Cold-surface photochemistry of selected organic nitrates

Article abstract of DOI:10.1021/jp5065424

Reflection-absorption infrared (RAIR) spectroscopy has been used to explore the low temperature condensed-phase photochemistry of atmospherically relevant organic nitrates for the first time. Three alkyl nitrates, methyl, isopropyl, and isobutyl nitrate together with a peroxyacyl nitrate, peroxyacetyl nitrate (PAN), were examined. For the alkyl nitrates, similar photolysis products were observed whether they were deposited neat to the gold substrate or codeposited with water. In addition to peaks associated with the formation of an aldehyde/ketone and NO, a peak near 2230 cm^-1 was found to emerge in the RAIR spectra upon UV photolysis of the thin films. Together with evidence obtained by thermal programmed desorption (TPD), the peak is attributed to the formation of nitrous oxide, N₂O, generated as a product during the photolysis. On the basis of the known gas-phase photochemistry for the alkyl nitrates, an intermediate pathway involving the formation of nitroxyl (HNO) is proposed to lead to the observed N₂O photoproduct. For peroxyacetyl nitrate, CO₂ was observed as a predominant product upon photolytic decomposition. In addition, RAIR absorptions attributable to the formation of methyl nitrate were also found to appear upon photolysis. By analogy to the known gas-phase and matrix-isolated-phase photochemistry of PAN, the formation of methyl nitrate is shown to likely result from the combination of alkoxy radicals and nitrogen dioxide generated inside the thin films during photolysis.

Full text of DOI:10.1021/jp5065424

Article
pubs.acs.org/JPCA
Cold-Surface Photochemistry of Selected Organic Nitrates
Daniel O’Sullivan,*,§ Ryan P. McLaughlin,*,‡ Kevin C. Clemitshaw,∥ and John R. Sodeau*,†
†Department of Chemistry, Centre for Research into Atmospheric Chemistry, University College, Cork, Ireland
‡Department of Chemistry, Seattle University, 901 12th Avenue, Seattle, Washington 98122, United States
∥Department of Earth Sciences, Royal Holloway, University of London Egham, Surrey TW20 0EX, U.K.
ABSTRACT: Reflection−absorption infrared (RAIR) spectroscopy has been used to explore the
low temperature condensed-phase photochemistry of atmospherically relevant organic nitrates for
the first time. Three alkyl nitrates, methyl, isopropyl, and isobutyl nitrate together with a peroxyacyl
nitrate, peroxyacetyl nitrate (PAN), were examined. For the alkyl nitrates, similar photolysis
products were observed whether they were deposited neat to the gold substrate or codeposited with
water. In addition to peaks associated with the formation of an aldehyde/ketone and NO, a peak
near 2230 cm−1 was found to emerge in the RAIR spectra upon UV photolysis of the thin films.
Together with evidence obtained by thermal programmed desorption (TPD), the peak is attributed to the formation of nitrous
oxide, N2O, generated as a product during the photolysis. On the basis of the known gas-phase photochemistry for the alkyl
nitrates, an intermediate pathway involving the formation of nitroxyl (HNO) is proposed to lead to the observed N2O
photoproduct. For peroxyacetyl nitrate, CO2 was observed as a predominant product upon photolytic decomposition. In
addition, RAIR absorptions attributable to the formation of methyl nitrate were also found to appear upon photolysis. By analogy
to the known gas-phase and matrix-isolated-phase photochemistry of PAN, the formation of methyl nitrate is shown to likely
result from the combination of alkoxy radicals and nitrogen dioxide generated inside the thin films during photolysis.
Talukdar et al. report the quantum yield of channel (1)
approaches unity (λ = 248 nm) for methyl nitrate,3 whereas for
the C3−C5 alkyl nitrates, Zhu et al. have also found quantum
yields of unity following photodissociation at 308 nm.16
At wavelengths above 240 nm, the UV absorption spectra of
the alkyl nitrates are dominated by an n−π* transition, with
absorption cross sections increasing with both the degree of
substitution and carbon chain length.2,17 Notably, however, the
absorption cross sections trail off rapidly in the actinic region (λ
> 290 nm), limiting the rates of photolysis of the alkyl nitrates
in the troposphere.2,3,6 Estimated surface level lifetimes (due to
both photolysis and reaction with hydroxyl radicals) at
Northern midlatitudes vary from weeks during the winter to
days during the summer.2 Consequently, alkyl nitrates remain
sufficiently long-lived to permit their long-range transport from
sources and can impact upon tropospheric photochemistry in
regions as remote as the poles.2,3 For instance, in the Arctic,
alkyl nitrates have been found to be substantial contributors to
the NOy budget, comprising 10−20%,9,10 whereas at the
Antarctic alkyl nitrates have also been found to represent a
major component of the total reactive nitrogen oxides.11,12
For peroxyacetyl nitrate (PAN), both gas-phase thermolysis
and photolysis play important roles in its removal from the
atmosphere, whereas reaction with hydroxyl radical is too slow
to be of importance throughout the troposphere.4,5 Thermol-
ysis has been extensively detailed18−21 and proceeds to yield
acylperoxy radicals and nitrogen dioxide via
INTRODUCTION
■
It is widely recognized that the oxides of nitrogen, NO and
NO2 (NO + NO2 ≡ NOx), play critical roles in the chemistry of
the lower atmosphere, participating in the production of ozone
and modulating the lifetime of the hydroxyl radical.1
Consequently, significant efforts have been devoted to
characterizing the atmospheric fates of species that can act as
precursors and sinks for NOx. Of particular interest to
researchers examining the chemistry of remote regions have
been organic nitrates, such as alkyl nitrates (RONO2) and
peroxyacyl nitrates (RC(O)OONO2). Owing to their relative
stability toward decomposition, these species can undergo long-
range transport away from sources of pollution.2−4 Upon
decomposition, alkyl and peroxyacyl nitrates can produce
reactive NOx in areas far removed from their formation. Given
the importance of the organic nitrates as temporary NOx
reservoirs, characterization of their photolysis pathways has
been the subject of a substantial number of studies.2,3,5−8
However, the potential role of ice surfaces in the photochemical
processing of organonitrates has not yet been investigated,
despite the fact that alkyl nitrates are thought to be a significant
contributor to the total reactive nitrogen oxides (NOy) at the
poles.9−12
In the troposphere, alkyl nitrates tend to be stable toward
thermolysis5 but are susceptible to gradual decomposition by
photolysis and reaction with the hydroxyl radical.2,3 To date,
the photolysis pathways and UV absorption cross sections of
several alkyl nitrates have been examined in the gas
phase.3,6,7,13−16 The main photodissociation channel for the
alkyl nitrates is thought to be3,8,13−15
Received: July 1, 2014
Revised: September 17, 2014
RONO2 + hv → RO + NO2
(1)
© XXXX American Chemical Society
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(EPIC 300, Hiden Analytical) to analyze gas-phase products
released from the thin films, using electron impact ionization
with 70 eV ionization potential. Thermal programmed
desorption (TPD) measurements were conducted at a ramp
rate of 0.8 K s−1. Photolysis experiments were conducted using
an unfiltered broadband Xe arc lamp source (Oriel, model
60010, λ > 230 nm), with the samples exposed through a quartz
window on the side of the chamber. Dosing of the vapors onto
the cold substrate (Tinitial ∼ 113 K) was conducted by means of
either manually operated valves or piezoelectric pulsed valves.
Estimating the number of monolayers from the exposure time
and pressure (10−6 Torr s = 1 Langmuir ≈ 1 monolayers)32
leads to an average of approximately 100 monolayers for the
experiments performed in this study. In codeposition experi-
ments, dosing was performed such that there was an excess of
the organic nitrate relative to water introduced to the chamber
(alkyl/peroxacyl nitrate:water ≈ 3:1). All samples were
degassed using three freeze−pump−thaw cycles on a vacuum
line prior to dosing into the UHV chamber. The duration of
each photolysis experiment was primarily limited by the length
of time the HgCdTe detector could be held at temperatures
low enough to permit spectral acquisition. The ability to
maintain the detector at sufficiently low temperatures by
refilling with liquid nitrogen deteriorated throughout the course
of an experiment, due to the condensation of water ice in the
cooling reservoir of the detector. With this constraint in mind,
the RAIR spectra indicated below have been selected to
highlight the largest spectral changes that were observed during
the course of photolysis experiments.
Samples of isopropyl nitrate (99%) and isobutyl nitrate
(95%) were purchased from Sigma and used without further
purification. Methyl nitrate was prepared via the low temper-
ature nitration of methanol, as described by Clemitshaw et al.2
Nitrogen gas was bubbled through a stirred mixture of 10 mL
of 99.5% nitric acid and 10 mL of 98% sulfuric acid held at 0
°C, before and during the slow dropwise addition of 7 mL of
methanol. The alkyl nitrate product was separated and washed
with Milli-Q purified water and then dried with anhydrous
magnesium sulfate. After the removal of water, the methyl
nitrate was filtered into a test tube that was stoppered and
stored in a dark freezer at −15 °C.
CH3C(O)O2NO2 + M ⇌ CH3C(O)O2 + NO2 + M
(2)
The rate of removal of PAN by thermolysis is critically
dependent on temperature. Under typical conditions found
near ground level (T = 290 K), the lifetime of PAN due to
thermal dissociation is on the order of minutes to hours,
whereas at temperatures representative of the upper tropo-
sphere, the lifetime is on the order of years (T = 250 K).22 The
gas-phase photochemistry of PAN has also been examined in
detail.23−26 Although a wide variety of photoproducts are
possible, the dominant decomposition channels for UV
photolysis are23,25,26
CH3C(O)O2NO2 + hν → CH3C(O)O2 + NO2
(3)
CH3C(O)O2NO2 + hν → CH3C(O)O + NO3
(4)
Both photolysis channels are expected to have quite different
outcomes in the atmosphere. Although the products of (3) can
recombine to re-form PAN, the instability of the acetoxy
product in pathway (4) leads to the irreversible loss of PAN
and the release of NOx following the photolysis of NO3. Mazely
and co-workers identified NO2 as the dominant photoproduct
(quantum yield, Φ
= 0.83
0.09) following pulsed laser
NO2
photolysis at 248 nm,25 whereas the wavelength-dependent
yield for NO3 has been found to vary from 0.19 to 0.41 across
the wavelength region 248−308 nm.23,24,27 However, the
measured absorption cross sections for PAN4,28 are sufficiently
low that photolysis is not an important process at altitudes <7
km, where thermolysis dominates.4 At low temperatures, PAN
is stable enough to permit the transport of NOx to remote
regions of the globe, including the poles. Anomalously high
contributions of PAN to NOy have been observed at both
poles, with [PAN]/[NOy] reaching levels of up to 0.5−0.9 in
the Arctic during springtime, as well as dominating the NOy
budget in the Antarctic.12,29,30
Past research has shown that heterogeneous transformations,
taking place at the surface of ices, can play important roles in
the atmospheric fates of important nitrogen oxides. However,
despite the significance of the organic nitrates as sources of
reactive nitrogen oxides at the poles, the heterogeneous
photochemistry of these compounds in the presence of water
ice has not been investigated to date. Here, by employing
modern surface science techniques such as reflection−
absorption infrared spectroscopy (RAIRS) in tandem with
thermal programmed desorption (TPD) measurements, the
condensed-phase photochemistry of three alkyl nitrates (R=
methyl, isopropyl, and isobutyl) and peroxyacetyl nitrate have
been examined.
Peroxyacetyl nitrate (PAN) was synthesized by the low-
temperature nitration of peroxyacetic acid as described by
Nikitas et al.33 Nitrogen gas was bubbled through a stirred
solution containing 3.3 mL of peroxyacetic acid and 2.6 mL of
98% sulfuric acid dissolved in 40 mL of tridecane held at 0 °C
in an ice bath for 5 min before, during, and for 10 min after the
dropwise addition of 0.7 mL of 99.5% nitric acid. To remove
unreacted acids after the nitration step, the PAN product (in
the tridecane solvent) was washed with Milli-Q purified water
held at 0 °C. After the product was dried by adding anhydrous
magnesium sulfate, the PAN solution was filtered into a test
tube, which was stoppered and stored in a dark freezer at −15
°C.
EXPERIMENTAL SECTION
■
Detailed descriptions of the experimental apparatus have been
discussed elsewhere,31 and therefore only a brief overview is
presented here. Thin film samples were dosed onto a gold
substrate contained within an ultrahigh vacuum (UHV)
chamber. The chamber pressure was maintained at <10−8
mbar between experiments, and the gold foil was routinely
cleaned between measurements by heating to 500 K. FTIR
spectra collected in the reflection absorption mode were
collected using a Bruker Vertex 70 FTIR spectrometer
equipped with a liquid-nitrogen-cooled HgCdTe detector.
Spectra were collected at a resolution of 2 cm−1. The UHV
chamber was also fitted with a quadrupole mass spectrometer
RESULTS
■
Methyl Nitrate. The RAIR spectra of methyl nitrate,
MeONO2, both deposited neat to the gold substrate and
codeposited with water, are illustrated in Figure 1. The spectral
assignments are presented in Table 1, where previous values
obtained from the literature are also listed for comparison.
A number of features characteristic of the alkyl nitrates are
immediately evident in the RAIR spectrum of methyl nitrate
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Figure 1. RAIR spectrum of methyl nitrate deposited neat (red trace)
and codeposited with water (blue trace) at 113 K.
Figure 2. RAIR difference spectrum illustrating the changes observed
upon photolysis of a thin methyl nitrate film that was codeposited with
water. Shown are the difference spectra after 1 h (red trace), 2 h (blue
trace), and 3 h (green trace).
Table 1. RAIR Normal Mode Assignments for a Thin Methyl
a
Nitrate Film Co-deposited with Water
is characteristic of a carbonyl stretching vibration,36 is also
accompanied by the development of bands at 1499 and 1249
cm−1, which grow at similar rates. Together, these three bands
were found to correspond well with transmission IR features
previously reported for solid formaldehyde.37 Two further weak
features not attributable to formaldehyde were also present at
1865 and 2230 cm−1. The former peak is readily identifiable as
the N−O stretching vibration of nitric oxide, NO.38 Notably,
the latter band at 2230 cm−1 has previously been found to arise
due to a photolysis product in low temperature RAIR spectra of
primary and tertiary alkyl nitrites (RONO), where it was shown
to result from the asymmetric stretch of nitrous oxide, N2O.39
Isopropyl Nitrate. The RAIR spectra of isopropyl nitrate,
both deposited neat to the gold substrate and codeposited with
water, are presented in Figure 3. The bands observed in the
RAIR spectra are assigned in Table 2, where vibrational
wavenumbers from the IR spectrum of the neat liquid40 are also
listed for comparison.
mode description
matrix isolated34
liquid35
RAIRS
10 K, N2, MR = 50:1 298 K
113 K, codep H2O
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
ν16
ν(C−H)
ν(C−H)
νas(NO2) 1652 vs
2965 w
2930 w
3040 mw 3030
2963 m
1634 vs
2964
1656 (1658)
δ(CH3)
δ(CH3)
νs(NO2)
ρ(CH3)
ν(C−O)
1465 w
1435 w
1293 vs
1177 w
1004 s
1458 mw 1454
1429 m
1282 vs
1425
1305 (1307)
1174 mw 1179
993 s
1000
884
ν(N−O) 860 m
ρ(NO2) 760 w
859 s
759 ms
765
a
Wavenumbers in brackets are those found for the neat thin films and
are listed when they differ from those of the co-deposited films.
codeposited with water. The most intense band arising from
the presence of MeONO2 is evident at 1656 cm−1 and is due to
the asymmetric NO2 stretching vibration.34,35 Similarly, the
symmetric NO2 stretching vibration is found to absorb at 1305
cm−1. The C−O stretching vibration, also common to all alkyl
nitrates, is located in the RAIR spectrum at 1000 cm−1. A
number of features characteristic of the methyl group are also
evident in the spectrum, the most intense of which is the CH3
deformation, centered at 1425 cm−1. The methyl nitrate peak
positions for both the codeposited and neat thin films were
found to change little, with the largest spectral shift observed
being a small red-shifting (Δv = −2 cm−1) of the NO -
stretching modes in the codeposited thin film relative to the
spectra of the neatly deposited films.
̃
2
Upon irradiation of the samples with the broadband source,
features assigned to MeONO2 were observed to slowly drop in
intensity, while a number of new peaks emerged. In Figure 2,
RAIR difference spectra of a codeposited thin film are
presented to illustrate the spectral changes resulting from
photolysis over the course of 3 h. These difference spectra were
obtained by subtraction of the spectra obtained prior to
irradiation (Figure 1), from those obtained at a given time
throughout the photolysis period. Consequently, increasing
peaks indicate the formation of new species, whereas decreasing
peaks indicate loss.
Figure 3. RAIR spectra of isopropyl nitrate deposited neat (red trace)
and codeposited with water (blue trace).
Whether deposited neat or codeposited with water, the main
RAIR band positions of isopropyl nitrate were found to be
similar. The largest spectral shift observed upon codeposition
Within the first hour of photolysis, bands assigned to
MeONO2 were observed to drop in intensity, while a new IR
band at 1716 cm−1 grew in. The appearance of this peak, which
was a small red-shifting (Δv = −4 cm−1) of the NO stretching
̃
2
modes relative to the spectra of the neatly deposited films.
More significant shifts in vibrational wavenumbers can be seen
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Table 2. Assignments of RAIR Wavenumbers (cm−1)
Observed for Isopropyl Nitrate Co-deposited with Water
a
assignment
liquid40
RAIR
298 K
113 K, codep H2O
2993
νas(CH3)
ν(C−H)
νas(CH3)
νas(NO2)
δ(CH3)
2993 mw
2944 w
2903 w
1627 vs
1471 w
1457 w
1446 w
1390 mw
1378 mw
1351 w
1332 w
1282 s
2946
2906
1630 (1634)
1472
δ(CH3)
1457
δ(CH3)
1444
νs(C−C−C)
νas(C−C−C)
ρ(C−C−C)
ρ(C−C−C)
νs(NO2)
1389
1376
1350
1333
Figure 4. RAIR difference spectra of a thin film of isopropyl nitrate,
codeposited with water. Shown are the spectra taken after photolysis
for 40 min (red trace), 80 min (green trace), and 120 min (black
trace).
1286 (1290)
1185
C−C−C wag
ν(C−C)
νas(C−C)
C−C−C wag
δ(CH3)
1183 w
1145 w
1124 w
1103 m
937 w
1149
1124
1104
shown in Figure 5. Here, the initial desorption of a volatile
species with m/z = 44, 28, and 16 can be seen, consistent with
937
ν(C−O)
910 m
913
ν(N)−O
881 s
896
ρ(NO2)
853 m
854
a
Wavenumbers in brackets are those found for a neat thin film and are
listed when they varied from those found for the co-deposited thin
films.
when the RAIR spectrum of isopropyl nitrate was codeposited
with water as compared with the analogous spectrum obtained
for methyl nitrate shown in Figure 1. Relative to methyl nitrate,
the ν (NO ) mode of isopropyl nitrate is red-shifted by Δv =
̃
as
2
−26 cm−1, while a similar change of Δv = −19 cm−1 is apparent
̃
by comparison to the νs(NO2) modes. The linking C−O
stretch absorbs at 913 cm−1, which is significantly red-shifted in
frequency (Δv = −87 cm−1) relative to that observed for the
̃
Figure 5. TPD profile obtained from a codeposited isopropyl nitrate/
H2O film subjected to 2 h of photolysis. Observed are fragments
methyl analogue (Table 1). In addition to bands attributable to
the nitric ester functionality (R−ONO2), features representa-
tive of an alkyl moiety are present in the RAIR spectra.
Consistent with the increased complexity of the alkyl group,
vibrational modes not seen in the RAIR spectra of MeONO2
are present for its isopropyl analogue. Such a feature is apparent
in the 1300−1400 cm−1 region of the isopropyl nitrate RAIR
spectrum, where new vibrations arise due to C−C−C
stretching and rocking modes.
Upon photolysis of the thin films of isopropyl nitrate, either
deposited neat or codeposited with water, bands associated
with the parent organic nitrate are observed to lose intensity in
the RAIR difference spectra (Figure 4). Concurrently, a positive
band at 1705 cm−1 develops, which is attributable to a CO
stretching vibration of an aldehyde or ketone.40 The appearance
of this band is also accompanied by the development of weaker
peaks at 1425, 1368, and 1242 cm−1, which grow at a similar
rate. Together, these emerging bands are found to correspond
well with RAIR vibrational wavenumbers previously reported
for propanone deposited to water ice films at 95 K.41 Similar to
what was observed in the RAIR spectra during the photolysis of
methyl nitrate (Figure 2), a further weak band at 2230 cm−1
was found to appear upon photolysis of the thin films,
identifiable as the asymmetric stretching vibration of nitrous
oxide.39,42 The presence of N2O within the thin films after
photolysis was further probed using TPD measurements, as
+
associated with N2O (N2O+, m/z = 44; N2 , m/z =28, O+, m/z = 16)
and a fragment associated with isopropyl nitrate and propanone
(CH3CO+, m/z = 43).
the loss of N2O from the films.43 Notably, the desorption of
these fragments peaks at two distinct temperatures (157 and
180 K), similar to what has been reported when water ice is
codeposited with other volatile species, such as CO2.44 The
initial desorption peaking at 157 K can be explained on the
basis of a “volcano” desorption as described by Smith et al.,45
where volatile species trapped in a water ice matrix are released
during the formation of connected desorption pathways as
amorphous solid water converts to crystalline ice. Subsequently,
the remaining trapped volatile species are then released during
the desorption of water ice at 180 K.
Isobutyl Nitrate. The RAIR spectra of isobutyl nitrate, both
deposited neat and codeposited with water, are presented in
Figure 6. In each case, the spectra were recorded directly after
deposition at 113 K. The wavenumbers of the RAIR band
centers are assigned in Table 3, where previous literature values
obtained from IR spectra of the neat liquid40 are listed for
comparison.
Apart from a small red-shifting of νas(NO2) mode by 5 cm−1,
the presence of water had little effect on the positions of the
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Figure 7. RAIR difference spectrum of a codeposited water/isobutyl
nitrate thin film, which was photolyzed for 3.5 h (blue trace). Also
shown is the RAIR spectrum of 2-methylpropanal codeposited with
N2O at 113 K for comparison (red trace), which has been scaled down
by a factor of 5 for comparison.
Figure 6. RAIR spectra of isobutyl nitrate deposited neat (red trace)
and codeposited with water (blue trace). Both spectra were recorded
directly after deposition at 113 K.
Table 3. Assignments of RAIR Features Observed for
a
Isobutyl Nitrate Co-deposited with Water
band in the RAIR spectra due to N2O also developed upon
irradiation (λ > 230 nm). A positive band at 1708 cm−1 also
develops during photolysis, indicating the formation of an
aldehyde or ketone.36 By analogy to methyl and isopropyl
nitrate, this absorption is most likely to arise from the
formation of 2-methylpropanal (isobutanal). However, the
major hydrocarbon modes of this aldehyde were difficult to
discern from the difference spectra. A comparison between the
RAIR spectrum of 2-methylpropanal codeposited with N2O
with the RAIR difference spectra taken during photolysis
provides an explanation for the apparent absence of further
hydrocarbon modes. As indicated in Figure 7, intense RAIR
features of the deposited aldehyde (e.g., the CH3 deformation
modes at 1470 and 1397 cm−1) coincide with those of isobutyl
nitrate. Furthermore, the remaining absorption features of 2-
methylpropanal that do not overlap with peaks from isobutyl
nitrate (e.g., 1140 and 1107 cm−1) are weak and therefore
become lost within the noise of the difference spectra. Table 4
shows a comparison of absorption features present in the
transmission IR spectrum of 2-methylpropanal with those
observed in the current RAIR study.
description
liquid40
RAIR
298 K, neat
2973 m
2940 w
2880 m
1630 vs
1472 m
1438 w
1395 m
1373 m
1356 m
1305 m
1281 s
113 K, codep H2O
νas(CH3)
νas(CH3)
νs(CH3)
2969
2936
2878
νas(NO2)
δ(CH3)
1633 (1638)
1472
δ(CH3)
1450
νs(C−C−C)
νas(C−C−C)
CH2 wag
νs(C−C)
νs(NO2)
1393
1377
1304
1286 (1289)
1246
CH2 rock
C−C−C wag
ν(C−O)
1249 w
1180 w
990 m
1181
989
ν(C−C)
962 m
960
δ(C−C−C)
ν(N−O)
ρ(NO2)
893 m
896
875 m
856 m
852
a
In Figure 8, the relative loss rates of the three alkyl nitrates
codeposited with water are compared using the decrease in
intensity from the νas(NO2) peak for each. The loss rates on
photolysis were found to follow the trend methyl < isobutyl <
isopropyl, which is the same trend found for the UV absorption
Wavenumbers in brackets are those found for a neat thin film, and are
listed when they varied from those found for the co-deposited thin
films.
RAIR peak maxima. In addition to the intense RAIR band due
to the νas(NO2) mode, further absorptions originating from the
presence of the nitrate functionality are again identifiable in the
spectra. The NO2 symmetric stretching, rocking, and wagging
vibrations are all found to be within ∼5 cm−1 of the analogous
absorptions observed for isopropyl nitrate, as shown in Table 2.
By contrast, the ν(C−O) mode of the nitric ester, which is
centered at 989 cm−1 for isobutyl nitrate, lies much closer to
that found for methyl nitrate (1000 cm−1) than was observed
for isopropyl nitrate (913 cm−1).
Table 4. Comparison of Absorption Features Present in the
Transmission IR Spectrum of 2-Methylpropanal with Those
Observed in the Current Study
a
mode
description
solid46
RAIR
neat, 90 K
1707
photoproduct, 113 K
ν6
ν(CO)
1708
ν23
ν9
δas(CH3)
1477
a
δs(CH3)
1397
a
Upon photolysis, the RAIR difference spectrum shown in
Figure 7 indicates that the behavior of isobutyl nitrate mirrors
that previously seen for both the methyl and isopropyl nitrates.
At 2230 cm−1, an absorption assigned to N2O is again found to
grow as the thin films are photolyzed. Notably, this photolysis
behavior mimics that found previously for the corresponding
alkyl nitrite (RONO, where R = i-But),39 where an identical
ν10
ν26
ν13
ν27
aldehydic (CH)
α CH bend
ρ(CH3)
1367
1367
1273
a
1138
νas(CCC)
1107
a
The peak coincides with an absorption of isobutyl nitrate and was
therefore not observable in the current study.
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Table 5. Assignments of the Vibrational RAIR Bands
Observed for Peroxyacetyl Nitrate, Together with Literature
a
Values for Comparison
mode description
gas47
matrix47
RAIR
Ar, 10 K, M.R.=
500:1
113 K, codep. with
water
ν4
ν(CO)
νas(NO2)
σas(CH3)
σs(CH3)
νs(NO2)
ν(CO)
ρ(CH3)
1842 s
1835 s
1812 (1826)
1735
1430
1368
1301
1165
1041
995
ν5
1741 vs 1731 vs
1430 w 1435 w
1371 m 1368 m
ν6−8
ν9
1302 s
1162 s
1300 s
1154 s
ν10
ν11,12
ν13
ν14
ν15
ν16
1055 w 1043 w
ν(CC)
ν(OO)
ν(NO)
σ(NO2)
990 m
930 m
821 m
792 s
988 m
927 m
821 m
789 s
Figure 8. Normalized intensity of the νas(NO2) absorption peaks
versus photolysis time for the three alkyl nitrates investigated during
this study. The intensities were determined from RAIR spectra of the
alkyl nitrates codeposited with water.
925
828
799
a
Wavenumbers in brackets are those found for the neat thin film and
are listed when they varied from those found for the co-deposited thin
film.
cross sections for these alkyl nitrates in the gas and liquid
phase.2,6,17
Peroxyacetyl Nitrate (PAN). In Figure 9, the RAIR spectra
of peroxyacetyl nitrate (PAN), deposited neat and codeposited
the growth of an absorption peak at 2344 cm−1, which could be
accounted for by the generation of carbon dioxide within the
thin films (Figure 10).44 No peaks associated with the RAIR
Figure 9. RAIR spectra of peroxyacetyl nitrate, deposited neat to the
gold substrate at 113 K (red trace) and codeposited with water (blue
trace).
Figure 10. RAIR difference spectra of a thin film of PAN, codeposited
with water. Shown is the difference spectrum recorded after 4 h of
photolysis (red trace). Also shown is the RAIR spectrum of the
codeposited thin film prior to photolysis for comparison (blue trace).
with water, are illustrated. In each case, the spectra were
recorded directly after deposition to the gold substrate at 113
K. Assignments of the RAIR spectral features are presented in
Table 5, together with IR absorption wavenumbers from the
literature for comparison.
spectrum of either NO or NO238 were observed to form during
photolysis. However, minor bands at 1642 and 879 cm−1 were
found to increase as a function of photolysis time. Notably, the
only strong absorptions of methyl nitrate that do not overlap
with those of PAN are found in these regions (Table 1). The
apparent formation of carbon dioxide and methyl nitrate as
photoproducts compares well with previous matrix isolation
studies of PAN, where the same photolysis products have been
reported upon irradiation at wavelengths below 300 nm.47 The
decrease with time of PAN due to photolysis, along with the
concurrent formation of the peak at 1642 cm−1 is depicted in
Figure 11.
Further evidence for the generation of MeONO2 within the
thin films was obtained from TPD measurements performed
subsequent to photolysis of the codeposited thin films (Figure
12). For both methyl nitrate and PAN, the parent molecular
ions are not observed in the (electron impact) mass spectra at
energies between 20 and 70 eV.5,48 Furthermore, the only
The RAIR spectra obtained for the neatly deposited PAN
films were found to be similar to spectra for when it had been
codeposited with water. In both spectra, the ν(CO)
stretching mode was found to split into two bands at 1826
and 1812 cm−1, with the splitting being more pronounced in
the codeposited films, and the 1826 cm −1 peak only appearing
as a shoulder when films were deposited neat. When these are
compared to previous gas-phase IR spectra of PAN,47 both red
and blue shifts of vibrational absorptions in the solid become
apparent. The largest difference of IR absorption wavenumbers
between the two phases occurs for the ν(CO) mode, with
Δv > −16 cm−1 in the solid relative to the gas phase.
̃
Upon photolysis of the codeposited thin film, the intensity of
peaks due to the peroxyacyl nitrate were observed to decrease.
The largest spectral change observed during the photolysis was
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the alkyl nitrates were codeposited with water onto the gold
substrate, few spectral shifts or changes in relative intensity
were observed in comparison with when deposited neat. One
exception to this was the νas(NO2) mode of the alkyl nitrates,
which was seen to red shift by up to 5 cm−1 for the alkyl
nitrates when codeposited with water. This red-shifting of the
vibrational frequency can be accounted for by intermolecular
interactions between water and the alkyl nitrates, which slightly
decreases the NO bond strength of the latter. Hydrogen
bonding interactions, where the NO group acts as an
electron density donor, and the H−O group of water the
acceptor provide a plausible explanation for this observation.
A number of spectral shifts also become apparent when the
different alkyl nitrates were compared to one another. For
instance, in the case of the νas(NO2) stretching vibration, it can
be seen that for each additional methyl group attached to the α-
carbon of the alkyl nitrate, the vibrational frequency is
progressively red-shifted. This trend has previously been
observed for the liquid phase by Urbanski,52 who attributed it
to the accumulative inductive effect of increasing the alkyl
substitution on the α-carbon. Accordingly, for each additional
alkyl group the NO bond strength is decreased due to the
decreasing positive character of the nitrogen atom.
Figure 11. Loss rate of peroxyacetyl nitrate (PAN) from the thin films
codeposited with water as tracked by the decrease of its ν(NO2)s
vibrational adsorption centered at 1301 cm−1. Also shown is the
growth rate for the ν(NO2)as peak of methyl nitrate at 1642 cm−1.
A dramatic reduction in the frequency (Δv = −87 cm−1) of
̃
the linking C−O stretching mode for isopropyl nitrate was
observed, relative to the methyl and isobutyl nitrates. In the
liquid phase, the region where this vibration occurs for all other
C1−C5 alkyl nitrates is around 990 cm−1.52 However, for
isopropyl nitrate, the region between 1080 and 940 cm−1 is
completely featureless, both in the RAIR spectra of the present
study (Figure 3) and in previous liquid-phase FTIR spectra.40,52
Although earlier researchers have simply left this mode
unassigned,52 McLaughlin et al. attributed a medium intensity
absorption at 910 cm−1 to the above stretching vibration.40 The
shifting of this band may be related to the unusual
conformation that isopropyl nitrate adopts in the liquid
phase. By contrast with the other alkyl nitrates, the plane in
which the NO3 group lies is thought to be twisted away from
the linking C−O bond axis in isopropyl nitrate.40 As a result,
the C−O bond itself may become strained, leading to the
observed red-shift in vibrational frequency.
Figure 12. Thermal desorption profile of a thin film of PAN
codeposited with water subsequent to photolysis. Shown are the major
+
fragments associated with PAN (m/z = 43, CH3CO+; m/z = 44, CO2 ;
+
m/z = 46, NO2 ), MeONO2 (m/z = 29, CHO+), and H2O (m/z = 18,
H2O+). The trace for water (m/z =18) has been scaled down by a
factor of 5 for comparison.
For all three of the alkyl nitrates, little information on their
orientation within the thin films could be discerned from the
RAIR measurements. Given the low symmetry of each of these
species in the liquid phase (Cs for R = Me, i-But and C1 for R =
i-Pr)34,40 the observed invariance of the RAIR absorption
intensities with respect to the liquid-phase IR spectra may be
expected. If, for example, the mirror planes of methyl and
isobutyl nitrate are lost in the solid phase, the point group is
reduced to the only subgroup of Cs, which is the trivial group
C1. In such a case the metal surface selection rule (MSSR),
which leads to the loss of absorptions with vibrational transition
dipole moments parallel to the surface,53 cannot stimulate
intensity changes of one vibration with respect to another.
Conversely, if the parent alkyl nitrate retains Cs symmetry, but
the solid phase is not highly ordered, no changes in the
absorption intensities will be apparent. Hence, in the current
study, RAIR spectra cannot distinguish between these two
potential scenarios.
fragment of methyl nitrate that has not been documented to
occur at the same mass to charge ratio for PAN is found at m/z
= 29 (CHO+).5,48−50 The desorption of PAN was monitored
via the generation of CH3CO+ and CO2+ fragments (m/z = 43
and 44, respectively).5,48 Consistent with previous TPD
measurements of pure water films deposited at low temper-
atures,51 water (H2O+ m/z =18) begins to desorb from
substrate at temperatures above 160 K. As the temperature is
raised further, ions attributable to PAN (CH3CO+, m/z = 43;
+
CO2 , m/z = 44) and methyl nitrate (CHO+, m/z = 29)
continue to be lost from the surface until around 200 K, where
desorption ends.
DISCUSSION
■
Methyl, Isopropyl, and Isobutyl Nitrates. In the current
study, the solid-phase IR spectra of some simple alkyl nitrates
(R = Me, i-Pr, i-But), deposited both neat and codeposited with
water have been reported for the first time. In general, the
spectra obtained were quite similar to those previously reported
for the liquid phase,40,52 with only minor frequency shifts
observed for the neatly deposited thin films. Additionally, when
During the current investigation, the photolysis of all three
alkyl nitrates proceeded in a similar fashion, leading to the
formation of an aldehyde or ketone, along with a minor band at
2230 cm−1, characteristic of nitrous oxide.39,42 For neatly
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deposited thin films, a weak broad feature in the region 3100−
3500 cm−1 was also observed to emerge during photolysis
experiments, consistent with the increasing presence of water
ice. It should be noted that it is unclear whether the growth of
peaks due to water ice in the neat thin films stemmed from
photolysis products or the condensation of trace amounts of
vapor in the UHV chamber during the course of an experiment.
Regardless of this, the observation of peaks due to nitrous oxide
together with those of an aldehyde or ketone in neatly
deposited thin films suggests that the formation of products was
insensitive to the relative excess of the organic nitrate over
water. The loss rate of the three alkyl nitrates upon photolysis
was found to proceed in the order R = methyl < isobutyl <
isopropyl, which mirrors the trend found for the magnitude of
the absorption cross sections in the gas2,6 and liquid phases.17
This increase in UV cross sections with increasing degree of
substitution and carbon number has previously been shown by
Csizmadia et al.17 and Roberts et al.6 to result from a
combination of both structural and electronic effects caused by
the alkyl substituents.
efficiently in the gas,55,56 liquid,62,63 and matrix-isolated
phases:59,61
2HNO → N2O + H2O
(10)
Accordingly, if the photolysis of the parent alkyl nitrate results
in the formation of the corresponding alkyl nitrite, reactions
8−10 will result in the formation of nitrous oxide. Alkyl nitrites
were not observed directly in the RAIR spectra of the
photolyzed alkyl nitrate thin films; this is likely a result of
their known photolability.54,57
Peroxyacetyl Nitrate (PAN). As a result of its low
symmetry (C1), the IR spectrum of PAN is rich in detail as
all 27 normal mode vibrations of this species are IR active.47
During the current RAIR investigation, the IR absorption
intensities of bands attributable to PAN were found to be
indistinguishable from those previously reported for the gas47,64
and matrix-isolated phases,47,65,66 which is accounted for by the
molecular asymmetry of PAN.
Relative to gas-phase and matrix-isolated IR spectra,47,64
differences in the carbonyl stretching frequency and band shape
were observed during the current investigation. When
deposited neat to the gold substrate, the absorption frequency
of this mode is observed to be centered at 1812 cm−1 and is
Past studies into the photolysis of alkyl nitrates have focused
almost exclusively on determining the quantum yields for the
primary processes.3,7,16 Indeed, by analogy to the known gas-
phase photolysis pathways for the alkyl nitrates,2,3,5,6 dissoci-
ation of the weak O−N bond is also likely to take place under
the conditions employed in the current study:
red-shifted by Δv = −30 cm−1 relative to the gas phase.47
A
̃
shoulder at 1826 cm−1 is also present in the spectrum of the
neatly deposited PAN and becomes more pronounced upon
codeposition with water. Although the latter peak at 1826 cm−1
is similar to that found in the Raman spectrum of the pure
liquid,47 the red-shifting of the carbonyl peak to 1812 cm−1
suggests that in the solid phase, the carbonyl group may be
influenced by intermolecular interactions between neighboring
PAN molecules.
RONO2 + hν → R′CH2O• + NO2
(5)
In the gas phase this process is highly efficient, with a quantum
yield approaching unity for the C3−C5 alkyl nitrates (λ = 308
nm).16 In the free troposphere, the alkoxy radical formed in (5)
is likely to react with oxygen, yielding an aldehyde or ketone
along with the hydroperoxy radical.54 However, in the
condensed phase under ultrahigh vacuum, this reaction with
oxygen is unlikely to be important.
Upon photolysis of ethyl nitrate, significant yields of ethyl
nitrite have been reported by both Rebbert7 and Zhu et al.15
Though Rebbert suggested that the alkyl nitrite was formed as a
primary product during photolysis, Zhu et al. have produced
evidence to the contrary. Through the use of cavity ring down
spectroscopy, the authors demonstrated that the alkyl nitrite
was formed via the secondary reactions:15
Upon photolysis of PAN, RAIR spectral features attributable
to the formation of carbon dioxide and methyl nitrate were
observed to develop (Figure 10). The formation of the latter
was further suggested through the observation of a peak with
m/z = 29 in the TPD spectra which is likely due to the CHO+
fragment of methyl nitrate (Figure 12). By analogy to known
thermal decomposition pathways of PAN in the gas phase,47,67
the formation of methyl nitrate could result from secondary
reactions of the peroxyacetyl radical formed via
•
CH3C(O)O2NO2 + hν → CH3C(O)O2 + NO2
(11)
NO2 + hν(λ < 420 nm) → NO + O(3P)
(6)
Following the formation of the peroxyacetyl radical, this
species may then undergo disproportionation, yielding the
acetoxy radical:47
R′CH2O• + NO → R′CH2ONO
(7)
The intermediate formation of alkyl nitrites via reactions 5−7
provides a plausible explanation for the apparent generation of
nitrous oxide during the photolysis of the alkyl nitrates, as
evidenced by both RAIR and TPD spectra obtained in the
current study. Notably, the low temperature photolysis of
primary and tertiary alkyl nitrites codeposited with water yields
an identical RAIR feature at 2230 cm−1, which begins to grow
immediately upon irradiation.39 In the gas,55−58 matrix
isolated,59−61 and solid phase,39 photolysis of alkyl nitrites
initially leads to the formation of an aldehyde or ketone, along
with the nitroxyl molecule, HNO:
•
•
2CH3C(O)O2 → 2CH3CO2 + O2
(12)
The methyl nitrate end product, together with carbon
dioxide, may then by formed via68
CH3CO2 → CH3• + CO2
CH3• + CH3C(O)O2 → CH3O• + CH3CO2
CH3O• + NO2 → CH3ONO2
•
(13)
•
•
(14)
(15)
Although reactions 13−15 were initially proposed to occur
during the gas-phase thermal decomposition of PAN,47,67 the
production of CO2 and methyl nitrate has also been previously
shown to occur during the UV photolysis of PAN trapped
within Ar matrices.47 This, along with evidence from the
current study, suggests that such a pathway is a viable route to
the observed end-products.
R′CH2ONO + hν → R′CH2O• + NO
(8)
R′CH2O• + NO → R′CHO + HNO
(9)
Nitroxyl, formed via reaction 9, may then condense to yield
nitrous oxide and water. This process is known to proceed
H
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International Exchange of Scholars and the Fulbright
Commission in Ireland.
CONCLUSIONS
■
The alkyl and peroxyacyl nitrates are both important reservoir
species of NOx in the troposphere. Owing to their relative
stabilities, these species allow for the long-range transport of
nitrogen oxides away from urbanized point sources, to the
“pristine” polar cryosphere, where they can accumulate during
the dark winter months. Although the gas-phase reactivities of
these species have been well established, the potential for
heterogeneous transformations of these species in frozen
environments is poorly understood. Accordingly, the major
low temperature loss process for alkyl and peroxyacyl nitrates in
the gas phase, i.e., photolysis, was examined under laboratory
conditions when these species are present within thin film
mimics of environmental ices.
When codeposited with water at low temperatures (T = 113
K), the alkyl nitrates investigated (R = Me, i-Pr, and i-Bu) were
found to undergo photolysis to form aldehydes or ketones and
nitrous oxide as end products. The relative loss rates of the
three alkyl nitrates investigated were found to mimic those in
the gas phase, which increase in the order R = Me < i-Pr < i-Bu.
The formation of nitrous oxide as the major nitrogeneous
photoproduct was rationalized by invoking the intermediacy of
the corresponding alkyl nitrites, generated as secondary
products during the photolysis of the alkyl nitrates. By analogy
to previous investigations, the nitrous oxide end product was
shown likely to result from the dimerization of the nitroxyl
molecule, HNO.
Although the N2O product observed here is stable in the
troposphere (and therefore not thought of as an NOy species in
the lower atmosphere),69 the vast majority of NOx in the
stratosphere originates from the dissociation of this long-lived
gas. Accordingly, in addition to its recognized role as a
greenhouse gas in the troposphere, N2O plays a crucial role in
stratospheric ozone chemistry. Indeed, as shown by Rav-
ishankara et al., N2O is predicted to destroy more stratospheric
ozone than any other reactive chemical family during the 21st
century.70
Upon exposure to UV light, the production of methyl nitrate
and carbon dioxide was observed to take place within thin films
of PAN codeposited with water ice. As the absorption cross
sections of PAN are known to increase with increasing
temperature,4 this process is likely to be even more favorable
under environmental temperatures compared with those used
in the current study. Hence, in the upper troposphere (T ≈
188−228 K) or at the poles (T ≈ 190−273 K)71,72 photolytic
decomposition of PAN adsorbed to cold surfaces via the
chemistry outlined above may arise via the pathways outlined.
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AUTHOR INFORMATION
Corresponding Authors
*D. O’Sullivan. E-mail: d.osullivan@leeds.ac.uk.
*R. P. McLaughlin. E-mail: mclaughlinr@seattleu.edu.
*J. R. Sodeau. E-mail: j.sodeau@ucc.ie.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
D.O.S. acknowledges IRCSET and SFI-RFP for their support.
R.P.M. acknowledges the Fulbright Program, Council for the
■
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