Gas-phase absorption cross sections of 2-nitrobenzaldehyde and benzaldehyde in the 285-400 nm region, and photolysis of 2-nitrobenzaldehyde vapor at 308 and 351 nm

Article abstract of DOI:10.1016/j.cplett.2009.04.056

We have measured the gas-phase absorption cross sections of 2-nitrobenzaldehyde and benzaldehyde in the 285-400 nm region by using cavity ring-down spectroscopy (CRD). Absorption cross sections of 2-nitrobenzaldehyde vapor at 290 and 400 nm are 10.7-fold

Full text of DOI:10.1016/j.cplett.2009.04.056

Chemical Physics Letters 474 (2009) 74–78
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Chemical Physics Letters
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Gas-phase absorption cross sections of 2-nitrobenzaldehyde and benzaldehyde
in the 285–400 nm region, and photolysis of 2-nitrobenzaldehyde vapor at 308
and 351 nm
*
Bin Xiang, Chengzhu Zhu, Lei Zhu
Wadsworth Center, New York State Department of Health, and Department of Environmental Health Sciences, State University of New York, Albany, NY 12201-0509, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
We have measured the gas-phase absorption cross sections of 2-nitrobenzaldehyde and benzaldehyde in
the 285–400 nm region by using cavity ring-down spectroscopy (CRD). Absorption cross sections of 2-
nitrobenzaldehyde vapor at 290 and 400 nm are 10.7-fold and 5.5-fold lower than 2-nitrobenzaldehyde
aqueous-phase cross section values. The HCO and the NO2 product channels, after 308 and 351 nm
photolysis of 2-nitrobenzaldehyde vapor, were examined. Neither HCO nor NO2 was detected. Upper
limits for the HCO and NO2 quantum yields are 62.2% and 66.4% at 308 nm, respectively. Preliminary
end-product study indicates that 2-nitrobenzaldehyde photodissociates at 308 nm but not at 351 nm.
Ó 2009 Elsevier B.V. All rights reserved.
Received 13 February 2009
In final form 17 April 2009
Available online 24 April 2009
1. Introduction
where photochemical thresholds were calculated from the corre-
sponding bond energy changes. Pathway (R1) is an intramolecular
2-Nitrobenzaldehyde (also called o-nitrobenzaldehyde; o-
C6H4(NO2)CHO) has been detected as a product of the oxidation
of toluene in a NOx-rich atmosphere [1]. It is used in the prepara-
tion of dyes, pesticides, non-linear optical materials, and pharma-
ceutical drugs [2], and is toxic. 2-Nitrobenzaldehyde has been
used successfully as an actinometer in photochemical studies
[3,4]. It can undergo photoisomerization to form o-nitrosobenzoic
acid, with a quantum yield of 0.5 in the 300–410 nm region in both
the liquid and solid phases [3,5,6]. The mechanism for the photo-
isomerization of 2-nitrobenzaldehyde in the aqueous-phase has
been postulated to involve a transient ketene [7] or a short-lived
triplet intermediate [8]. The UV/visible absorption spectrum of 2-
nitrobenzaldehyde has been measured in non-polar solvents
[9,10] in the 290–420 nm region. Due to the compound’s low vapor
pressure (ꢀ0.003 Torr at 298 K), the gas-phase absorption cross
sections, the photolysis product channels and quantum yields of
2-nitrobenzaldehyde vapor have not been determined. Since re-
moval of 2-nitrobenzaldehyde by tropospheric gas-phase photo-
chemistry is a highly probable process, further work is needed to
evaluate the potential importance of photolysis.
photoisomerization channel. Pathway (R2) is an NO2 formation
channel. Pathway (R3) is a radical formation channel. Pathway
(R4) is a molecular elimination channel.
In this Letter, we report results obtained from absorption cross
section measurements of 2-nitrobenzaldehyde in the 285–400 nm
region. Absorption cross sections of benzaldehyde have also been
measured over the same wavelength range for comparative pur-
poses. We searched for possible HCO and NO2 products from 308
and 351 nm photolysis of 2-nitrobenzaldehyde by using excimer
laser photolysis combined with cavity ring-down spectroscopy
[11,12]. Neither HCO nor NO2 was detected. We have estimated
the atmospheric photolysis rate constants of 2-nitrobenzaldehyde
vapor using the gas-phase absorption cross section data of 2-nitro-
benzaldehyde determined in this work.
2. Experimental
The gas-phase absorption cross sections of 2-nitrobenzaldehyde
and benzaldehyde in the 285–400 nm region were measured in a
50 cm long cell by cavity ring-down spectroscopy [11,12]. A pair
of highly reflective cavity mirrors vacuum-seal the two ends of
the cell. Four pairs of high-reflectance cavity mirrors with center-
ing wavelengths at 285, 310 nm (covering 290–330 nm region),
320 nm (covering 300–340 nm region), and 375 nm (covering
340–400 nm region) were used in the cross section measurements.
The fundamental or the second harmonic output from a XeCl exci-
mer-pumped dye laser (at a much reduced fluence level) was
transmitted into the ring-down cavity through the front cavity
mirror. The laser dyes used to cover the 285–400 nm region were
The photolysis of 2-nitrobenzaldehyde can occur through the
following pathways:
o-C6H4ðNO2ÞCHO þ h
v
! o-C6H4ðNOÞCOOH
ðR1Þ
ðR2Þ
ðR3Þ
ðR4Þ
! C6H4CHO þ NO2 ðk ꢁ 403 nmÞ
! C6H4NO2 þ HCO ðk ꢁ 326 nmÞ
! C6H5NO2 þ CO ðk ꢁ 600 nmÞ
* Corresponding author. Fax: +1 518 473 2895.
E-mail address: zhul@wadsworth.org (L. Zhu).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.04.056

B. Xiang et al. / Chemical Physics Letters 474 (2009) 74–78
75
coumarin 153, rhodamine 6G, rhodamine B, DCM, PTP, and BBQ.
The photon intensity decay inside the cavity was monitored by a
photomultiplier tube (PMT) inserted behind the rear cavity mirror.
The PMT output was amplified, digitized, and transferred to a com-
puter. The decay curve was fitted to a single-exponential decay
of the photolysis/probe laser overlap region is defined by (beam
width) ꢃ (sin 15°)ꢂ1. For a 12 mm wide photolysis beam, the length
of the photolysis/probe laser overlap region is about 4.6 cm. We
probed for the NO2 product from the photolysis of 2-nitrobenzalde-
hyde in the 439–448 nm region, and we probed for the HCO product
from the photolysis of 2-nitrobenzaldehyde in the 613–617 nm
region (HCO X2A00 (0, 0, 0) ? A2A0 (0, 9, 0) transition). A pulse/delay
generator was used to vary the delay time between the firings of the
photolysis laser and the probe laser. The end-products from the
308 nm photolysis of 2-nitrobenzaldehyde were quantified using
a Fourier-transform infrared spectrometer (Bruker IFS 66v).
2-Nitrobenzaldehyde is a light-yellow fine crystal at 295 K, and
was obtained from Aldrich (P98% purity). It was pumped over-
night and stored in a gas bulb. 2-Nitrobenzaldehyde in the gas bulb
was pumped for at least 30 min before being introduced into the
cell. Displayed in Fig. 1 is an IR absorption spectrum of 2-nitro-
benzaldehyde vapor in the 500–3500 cmꢂ1 region (0.5 cmꢂ1 reso-
lution). Benzaldehyde is a liquid at 295 K and was obtained from
Aldrich (P99.5% purity). All experiments were carried out at an
ambient temperature of 295 2 K.
function, from which the ring-down time constant (s) and the total
loss (U) per optical pass were calculated. When cavity mirrors were
properly aligned, the maximum uncertainty for fitting the ring-
down decay curve to a single-exponential decay function was 5%.
Optical loss due to absorption by 2-nitrobenzaldehyde vapor was
determined through measurement of the cavity losses with and
without 2-nitrobenzaldehyde in the cavity. Ring-down decay times
for an empty cavity were about 0.31
310 nm, 1.19 s at 320 nm, and 2.71 s at 375 nm. With 2-nitro-
benzaldehyde vapor in the cavity, ring-down decay times short-
ened to as short as 0.21 s at 285 nm, 0.53 s at 310 nm, 0.76
at 320 nm, and 2.05 s at 375 nm. The gas pressure inside the cell
ls at 285 nm, 0.95 ls at
l
l
l
l
ls
l
was read by an MKS Baratron capacitance manometer (1 Torr full
scale), which can measure pressures down to 10ꢂ4 Torr (pressure
measurement accuracy is about 25% at 0.4 mTorr, 10% at 1 mTorr,
and 5% at 2 mTorr). The cell was evacuated to 10ꢂ5 Torr (measured
with a cold cathode vacuum gauge) with a combination of rotary
and diffusion pumps before each experiment. The cell had an out-
gassing rate of 5 ꢃ 10ꢂ5 Torr/min. For absorption cross section
measurements, 2-nitrobenzaldehyde pressure inside the cell was
varied between 4 ꢃ 10ꢂ4 Torr and 2 ꢃ 10ꢂ3 Torr; absorptions of
the probe beam by 2-nitrobenzaldehyde at six different pressures
were measured for each cross section run. The cell was pumped
out before 2-nitrobenzaldehyde at a different pressure was al-
lowed to flow into the cell, so as to minimize the effect of outgas-
sing. The cross section measurements were made under static
condition. It took up to 4 min for 2 ꢃ 10ꢂ3 Torr of 2-nitrobenzalde-
hyde to fill the cell. Once the cell was filled with a sample, about 8 s
were needed to perform a ring-down measurement, with a laser
repetition rate of 1 Hz and with 8 count average. With the cell
degassing rate and the sample filling time as well as the accuracy
of the pressure read-out taken into account, the uncertainty in
the 2-nitrobenzaldehyde concentration measurements is 635% at
4 ꢃ 10ꢂ4 Torr, 620% at 1 ꢃ 10ꢂ3 Torr, and 615% at 2 ꢃ 10ꢂ3 Torr.
Benzaldehyde absorption cross section measurements were made
under static conditions for all wavelengths except 285 nm; at that
wavelength, the cross section measurement was made under slow-
flow condition. Absorptions of the probe beam by benzaldehyde at
six different pressures were determined for each cross section run.
The vapor pressure of benzaldehyde is about 0.85 Torr at room
temperature. Pressure measurement uncertainty due to outgassing
can be neglected, since the benzaldehyde sample quickly filled the
cell. Pressures of benzaldehyde used in the cross section measure-
ments ranged from 0.6–2.3 mTorr at 285 nm to 0.22–0.83 Torr at
400 nm. The maximum uncertainty in the determination of the
benzaldehyde concentration was about 17% at 285 nm, and about
1% at 400 nm.
3. Results and discussion
3.1. Absorption cross sections of 2-nitrobenzaldehyde vapor in the
285–400 nm region
We have determined the gas-phase UV absorption cross section
of 2-nitrobenzaldehyde in the 285–400 nm region by use of cavity
ring-down spectroscopy. The validity of using cavity ring-down
spectroscopy for accurate cross section determination of low vapor
pressure compound has been demonstrated in our recent work
[16], in which we measured absorption cross section of E,E-2,4-
hexadienedial (HCO–CH@CH–CH@CH–CHO) in the 290–430 nm
region. The room temperature vapor pressure of E,E-2,4-hexadi-
enedial is ꢀ4 mTorr, comparable in magnitude to the room tem-
perature vapor pressure of 2-nitrobenzaldehyde. The cross
section values of E,E-2,4-hexadienedial in the 315–345 nm region
determined using ring-down technique agree with cross section
values of E,E-2,4-hexadienal (CH3–CH@CH–CH@CH–CHO) deter-
mined using diode-array spectrometer [17] to within 10%. The near
UV/visible band of E,E-2,4-hexadienedial is broader than that of
E,E-2,4-hexadienal, and the cross section data of E,E-2,4-hexadi-
enedial obtained using ring-down technique in the 350–400 nm
region are larger than cross section data of E,E-2,4-hexadienal since
the n ? p* transitions arise from symmetric and antisymmetric
combinations of the n(O) orbitals on the two carbonyl centers for
Photolysis of 2-nitrobenzaldehyde occurred in a stainless steel
cell. Detailed descriptions about our experimental setup can be
found elsewhere [13–15]. The output from an excimer laser was di-
rected into the reaction cell at an angle of 15° to the main cell axis,
through a side arm. The probe laser beam, used to monitor NO2 and
HCO generated from the photolysis process, entered the cell along
the main cell axis. The cell had been vacuum-sealed with a pair of
highly reflective cavity mirrors. The probe laser beam overlapped
with the photolysis beam at the center of the cavity. The photoly-
sis/probe laser overlap region can be envisioned as a rectangular so-
lid with its center overlapping that of the cell, with its width and
height defined by those of the photolysis beam, and with its length
defined by (beam width) ꢃ (tan 15°)ꢂ1, where 15° is the crossing
angle between the pump and the probe laser beams. The length
Fig. 1. FTIR spectrum of 2 mTorr of 2-nitrobenzaldehyde vapor.

76
B. Xiang et al. / Chemical Physics Letters 474 (2009) 74–78
Table 1
r
, in units of cm2/molecule, base e) of 2-nitrobenzaldehyde
E,E-2,4-hexadienedial. In the present study, absorption cross sec-
tion of 2-nitrobenzaldehyde at each wavelength was determined
by measurement of the round-trip cavity losses due to absorption
as a function of 2-nitrobenzaldehyde pressure in the cavity, plot-
ting of the absorption against 2-nitrobenzaldehyde pressure, and
extraction of the absorption cross section from the slope of the
plot. Fig. 2 shows round-trip absorption plotted against 2-nitro-
benzaldehyde pressure at 350 nm. 2-Nitrobenzaldehyde absorp-
tion cross section of 1.78 ꢃ 10ꢂ19 cm2/molecule was derived from
the slope of the plot. 2-Nitrobenzaldehyde gas-phase absorption
cross section at each wavelength was independently measured
for 3–8 times. Cross section values for 2-nitrobenzaldehyde vapor
as a function of wavelength thus obtained are listed in Table 1 and
shown in Fig. 3. The absorption spectrum of 2-nitrobenzaldehyde
in the 285–400 nm region could originate from several n ? p*
transitions [10] from the lone pairs of nitro and aldehyde groups
into p* orbitals localized either in the benzene ring or in the NO2
Absorption cross sections (
(2-NBA) and benzaldehyde (BA) as a function of wavelength (k).
k
r
r
(nm)
(cm2/molecule, 2-NBA)
(cm2/molecule, BA)
285
290
295
300
305
310
315
320
325
330
335
340
345
350
355
360
365
370
375
380
385
390
395
400
(1.04 0.05a,b) ꢃ 10ꢂ18
(5.98 0.01) ꢃ 10ꢂ19
(3.31 0.23) ꢃ 10ꢂ19
(3.11 0.21) ꢃ 10ꢂ19
(3.10 0.17) ꢃ 10ꢂ19
(3.03 0.12) ꢃ 10ꢂ19
(2.67 0.04) ꢃ 10ꢂ19
(2.07 0.21) ꢃ 10ꢂ19
(2.00 0.07) ꢃ 10ꢂ19
(2.08 0.06) ꢃ 10ꢂ19
(1.98 0.10) ꢃ 10ꢂ19
(1.92 0.16) ꢃ 10ꢂ19
(1.82 0.01) ꢃ 10ꢂ19
(1.78 0.05) ꢃ 10ꢂ19
(1.50 0.08) ꢃ 10ꢂ19
(1.34 0.10) ꢃ 10ꢂ19
(1.21 0.06) ꢃ 10ꢂ19
(8.28 0.48) ꢃ 10ꢂ20
(6.65 0.55) ꢃ 10ꢂ20
(5.12 0.57) ꢃ 10ꢂ20
(4.36 0.32) ꢃ 10ꢂ20
(2.85 0.03) ꢃ 10ꢂ20
(2.35 0.27) ꢃ 10ꢂ20
(1.80 0.10) ꢃ 10ꢂ20
(2.12 0.01a,c) ꢃ 10ꢂ18
(9.01 0.06) ꢃ 10ꢂ20
(4.61 0.09) ꢃ 10ꢂ20
(3.85 0.16) ꢃ 10ꢂ20
(5.18 0.07) ꢃ 10ꢂ20
(4.50 0.05) ꢃ 10ꢂ20
(6.06 0.42) ꢃ 10ꢂ20
(5.71 0.32) ꢃ 10ꢂ20
(5.98 0.06) ꢃ 10ꢂ20
(7.23 0.51) ꢃ 10ꢂ20
(6.03 0.07) ꢃ 10ꢂ20
(6.94 0.08) ꢃ 10ꢂ20
(5.71 0.12) ꢃ 10ꢂ20
(3.65 0.11) ꢃ 10ꢂ20
(4.60 0.04) ꢃ 10ꢂ20
(6.95 0.18) ꢃ 10ꢂ20
(5.96 0.15) ꢃ 10ꢂ20
(1.30 0.02) ꢃ 10ꢂ20
(4.28 0.05) ꢃ 10ꢂ21
(7.36 0.04) ꢃ 10ꢂ22
(2.36 0.04) ꢃ 10ꢂ22
(1.02 0.04) ꢃ 10ꢂ22
(2.46 0.01) ꢃ 10ꢂ22
(7.06 0.48) ꢃ 10ꢂ23
and CHO moieties. The errors quoted (1r) represent the estimated
precision of cross section measurements obtained from the stan-
dard deviation of 3–8 cross section measurements. Systematic
uncertainties in the cross section data arise mainly from errors in
pressure measurements (up to 35–17% for 2-nitrobenzaldehyde
pressure in the 4 ꢃ 10ꢂ4–2 ꢃ 10ꢂ3 Torr range) and to a lesser ex-
tent from errors in the determination of absorption (5%). The rela-
tive absorption cross sections as a function of wavelength are
determined more accurately than the absolute cross section values
as many of the systematic errors are about the same for the mea-
surements made at different wavelengths. With both random er-
rors and systematic errors taken into account, the overall
uncertainties in the determination of 2-nitrobenzaldehyde cross
sections are 40% at 290 and 345 nm; 45% at 285, 350, 355, 365,
390 nm, and in the 305–315 nm and 325–335 nm regions; 50% at
295, 300, 320, 340, 360, 370, 375, 385, and 400 nm; and 55% at
380 and 395 nm. Cross section values of 2-nitrobenzaldehyde va-
por in the 285–400 nm region range from 1.04 ꢃ 10ꢂ18 to
1.80 ꢃ 10ꢂ20 cm2/molecule.
a
Errors quoted are standard deviations in the precision of the measurements.
The overall uncertainties in the determination of 2-nitrobenzaldehyde cross
b
sections are 40% at 290 and 345 nm; 45% at 285, 350, 355, 365, 390 nm, and in the
305–315 nm and 325–335 nm regions; 50% at 295, 300, 320, 340, 360, 370, 375,
385, and 400 nm; and 55% at 380 and 395 nm.
c
The overall uncertainties in the determination of benzaldehyde cross sections
are 10% in the 290–310 nm, 335–345 nm, 355–365 nm, and 380–395 nm region,
and at 325 and 375 nm; 15% at 315, 320, 350, 370, and 400 nm; 20% at 330 nm; and
25% at 285 nm.
Shown in Fig. 3 for comparison are previously reported aque-
ous-phase absorption cross sections of 2-nitrobenzaldehyde [9]
in non-polar solvents in the 290–400 nm region. The aqueous-
phase absorption cross sections of 2-nitrobenzaldehyde ranged
from 6.39 ꢃ 10ꢂ18 cm2/molecule at 290 nm to 9.90 ꢃ 10ꢂ20 cm2/
molecule at 400 nm. The gas-phase absorption cross sections of
2-nitrobenzaldehyde determined in our study ranged from
Fig. 3. Absorption cross sections of 2-nitrobenzaldehyde in the 285–400 nm region.
Circles represent gas-phase cross section data determined in this work. Precision of
the measurement is shown for all wavelengths. Absolute error bars are shown for
data points at 285 and at 350 nm. Solid line represents aqueous-phase cross section
data obtained for 2-nitrobenzaldehyde in non-polar solvents [9].
5.98 ꢃ 10ꢂ19 cm2/molecule at 290 nm to 1.80 ꢃ 10ꢂ20 cm2/mole-
cule at 400 nm. The liquid-phase absorption cross sections of 2-
nitrobenzaldehyde at 290 and 400 nm are 10.7-fold and 5.5-fold
greater than those in the gas-phase. The difference between the li-
quid-phase and gas-phase UV absorption cross sections of 2-nitro-
benzaldehyde can be partially attributed to the red shift of the
liquid-phase absorption spectrum of 2-nitrobenzaldehyde, com-
pared with the gas-phase spectrum. Solution-phase cross section
Fig. 2. Round-trip absorption (base e) plotted against 2-nitrobenzaldehyde
pressure at 350 nm. The solid line is a linear least-squares fit of the data points.
An absorption cross section of 1.78 ꢃ 10ꢂ19 cm2/molecule was derived from the
slope of the plot.

B. Xiang et al. / Chemical Physics Letters 474 (2009) 74–78
77
data have been used to model the gas-phase photolysis of 2-nitro-
benzaldehyde since gas-phase spectroscopic information in the
UV/visible region are lacking [18]. Our current data show a large
difference exists between the gas-phase and aqueous-phase UV
absorption cross sections of 2-nitrobenzaldehyde. Thus, only gas-
phase cross section data should be used to model the gas-phase
photolysis of 2-nitrobenzaldehyde.
changes in the photolysis fluence transmitted through the cell; they
reported benzaldehyde cross section values of 1.8 ꢃ 10ꢂ19
,
3.2 ꢃ 10ꢂ19
,
and 3.8 ꢃ 10ꢂ20 cm2/molecule at 280, 285, and
308 nm, respectively. The benzaldehyde absorption cross section
reported in the present work agrees to within 20% with the one re-
ported by Zhu and Cronin [20] at 308 nm. However, at 285 nm, the
benzaldehyde absorption cross section determined in the present
work is 6.6-fold greater than that reported by Zhu and Cronin. This
disparity likely results from the combination of the low vapor pres-
sure of benzaldehyde (ꢀ0.85 Torr at 295 K) and the use of an insen-
sitive transmitted photolysis fluence measurements to determine
benzaldehyde absorption cross sections in the previous study. Thus,
the benzaldehyde cross section data reported here supersede the
previous cross section data reported by our group. Nozière et al.
[21] obtained benzaldehyde cross section values of 2.1 ꢃ 10ꢂ18
and 1.65 ꢃ 10ꢂ18 cm2/molecule at 290 and 300 nm, respectively.
The benzaldehyde cross section data at 290 and 300 nm determined
in the present study are 23.3-fold and 42.9-fold lower than those re-
ported by Nozière et al. [21]. Such a disparity probably resulted
from the presence of absorbing benzaldehyde photolysis products
in the study of Nozière et al.
3.2. Comparison between gas-phase absorption cross section values of
2-nitrobenzaldehyde and of benzaldehyde, in the 285–400 nm region
We have determined the gas-phase absorption cross sections of
benzaldehyde in the 285–400 nm region, in order to compare them
with the cross section data of 2-nitrobenzaldehyde. The benzalde-
hyde cross section data determined in this study are listed in Table
1. The errors quoted (1r) are the estimated precision of cross sec-
tion measurements obtained from the standard deviation of at least
three independent cross section measurements. With both random
errors and systematic errors taken into account, the overall
uncertainties in the determination of benzaldehyde cross sections
are 10% in the 290–310 nm, 335–345 nm, 355–365 nm, and 380–
395 nm region, and at 325 and 375 nm; 15% at 315, 320, 350,
370, and 400 nm; 20% at 330 nm; and 25% at 285 nm. Displayed
in Fig. 4 are UV absorption cross sections of 2-nitrobenzaldehyde
and benzaldehyde as a function of wavelength. In addition to the
gas-phase benzaldehyde cross section data determined in this
study, previously reported benzaldehyde cross section values of
Thiault et al. [19], of Zhu and Cronin [20], and of Nozière et al.
[21] are included. Our benzaldehyde cross section data agreed with
those obtained by Thiault et al. to within 5% at 320 nm; to within
15% in the 305–315 nm region and at 325 nm; to within 20% at
300 nm and in the 330–340 nm region; to within 25% at 295 nm;
to within 30% at 285 nm; to within 35% at 345 and 355 nm; to with-
in 40% at 290 nm; and to within 45% at 350 nm. Our benzaldehyde
cross section data at 360 and 365 nm are 2.9-fold and 2.5-fold
greater than the cross section values reported by Thiault et al. The
cross section data obtained by Thiault et al. in the 345–365 nm re-
gion appear to be limited by detection sensitivity of the diode-array
spectrometer that was used. Zhu and Cronin [20] previously deter-
mined absorption cross sections of benzaldehyde, by varying benz-
aldehyde pressure in the cell and monitoring the corresponding
A comparison of the UV absorption spectra of benzaldehyde and
2-nitrobenzaldehyde in the 285–400 nm range shows that (i) the
UV absorption spectrum of 2-nitrobenzaldehyde is broader than
that of benzaldehyde; and (ii) the absorption cross sections of 2-
nitrobenzaldehyde are larger than those of benzaldehyde in this
region. Both characteristics could result from the contribution of
an additional NO2 moiety to the conjugated
benzaldehyde.
p system of
3.3. Time-resolved studies of the photolysis of 2-nitrobenzaldehyde at
308 and 351 nm
We investigated the photolysis of 2-nitrobenzaldehyde at 308
and 351 nm by probing for possible photodissociation products,
such as HCO and NO2, immediately after the photolysis process.
We probed for the HCO radical in the 613–617 nm region (HCO
X2A00 (0, 0, 0) ? A2A0 (0, 9, 0) transition), and for NO2 product in
the 439–448 nm region. However, neither HCO nor NO2 was de-
tected after 308 nm or 351 nm photolysis of 2-nitrobenzaldehyde.
Fig. 4. Gas-phase absorption cross sections of 2-nitrobenzaldehyde and benzalde-
hyde in the 285–400 nm region. Circles represent 2-nitrobenzaldehyde cross
section data determined in this work. Benzaldehyde cross section data included
in the figure are those obtained by Zhu and Cronin [20] (hexagons), by Thiault et al.
[19] (solid line), by Nozière et al. [21] (diamonds), and in this work (triangles).
Fig. 5. Estimated atmospheric photolysis rate constants of 2-nitrobenzaldehyde as
a function of solar zenith angle under cloudless conditions, at sea level, and for best-
estimate albedo. At a solar time of 12:00 p.m. and at a latitude of 40°N, the solar
zenith angle [22] is 63.0° on January 1 and 16.9° on July 1.

78
B. Xiang et al. / Chemical Physics Letters 474 (2009) 74–78
hyde for zenith angles in the 0–60° range are 5.6 ꢃ 10ꢂ4 to
2.3 ꢃ 10ꢂ4 sꢂ1; these values correspond to photolysis lifetimes of
0.5–1.2 h. The rate constant for reaction of 2-nitrobenzaldehyde va-
por with OH or with NO3 radical has not been previously reported.
Assuming the OH/2-nitrobenzaldehyde and the NO3/2-nitrobenzal-
dehyde reaction rate constants are the same as rate constants for
the OH/benzaldehyde and NO3/benzaldehyde reactions [24]
Since our preliminary end-product study indicates that 2-nitro-
benzaldehyde vapor was not photolyzed at 351 nm, we provide
here the estimated maximum HCO and NO2 quantum yields after
308 nm photolysis of 2-nitrobenzaldehyde.
The minimum round-trip HCO absorption that can be detected
with our current cavity ring-down setup is 1 ppm. The length of
the photolysis/probe laser overlap region is 4.6 cm, with a 12 mm
wide excimer beam. With an HCO absorption cross section [14]
of ꢀ2.0 ꢃ 10ꢂ18 cm2/molecule at 613.80 nm (R bandhead), the ab-
sence of detectable levels of HCO at the 308 nm photolysis wave-
length suggests that the HCO concentration in the photolysis/
probe laser overlap region was lower than 5.4 ꢃ 1010 molecules/
cm3. The absorbed photon density from 308 nm photolysis of
0.003 Torr 2-nitrobenzaldehyde was about 2.5 ꢃ 1012 molecules/
cm3, with an incident photolysis fluence of about 0.055 J/cm2 at
this wavelength. The lack of detection of HCO at 308 nm suggests
that the upper limit for the HCO quantum yield is below 2.2% at
this wavelength. A similar calculation suggests that the maximum
NO2 quantum yield from 308 nm photolysis of 2-nitrobenzalde-
hyde is 6.4%. The estimated maximum HCO quantum yield from
the 308 nm photolysis of 2-nitrobenzaldehyde, 2.2%, is much smal-
ler than the HCO quantum yield of 29% reported previously by our
group [20] for the 308 nm photolysis of benzaldehyde.
(kOH+benzaldehyde = 1.4 ꢃ 10ꢂ11 cm3 moleculeꢂ1 sꢂ1
,
kNO3+benzaldehyde
= 4.3 ꢃ 10ꢂ15 cm3 moleculeꢂ1 sꢂ1) and using 12 h daily average
OH concentration [25] of 1.6 ꢃ 106 molecule/cm3 and an average
NO3 concentration [26] of 5 ꢃ 108 molecule/cm3, the tropospheric
lifetimes of 2-nitrobenzaldehyde vapor due to reaction with OH
and NO3 are about 12.4 h and 5.4 days. Photolysis is a major process
for removing 2-nitrobenzaldehyde from the atmosphere.
Acknowledgments
We thank Professor Ted Dibble for helpful discussions. We are
grateful for the support provided by the National Science Founda-
tion under grant #ATM-0653761.
References
[1] B.E. Dumdei, R.J. O’Brien, Nature 311 (1984) 248.
[2] C.I. Sainz-Díaz, Monatshefte für Chemie 133 (2002) 9.
[3] J.N. Pitts Jr., J.K.S. Wan, E.A. Schuck, J. Am. Chem. Soc. 86 (1964) 3606.
[4] G.J. Phillips, W.R. Simpson, J. Geophys. Res. 110 (2005) D08306, doi:10.1029/
2004JD005552.
3.4. Atmospheric photolysis rate constant of 2-nitrobenzaldehyde
vapor
[5] P.A. Leighton, F.A. Lucy, J. Chem. Phys. 2 (1934) 756.
[6] E.J. Bowen, H. Hartley, W.D. Scott, H.G. Watts, J. Chem. Soc. 125 (1924) 1218.
[7] S. Laimgruber, W.J. Schreier, T. Schrader, F. Koller, W. Zinth, P. Gilch, Angew.
Chem., Int. Ed. 44 (2005) 7901.
[8] M.V. George, J.C. Scalano, J. Phys. Chem. 84 (1980) 492.
[9] L. Láng, Absorption Spectra in the Ultraviolet and Visible Region, Academic
Press, New York, 1961–1971.
[10] V. Leyva et al., J. Phys. Chem. A 112 (2008) 5046.
[11] A. O’Keefe, D.A.G. Deacon, Rev. Sci. Instrum. 59 (1988) 2544.
[12] A. O’Keefe, J.J. Scherer, A.L. Cooksy, R. Sheeks, J. Heath, R.J. Saykally, Chem.
Phys. Lett. 172 (1990) 214.
[13] T.J. Cronin, L. Zhu, J. Phys. Chem. A 102 (1998) 10274.
[14] Y. Tang, L. Zhu, J. Phys. Chem. A 108 (2004) 8307.
[15] L. Zhu, G. Johnston, J. Phys. Chem. 99 (1995) 15114.
[16] B. Xiang, L. Zhu, Chem. Phys. Lett. 450 (2007) 31.
[17] M.P. O’Connor, J.C. Wenger, A. Mellouki, K. Wirtz, A. Munoz, Phys. Chem. Chem.
Phys. 8 (2006) 5236.
[18] J.G. Calvert, R. Atkinson, K.H. Becker, R.M. Kamens, J.H. Seinfeld, T.J.
Wallington, G. Yarwood, The Mechanisms of Atmospheric Oxidation of
Aromatic Hydrocarbons, Oxford University Press, New York, 2002.
[19] G. Thiault, A. Mellouki, G. Le Bras, A. Chakir, N. Sokolowski-Gomez, D.
Daumont, J. Photochem. Photobiol. A: Chem. 162 (2004) 273.
[20] L. Zhu, T.J. Cronin, Chem. Phys. Lett. 317 (2000) 227.
[21] B. Nozière, R. Lesclaux, M.D. Hurley, M.A. Dearth, T.J. Wallington, J. Phys. Chem.
98 (1994) 2864.
[22] K.L. Demerjian, K.L. Schere, J.T. Peterson, Adv. Environ. Sci. Technol. 10 (1980)
369.
[23] K.L. Coulson, D.W. Reynolds, J. Appl. Meteorol. 10 (1971) 1285.
[24] G.M. Clifford, L.P. Thüner, J.C. Wenger, D.E. Shallcross, J. Photochem. Photobiol.
A: Chem. 176 (2005) 172.
We have estimated the atmospheric photolysis rate constants
(kphotolysis) of 2-nitrobenzaldehyde vapor with the following
formula:
X
kphotolysis
¼
rðkÞ ꢄ
u
ðkÞ ꢄ JðkÞ
Dk
where J(k) represents actinic solar flux,
r(k) represents the gas-
phase absorption cross sections of 2-nitrobenzaldehyde determined
in this work, and u(k) represents the photolysis quantum yield for
2-nitrobenzaldehyde. J(k)Dk values reported by Demerjian et al.
[22] were used. In the photolysis rate calculation, u(k) was assumed
to be unity in the 290–310 nm region, and 0 at 350 nm; in the 310–
350 nm region, u(k) was assumed to decrease linearly, from 1 at
310 nm to 0 at 350 nm. (Our preliminary end-product study indi-
cates that 2-nitrobenzaldehyde photodissociates at 308 nm with a
photolysis quantum yield of about unity. Our end-product study
also indicates that 2-nitrobenzaldehyde vapor does not undergo
photodissociation at 351 nm photolysis wavelength.) Note that
u(k) may not actually vary linearly with wavelength in the 310–
350 nm region; we made this assumption solely to simplify the
photolysis rate estimation. The photolysis rate constants for 2-
nitrobenzaldehyde were estimated as a function of the zenith angle
under cloudless conditions, at sea level, and for best-estimate albe-
do [23] (5% in the 290–350 nm region); the results are shown in
Fig. 5. Our estimated photolysis rate constants for 2-nitrobenzalde-
[25] R. Atkinson, J. Phys. Chem. Ref. Data (1994) (Monograph No. 2).
[26] Y. Shu, R. Atkinson, J. Geophys. Res. 100 (1995) 7275.