The Journal of Physical Chemistry A
ARTICLE
ꢀ
3
concentrations (in molecule cm ) were CH ONO and NO,
3
1
4
∼
2.4 ꢁ 10 each; and organic and reference compound, ∼2.4 ꢁ
1
3
1
0 each. n-Octane was used as the reference compound, apart
from experiments with cycloheptane, which coeluted with n-
octane on the gas chromatographic column used. The rate
constant for cycloheptane was therefore determined relative to
that for cyclooctane. Irradiations were carried out at 20% of the
maximum light intensity for up to 16ꢀ27 min, resulting in up to
2
8ꢀ67% of the initially present organic or reference compound
being consumed by reaction.
The concentrations of the organics and reference compounds
were measured during the experiments by gas chromatography
3
with flame ionization detection (GC-FID). Gas samples of 100 cm
volume were collected from the chamber onto Tenax-TA adsor-
bent, with subsequent thermal desorption at ∼205 °C onto a
3
0 m DB-1701 megabore column, initially held at ꢀ40 °C and
ꢀ
1
then temperature-programmed to 250 °C at 8 °C min . Dur-
ing each experiment, the following GC-FID analyses were
conducted: at least two replicate analyses prior to reaction,
one analysis after each of three irradiation periods, and a replicate
analysis after the third (and last) irradiation period. Replicate
analyses of the cycloalkanes, cycloketones, and n-octane showed
that the measurement uncertainties were typically <3%.
Figure 1. Plots of eq I for reactions of OH radicals with (4) cyclo-
octane and (0) cyclodecane, with n-octane as the reference compound.
Data are from three experiments for each cycloalkane.
were sampled through a 25 mm diameter, 75 cm length, Pyrex
ꢀ
1
Product Studies. Analyses by Gas Chromatography. OH
radical-initiated reactions of cycloheptane, cyclooctane, and
cyclodecane were carried out to measure the formation yields
of the corresponding cycloketones, with reactants and products
being collected onto Tenax solid adsorbent and analyzed by
GC-FID as described above. The initial concentrations (mole-
tube at ∼20 L min directly into the atmospheric pressure
ionization (API) MS source. The operation of the API-MS in the
MS (scanning) and MS/MS [with collision-activated dissocia-
6
,11
tion (CAD)] modes has been described previously.
positive and negative ion modes were used in this work. In
positive ion mode, protonated water hydrates [H O (H O) ]
generated by the corona discharge in the chamber diluent
air were responsible for the formation of protonated molecules
([M + H] ), water adduct ions [M + H + H
Both
+
3 2 n
ꢀ
3
cule cm ) of CH ONO, NO, and cycloalkane were ∼2.4 ꢁ
3
1
4
14
13
1
0 , ∼2.4 ꢁ 10 , and (2.20ꢀ2.41) ꢁ 10 , respectively.
+
+
Irradiations were carried out at 20% of the maximum light
2
O] , and protonated
11
ꢀ
intensity for up to 3ꢀ24 min.
homo- and heterodimers, while in negative ion mode, O
2
,
ꢀ
ꢀ
ions were responsible for formation of adduct
3
An additional series of CH ONOꢀNOꢀair irradiations of
NO
ions.
2
1, and NO
3
1
cyclohexane (dry), cycloheptane (dry and at 50% RH), cyclo-
octane (dry), cyclodecane (dry), cyclooctane + cyclodecane
ꢀ
3
The initial concentrations (molecule cm ) were CH ONO,
3
13
13
13
(
50% RH), and cyclohexane + cycloheptane + cyclooctane +
∼4.8 ꢁ 10 ; NO, ∼4.8 ꢁ 10 ; and cycloalkane, ∼2.4 ꢁ 10 .
The reactant mixtures were irradiated for 3ꢀ15 min at 20% of
maximum light intensity, resulting in 14ꢀ35% consumption of
the initially present cycloalkane as measured by GC-FID; note
that experiments were carried out at low extents of reaction
(14ꢀ19%) for each cycloalkane.
cyclodecane (dry and at 50% RH) were carried out, with a single
irradiation period. GC-FID analyses of the cycloalkanes and of
their cycloketone products were conducted as described above.
For the analysis of cyclohexane, gas samples were also collected
into a 100 cm all-glass gastight syringe and transferred via a
1
3
3
cm gas sampling loop onto a 30 m DB-5 megabore column
Chemicals. The chemicals used, and their stated purity levels,
were cyclohexane (99.9+%), cycloheptane (98%), cyclooctane
(99+%), cyclodecane (samples of 95% and 98% purity were
used), cyclohexanone (99.8%), cycloheptanone (99%), cyclo-
octanone (99%), cyclodecanone (97%), and n-octane (99+%),
from Aldrich; O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine
hydrochloride (99+%), from Lancaster; cyclohexyl nitrate, from
Fluorochem, Inc.; and NO (g99.0%), from Matheson Gas
Products. Methyl nitrite was prepared as described by Taylor
initially held at ꢀ25 °C and then temperature-programmed at
ꢀ
1
8
°C min . Samples were also collected, starting immediately
ꢀ
1
after the lights were turned off, for 60 min at 15 L min
by use of an XAD resin-coated denuder, further coated with
O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) prior
to sampling to derivatize carbonyls to their oximes, and extracted as
10
described previously. The extracts were analyzed by combined gas
chromatographyꢀmass spectrometry in both positive chemical ioni-
zation (PCI GC-MS) and negative chemical ionization (NCI GC-
MS) modes and by GC-FID, with all analyses using 30 m DB-5
columns. The GC-MS analyses used an Agilent 5973 mass-selective
detector operated in the scanning mode with methane as the reagent
gas. Each carbonyl group derivatized to an oxime adds 195 mass units
to the compound’s molecular weight (MW), and methane-PCI gives
12
et al. and stored at 77 K under vacuum.
’
RESULTS AND DISCUSSION
Kinetic Experiments. The experimental data from CH O-
3
NOꢀNOꢀorganicꢀreference compoundꢀair irradiations are
plotted in accordance with eq I in Figures 1ꢀ3, and the rate
constant ratios k /k obtained from least-squares analyses of
+
characteristic protonated molecules ([M + H] ) and smaller adduct
+
+
ions at [M + 29] and [M + 41] . In addition, hydroxycarbonyls have
4
5
+
characteristic [M + H ꢀ H O] fragment ions.
these data are given in Table 1. The rate constant ratios k4/
k (n-octane) are placed on an absolute basis by use of a value of
k5(n-octane) = 8.11 ꢁ 10
2
Analyses by API-MS/MS. CH ONOꢀNOꢀcycloalkaneꢀair
3
5
ꢀ
12
3
ꢀ1 ꢀ1
3
irradiations were carried out during which the chamber contents
cm molecule
s
at 298 K,
1
4454
dx.doi.org/10.1021/jp209654h |J. Phys. Chem. A 2011, 115, 14452–14461