iments. At long delay times there are three bands, labeled as A0, B0,
and C0, that are still clearly visible. Long lived spectral lines were
observed in our previous investigation of tertiary radicals [26]. In
that study a much slower self-reaction rate constant was found
for the t-butyl peroxy radical than for its secondary and primary
isomer counterparts. These additional observations strongly sup-
port the assignment of bands A0, B0, C0 to the tertiary decyl peroxy
isomer.
C’
B
B’
7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 9200
Wavenumber (cm-1)
e
e
Figure 5. Time delay tests of A—X spectrum of iso-octyl peroxy. Red (top) trace is
taken at a 5 ms delay of NIR probe from the excimer photolysis beam, blue trace
taken at 500 ms delay of NIR probe, and black trace taken at 1 ms delay of NIR probe.
Labeling of bands signifies assignment of transition to primary or tertiary isomer. A
and B bands are tentatively assigned to primary isomer while A0, B0, and C0 bands
belong to tertiary isomer. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this Letter.)
Additional information about the isomers can be obtained by
estimating the initial radical concentrations. Melnik et al. [34]
found their chlorine atom concentration to be 2 ꢂ 1015 mole-
cules/cmꢁ3; since we are using similar conditions that should be
a good approximation for the concentration of chlorine atoms in
our current Letter. Looking at Figure 4, the two bands near 7500
and 7800 cmꢁ1 have nearly the same intensity. If we assume equal
oscillator strengths we can infer the isomers are formed in a 1:1
ratio. Given our initial chlorine atom concentration, this will yield
a peroxy radical concentration of 1 ꢂ 1015 molecules/cm3 for each
of the species present in our spectrum.
peroxy radicals do not seem to decay at all. This consistent with
a reported self-reaction rate constant of 2 ꢂ 10ꢁ17 cm3 mole-
culeꢁ1 sꢁ1 [21] for tertiary peroxy radicals, which would make
their self reaction not observable on our millisecond timescale,
with comparable observations being obtained in similar experi-
ments [26,21]. From Figure 5 we can estimate a half-life of
ꢀ500
ls for bands A and B and obtain self reaction rate constants
of ꢀ2 ꢂ 10ꢁ12 cm3 moleculeꢁ1 sꢁ1. Unfortunately this rate constant
does not allow us to differentiate between isomers, since it is
roughly consistent with either primary or secondary peroxy radi-
cals. Based on a combination of the self-reaction rates and the
spectral/structural relationships our conclusion therefore is that
We can estimate the half-life of the peroxy radicals from the
data in Figure 5. The bands that we have assigned as the tertiary

154
N.D. Kline, T.A. Miller / Chemical Physics Letters 601 (2014) 149–154
Table 2
References
Spectral assignments for iso-octyl peroxy.
[1] S. Wang, D.L. Miller, N.P. Cernansky, H.J. Curran, W.J. Pitz, C.K. Westbrook,
Band
Frequency
Assignment
Combust. Flame 118 (1999) 415.
A
7503
7804
8230
7597
8734
Origin of primary isomer
Origin of tertiary isomer
COO bend of tertiary isomer
OO Stretch of primary isomer
OO Stretch of tertiary isomer
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A0
B0
B
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C0
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we can definitively assign bands A0, B0, and C0 to the tertiary isomer.
Using the spectral/structural relationships, we feel it is more likely,
but not certain, that bands A and B belong to the primary isomer.
These results are summarized in Table 2.
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4. Conclusion
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e
e
The A ꢁ X absorption spectra of hexyl–decyl (straight chain)
peroxy radicals and iso-octyl peroxy have been recorded and
assigned. Spectral assignments were made through the spectral/
structural relationships that have been determined through previ-
ous work on peroxy radicals. All of the straight chain peroxy radi-
cals studied were found to have origin values between 7550 and
7600 cmꢁ1 and OO stretch bands about 900 cmꢁ1 to the blue of
the origin, with hexyl peroxy also exhibiting evidence of a COO
bending band. The positions of the origin bands of the straight
chain peroxy radicals indicate that the main carriers of the spectra
are the secondary peroxy isomers. The iso-octyl peroxy spectrum
shows evidence that both the primary and tertiary peroxy isomers
are formed. Assignments of the iso-octyl peroxy spectrum were
further confirmed by examining the temporal behavior of the
bands in the spectrum and finding that some of the bands were
long lived (>1 ms) which is expected for the tertiary peroxy
isomers.
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Acknowledgement
The authors acknowledge the financial support of this Letter by
the US Department of Energy, via grant DE-FG02–01ER14172.