Photochemical and photosensitised reactions involving 1-nitronaphthalene and nitrite in aqueous solution

Article abstract of DOI:10.1039/c0pp00311e

The excited triplet state of 1-nitronaphthalene, 1NN, (³1NN) is able to oxidise nitrite to NO₂, with a second-order rate constant that varies from (3.56 ± 0.11) × 10⁸ M^-1 s^-1 (μ ± σ) at pH 2.0 to (3.36 ± 0.28) × 10⁹ M^-1 s^-1 at pH 6.5. The polychromatic quantum yield of NO₂ photogeneration by 1NN in neutral solution is Φ_NO2^1NN ≥ (5.7 ± 1.5) × 10⁷ × [NO₂^-]/{(3.4 ± 0.3) × 10⁹ × [NO₂^-] + 6.0 × 10⁵} in the wavelength interval of 300-440 nm. Irradiated 1NN is also able to produce OH, with a polychromatic quantum yield Φ_OH^1NN = (3.42 ± 0.42) × 10^-4. In the presence of 1NN and NO ₂^-/HNO₂ under irradiation, excited 1NN (probably its triplet state) would react with NO₂ to yield two dinitronaphthalene isomers, 15DNN and 18DNN. The photonitration of 1NN is maximum around pH 3.5. At higher pH the formation rate of NO₂ by photolysis of NO₂^-/HNO₂ would be lower, because the photolysis of nitrite is less efficient than that of HNO₂. At lower pH, the reaction between ³1NN and NO₂ is probably replaced by other processes (involving e.g.³1NN-H⁺) that do not yield the dinitronaphthalenes. The Royal Society of Chemistry and Owner Societies 2011.

Full text of DOI:10.1039/c0pp00311e

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PAPER
Photochemical and photosensitised reactions involving 1-nitronaphthalene and
nitrite in aqueous solution†
Pratap Reddy Maddigapu,a Claudio Minero,a Valter Maurino,a Davide Vione,*a,b Marcello Brigante,*c,d
Tiffany Charbouillot,c,d Mohamed Sarakhac,d and Gilles Mailhotc,d
Received 20th October 2010, Accepted 3rd January 2011
DOI: 10.1039/c0pp00311e
The excited triplet state of 1-nitronaphthalene, 1NN, (31NN) is able to oxidise nitrite to ∑NO2, with a
second-order rate constant that varies from (3.56 0.11) ¥ 108 M-1 s-1 (m s) at pH 2.0 to (3.36 0.28) ¥
109 M-1 s-1 at pH 6.5. The polychromatic quantum yield of ∑NO2 photogeneration by 1NN in neutral
1NN
-
-
∑
solution is U
≥ (5.7 1.5) ¥ 107 ¥ [NO2 ]/{(3.4 0.3) ¥ 109 ¥ [NO2 ] + 6.0 ¥ 105} in the wavelength
NO
2
interval of 300–440 nm. Irradiated 1NN is also able to produce ∑OH, with a polychromatic quantum
1NN
-
∑
yield U
= (3.42 0.42) ¥ 10-4. In the presence of 1NN and NO2 /HNO2 under irradiation, excited
OH
1NN (probably its triplet state) would react with ∑NO2 to yield two dinitronaphthalene isomers,
15DNN and 18DNN. The photonitration of 1NN is maximum around pH 3.5. At higher pH the
∑
-
formation rate of NO2 by photolysis of NO2 /HNO2 would be lower, because the photolysis of nitrite
is less efficient than that of HNO2. At lower pH, the reaction between 31NN and ∑NO2 is probably
replaced by other processes (involving e.g. 31NN-H+) that do not yield the dinitronaphthalenes.
consistent with a condensed-phase nitration process that takes
place in situ.9
Introduction
1-Nitronaphthalene (1NN) is a genotoxic atmospheric pollutant1,2
that is frequently detected in urban air3 despite its fast degradation
by direct photolysis.4,5 The main sources of 1NN are the direct
emission upon combustion processes and the atmospheric nitra-
tion of naphthalene.6,7 The very fast photolysis of 1NN (half-life
time of less than 1 h in the atmosphere)8 would make its long-range
transport very unlikely. However, significant amounts of 1NN
(and of 2NN) have been detected in Antarctic airborne particulate
matter.9 While the long-range transport from the continents would
be excluded, a possible explanation is the gas-phase nitration of
naphthalene (probably by ∑NO3 + ∑NO2), followed by partitioning
of the nitronaphthalenes on the particles at the low temperatures
of the Antarctic.10 Various dinitronaphthalenes have also been
detected on the airborne particles in the Antarctic, which is
The nitration of the nitroaromatic compounds is an interesting
issue; in the case of the formation of 2,4-dinitrophenol, it has been
shown that the reaction takes place between the excited mononi-
trophenols and ∑NO2.11 The case of excited 1NN is potentially very
interesting because of the elevated quantum yield for the formation
of the excited triplet state, 31NN.12,13 Moreover, the chemistry
of 31NN is of interest because this species is able to oxidise
the halogenide anions to the corresponding radical species, and
to produce OH via photoinduced generation of O2 -/HO2 and
probably via water oxidation.14,15 The photosensitised processes
in the atmospheric aqueous phase and on particles have recently
gained interest because of the role they play in the atmospheric
processing of humic-like substances.16,17
This work studies the photochemical reactions that involve 1NN
in the presence of nitrite, a major photochemical source of ∑NO2
in solution.18 Particular interest is focused on the photoinduced
formation of the dinitronaphthalenes. To this purpose, a combi-
nation of laser flash photolysis runs and steady-state irradiation
experiments was adopted.
∑
∑
∑
aDipartimento di Chimica Analitica, Universita` di Torino, Via P. Giuria 5,
10125, Torino, Italy. E-mail: davide.vione@unito.it; Fax: +39-011-6707615;
Tel: +39-011-6707838; Web: http://www.chimicadellambiente.unito.it
bCentro Interdipartimentale NatRisk, Universita` di Torino, Via Leonardo da
Vinci 44, 10095, Grugliasco (TO), Italy
cClermont Universite´, Universite´ Blaise Pascal, Laboratoire de Photochimie
Mole´culaire et Macromole´culaire, BP 10448, F-63000, Clermont-Ferrand,
France. E-mail: marcello.brigante@univ-bpclermont.fr
Experimental
Reagents and materials
dCNRS, UMR 6505, Laboratoire de Photochimie Mole´culaire et Macro-
mole´culaire, F-63177, Aubie`re, France
1-Nitronaphthalene (1NN, purity grade 99%), 1,3-dinitron-
aphthalene (13DNN, 98%), 1,5-dinitronaphthalene (15DNN,
99%), 1,8-dinitronaphthalene (18DNN, 98%), phenol (>99%),
2-nitrophenol (98%) and 4-nitrophenol (>99%) were purchased
† Electronic supplementary information (ESI) available: Effects of nitrite
and pH on the decay of 31NN, pH trend of 1NN transformation rate,
effects of 2-propanol and oxygen on the photonitration of 1NN. See DOI:
10.1039/c0pp00311e
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from Aldrich, NaNO2 (>97%) and (NH4)2Ce(NO3)6 (98%) from
Carlo Erba, acetonitrile (LiChrosolv gradient grade), 2-propanol
(LiChrosolv gradient grade), benzene (for gas chromatography),
HClO4 (70%) and H3PO4 (85%) from VWR Int. All reagents were
used as received, without further purification. The g-MnOOH was
synthesised following the procedure of Brauer.19
Analytical determinations
After irradiation the solutions were allowed to cool for 10–15 min
under refrigeration, to minimise the volatilisation of 1NN and,
when applicable, that of benzene. Analysis was then carried out
by High Performance Liquid Chromatography coupled with UV-
Vis detection (HPLC-UV). The adopted Merck-Hitachi instru-
ment was equipped with AS2000A autosampler (100 mL sample
volume), L-6200 and L-6000 pumps for high-pressure gradients,
Merck LiChrocart RP-C18 column packed with LiChrospher 100
RP-18 (125 mm ¥ 4.6 mm ¥ 5 mm), and L-4200 UV-Vis detector
(detection wavelength 220 nm). The adopted gradient of CH3CN :
aqueous H3PO4 (pH 2.8) was the following: 40 : 60 for 10 min,
then to 60 : 40 in 1 min and keep for 8 min, back to the initial
conditions in 1 min and keep for 8 min. With an eluent flow rate
of 1.0 mL min-1 the retention times were (min): phenol (2.55), 4-
nitrophenol (3.20), 2-nitrophenol (5.15), benzene (8.20), 18DNN
(9.06), 15DNN (14.05), 13DNN (16.50), 1NN (17.65). The column
dead time was 0.90 min.
Irradiation experiments
Two different lamp set-ups were used for the irradiation exper-
iments: a set of three 40 W Philips TL K05 UVA lamps, with
emission maximum at 365 nm, and one 100 W Philips TL 01
lamp with emission maximum at 313 nm. The samples (5 mL
total volume) were placed into cylindrical Pyrex glass cells (4.0 cm
diameter, 2.3 cm height) closed with a lateral screw cap, and were
magnetically stirred during irradiation. The incident radiation
reached the cells mainly from the top, and the optical path length
of the solution was b = 0.4 cm. The photon flux incident into the
solutions was actinometrically determined using the ferrioxalate
method, by taking into account the absorption spectrum of
3-
Fe(C2O4)3 and the variation with wavelength of the quantum
Kinetic treatment of the data
yield of Fe2+ generation.20 If one knows, as a function of the
wavelength, the fraction of radiation absorbed by Fe(C2O4)33-, the
quantum yield of Fe2+ photoproduction and the shape of the lamp
spectrum (vide infra), it is possible to use the measured formation
rate of Fe2+ to fix the value of the incident spectral photon flux
density p0(l). The photon flux
The time evolution data of 1NN were fitted with pseudo-first
order equations of the form Ct = Coexp(-kt), where Ct is the
concentration of 1NN at the time t, Co its initial concentration,
and k the pseudo-first order degradation rate constant. The initial
transformation rate of 1NN is Rate1NN = kCo. The time evolution
of the intermediates (15DNN and 18DNN from 1NN, phenol
from benzene, 2- and 4-nitrophenol from phenol) was fitted with
C¢t = kfICo(kd - kd )-1[exp(-kd t) - exp(-kd t)], where C¢t is the
P =
p0 (l)dl
0
∫
I
S
S
I
l
concentration of the intermediate at the time t, Co the initial
concentration of the substrate, kfI and kd the pseudo-first order
I
was 4.4 ¥ 10-5 einstein L-1 s-1 for the TL K05 and 3.2 ¥ 10-6
einstein L-1 s-1 for the TL 01 lamp. In both cases the irradiation
temperature was around 303 3 K. Fig. 1 reports the emission
spectra of the adopted lamps, measured with an Ocean Optics SD
2000 CCD spectrophotometer and normalised to the actinometry
results. The Figure also reports the absorption spectra of 1NN,
nitrite and HNO2, taken with a Varian Cary 100 Scan UV-Vis
spectrophotometer.
formation and transformation rate constants of the intermediate,
respectively, and kd the pseudo-first order transformation rate
S
constant of the substrate. The initial formation rate of the
intermediate is RateI = kfICo. The reported errors on the rates were
derived from the scattering of the experimental data around the
fitting curve, and represent m s. The reproducibility of repeated
runs was around 10–15%.
Radiation absorption calculations
Assume a dissolved species A with concentration cA and molar
absorption coefficient eA(l), which is irradiated under a lamp
with incident spectral photon flux density p0(l), in a solution of
optical path length b. The spectral photon flux density absorbed
paA (l) = p0 (l)[1−10−e (l)bc
]
A
A
by A at the wavelength l is
. The
PA
=
paA (l)dl
a
all-wavelength photon flux absorbed by A is
.
∫
l
If the solution contains two light-absorbing species, A and
B, the absorbances are additive but the absorbed photon flux
densities pa (l) (i = A or B) are not. However, at each wavelength
i
l the ratio of the spectral photon flux densities would be
equal to the ratio of the respective absorbances.24 Therefore,
paA(l) = pa (l)AA(l)[AB(l)]-1, where AA(l) = eA(l)bcA and AB(l) =
B
Fig. 1 Emission spectra (spectral photon flux densities p0(l)) of the
adopted lamps (TL K05 with emission maximum in the UVA, TL 01
with emission maximum in the UVB). Absorption spectra of 1NN, nitrite
and nitrous acid.
eB(l)bcB. It would also be paA(l) = patot(l)AA(l)[Atot(l)]-1, where
patot (l) = p0 (l)(1−10−A (l)
)
is the total spectral photon flux
tot
density absorbed by the solution, and Atot(l) = AA(l) + AB(l).24
602 | Photochem. Photobiol. Sci., 2011, 10, 601–609
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B
3
A similar expression would also hold for pa (l). For the absorbed
pulse (41 ns), the spectrum of 1NN appears with two intense
absorptions peaks at 620 and 400 nm, in agreement with previously
reported studies.15 At 0.9 ms, after complete relaxation of the triplet
state it can be observed a new intense band centred at 380 nm,
which can be attributed mainly to 1NN∑-.15 Moreover we noticed
that, in the absence of nitrite ions, the maximum absorbance
reached at 380 nm (A380) was about 10 times lower than the corre-
sponding A620 of 31NN. Conversely, in the presence of nitrite, the
two absorbance values were similar. This finding provides evidence
that the addition of nitrite enhances the formation of 1NN∑-.
Fig. 3A displays the absorbance of 31NN monitored at 620 nm,
in the presence of different nitrite concentration values at pH 6.5.
It is shown that 31NN is quantitatively quenched by nitrite and that
its pseudo-first order decay constant increases from ~6.0 ¥ 105 s-1
Pi =
pai (l)dl
a
photon flux one gets
, where i = A or B.
∫
l
Laser flash photolysis experiments
A Nd:YAG laser system instrument (Quanta Ray GCR 130-
01) operated at 355 nm (third harmonic) with typical energies
of 60 mJ (the single pulse was ~9 ns in duration) was used
to investigate the photosensitised reaction between the excited
state of 1NN and nitrite in aqueous solution as a function of
pH. Individual cuvette samples (3 mL volume) were used for a
maximum of two consecutive laser shots. The transient absorbance
at the pre-selected wavelength was monitored by a detection system
consisting of a pulsed xenon lamp (150 W), monochromator and a
photomultiplier (1P28). A spectrometer control unit was used for
synchronising the pulsed light source and programmable shutters
with the laser output. The signal from the photomultiplier was
digitised by a programmable digital oscilloscope (HP54522A). A
32 bits RISC-processor kinetic spectrometer workstation was used
to analyse the digitised signal.
to 3.5 ¥ 107 s-1 in pure water and in the presence of 10 mM NO2 ,
-
respectively (see insert in Fig. 2A). Regarding the absorbance trend
followed at 380 nm reported in Fig. 3B, it is interesting to note
the enhancement of the formation rate in the presence of nitrite.
The fast triplet state quenching by nitrite ions, which leads, to
our knowledge, mainly to the formation of 1NN∑-, is compatible
Solutions of both 1NN and NaNO2 were prepared in Milli-Q
water and their stability was regularly checked by means of UV
spectroscopy. The decay of the triplet state of 1NN (31NN) and
the formation of the radical anion (1NN∑-) were monitored at
620 and 380 nm, respectively. The pseudo-first order decay and
growth constants were obtained by fitting the absorbance vs. time
data with single or double exponential equations. The error was
calculated as 1s from the fit of the experimental data; all the
experiments were performed at ambient temperature (295 2 K)
in aerated solution.
Results
Laser flash photolysis experiments
Fig. 2 shows the transient absorption spectra produced upon LFP
excitation (355 nm, 65 mJ) of 1NN (5 ¥ 10-5 M) and NO2
-
(2 ¥ 10-3 M) solution at pH 6.5. Immediately after the laser
Fig. 3 Transient profiles obtained following LFP (355 nm, 60 mJ) of 1NN
(5 ¥ 10-5 M) in aerated solution. (A) Decay at 620 nm corresponding to the
triplet state of 1NN (31NN) in pure water and with different concentrations
of NO2-. Insert: pseudo-first order decay constant of 31NN followed at
620 nm, in the presence of variable [NO2-]. (B) Growth curve of the
transient absorbance at 380 nm in the presence of three [NO2-] values.
Fig. 2 Transient absorption spectra obtained after 355 nm excitation of
-
5 ¥ 10-5 M 1NN and 2 ¥ 10-3 M NO2 in aqueous solution, at pH 6.5 and
T = 295 2 K.
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3
with the electron-transfer reaction (reaction (1)) between 1NN
of 31NN-H+ is ~0.66 in water–ethanol solution,14 and it is difficult
to figure out how this species could be able to affect the triplet
state reactivity at pH 5. We can argue that part of the 380 nm
signal could be attributed to the formation, in addition to 1NN∑-,
of unidentified transient species. If this is the case, the kinetic
analysis of the 380 nm signal would be next to impossible and
no definite conclusion could be derived. Therefore, the following
discussion will only be based on the pH trend of the bimolecular
rate constant between 31NN and nitrite, reported in Fig. 4.
and nitrite to yield ∑NO2.
∑
∑
31NN + NO2 → 1NN - + NO2
-
(1)
∑
Unfortunately we have not been able to directly detect NO2
because of its low molar absorption coefficient (e400 nm
=
201 M-1 cm-1).21
An additional effect of nitrite/HNO2 would be their ability to
absorb laser radiation at 355 nm, thereby competing with 1NN for
the incident photons. To account for this effect, we investigated
Generation of ∑NO2 by irradiated 1NN
3
the variation of the 1NN absorbance soon after formation as
a function of nitrite concentration at different pH values. The
corresponding “screen” effect of nitrite on 1NN excitation has
been estimated to be linearly dependent on the concentration of
nitrite/nitrous acid. For instance, at pH 6.5 the absorbance of
Steady irradiation was carried out to test the hypothesis that the
reaction between 31NN and nitrite, observed by LFP, really yields
∑NO2. Phenol nitration into 2- and 4-nitrophenol was adopted as
a probe reaction for the nitrogen dioxide radical, which is a rather
effective nitrating agent for phenolic compounds in the aqueous
solution.22,23 Irradiation took place under the TL 01 lamp, with the
purpose of achieving a more efficient excitation of 1NN compared
to nitrite (although the two absorption spectra are quite similar in
the near UV range, see Fig. 1).
Fig. 5 reports the time evolution of 2- and 4-nitrophenol (2NP,
4NP) upon irradiation of 0.1 mM 1NN, 1 mM phenol, and 10 mM
NaNO2. The Figure also reports by comparison the time trend of
the nitrophenols upon irradiation of phenol and NaNO2, without
1NN (in which case ∑NO2 is formed by reactions (2) and (3)).18 The
significant enhancement of phenol nitration by 1NN is consistent
with the formation of ∑NO2 upon reaction (1) between 31NN and
nitrite.
31NN was decreased by 25 5% in the presence of 10 mM NO2 ,
-
compared with pure water. Nevertheless, the competition for
irradiance between nitrite and 1NN does not modify the obtained
pseudo-first order decay constants, which are not dependent on
the triplet state concentration.
Experimental data like those reported in Fig. 3 allowed us to
determine the bimolecular rate constants for the quenching of
31NN by nitrite (Fig. 4). The corresponding trends with [NO2 ]
-
3
of the pseudo-first order rate constants of 1NN are reported in
3
-
Fig. ESI1 in the ESI.† The bimolecular rate constant k
1NN,NO
2
decreased from (3.36 0.28) ¥ 109 M-1 s-1 at pH 6.5 to (3.56
0.11) ¥ 108 M-1 s-1 at pH 2.0, showing that the reactivity of 31NN
towards nitrite/HNO2 decreases significantly with pH.
∑
∑
NO2 + hn + H+ → NO + OH
-
(2)
(3)
∑
∑
NO2 + OH → NO2 + OH- [k3 = 1.0 ¥ 1010 M-1 s-1]
-
3
3
-
Fig. 4 Bimolecular rate constants for the quenching of 1NN (k 1NN,NO
)
2
as a function of pH, in aerated aqueous solution at T = 295 2 K, in the
presence of NO2-. pH was adjusted with HClO4. The dotted vertical line
shows the pKa of HNO2.25
At pH 3.5 and 5.0, the rate constants for the quenching of 31NN
were surprisingly lower than those of formation of the transient
monitored at 380 nm. This difference could not be explained on
the basis of the electron transfer reaction reported before (reaction
(1)). Martins and co-workers14 reported that pH plays a central
Fig. 5 Time evolution of nitrophenols upon irradiation of 0.1 mM 1NN,
1 mM phenol and 10 mM NaNO2 (open symbols), and of 1 mM phenol +
10 mM NaNO2 (solid symbols). Irradiation under the TL 01 lamp, at
pH 6.5 and in aerated solution.
3
role in the electron-transfer reactions from halide ions to 1NN,
The formation after 4 h irradiation of ~20 mM 2NP and
4NP, with pKa ~ 7.2 could potentially decrease the solution pH
to around 5.7. Such a pH change was not observed, however,
probably because of the contemporary consumption of H+ in
reaction (2).
via formation of a protonated triplet state (31NN-H+). The species
3
31NN-H+ is considerably more reactive than 1NN toward, for
example, halides,15 and a similar effect can also be expected with
nitrite/HNO2. However, as reported in a previous study the pKa
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It is also possible to calculate a lower limit for the polychromatic
quantum yield of ∑NO2 generation by 1NN, under the hypothesis
that all NO2 reacts with phenol and that the nitration yield of
irradiation wavelength was set to 365 nm (by using a 1000 W
xenon lamp coupled with a monochromatic system). From the
experimental results (see Fig. 6), a 3-fold decrease of photoformed
∑OH moles was estimated in the absence of oxygen, under which
conditions reaction (5) would be strongly inhibited. Such a result
suggests that both processes (oxidation of water and photolysis
∑
phenol by ∑NO2 is unity. In the studied system it is Pa
= 3.42 ¥
1NN
10-7 einstein L-1 s-1 and Pa - = 1.68 ¥ 10-7 einstein L-1 s-1. By com-
parison, 10 mM nitrite alone absorbs 1.87 ¥ 10-7 einstein L-1 s-1.
The overall formation rate of the two nitrophenols with 1NN +
nitrite is (6.7 0.9) ¥ 10-9 M s-1, to be compared with (9.8 0.9) ¥
10-10 M s-1 in the presence of nitrite alone. The processes induced by
nitrite alone would contribute to the formation of the nitrophenols
also in the system containing 1NN, and the corresponding reaction
rate is expected to be proportional to the photon flux absorbed
by nitrite. Accordingly, the contribution of nitrite photolysis to
phenol nitration would be slightly lower in the presence of 1NN +
NO2- than with NO2- alone. Given these premises, the reaction (1)
NO
2
∑
of photogenerated H2O2) may account for the formation of OH
in the studied system. However, additional experiments will be
required to further test the hypothesis, including the quantification
of photogenerated H2O2.
between excited 1NN and nitrite in the studied system is expected
= (5.8 1.0) ¥ 10-9 M s-1 to nitrophenol
1NN
to contribute RateNP
formation, which corresponds to a polychromatic quantum yield
1NN
1NN -1
UNP
= RateNP1NN(Pa
)
= (1.7 0.3) ¥ 10-2. That would be
∑
the lower limit for the polychromatic quantum yield of NO2
production by 1NN under irradiation, U
1NN, in the presence of
∑
NO
2
10 mM nitrite. The LFP results (see insert in Fig. 2A) also suggest
that 10 mM nitrite is able to completely quench 31NN. Under such
circumstances, practically all 31NN would react with NO2- to yield
∑
1NN
-
∑
NO2, and U
would be independent of [NO2 ]. In contrast,
NO
2
at very low [NO2 ] the reaction with nitrite would scavenge 31NN
to a lesser extent, and U
-
1NN
∑
would be directly proportional to
NO
2
Fig. 6 Time evolution of photoformed ∑OH moles upon monochromatic
irradiation (365 nm) of 3.5 ¥ 10-5 M 1NN + 4.0 ¥ 10-4 M TA, in aerated
and argon-saturated solutions. The experiments were performed at pH 6.5
and T = 295 2 K.
-
[NO2 ]. Based on these considerations and on the fact that the
first-order decay constant of 31NN without nitrite is 6.0 ¥ 105 s-1,
while the second-order rate constant between 31NN and NO2 is
-
(3.36 0.28) ¥ 109 M-1 s-1 at pH 6.5, one would get the following
1NN
-
∑
trend for U
vs. [NO2 ]:
NO
2
Fig. 7 reports the time evolution of phenol upon irradiation of
0.1 mM 1NN + 4 mM benzene, in aerated solution under the TL
01 lamp. The formation of phenol from benzene is a suitable probe
reaction to determine the generation rate of ∑OH from irradiated
1NN, as well as the relevant polychromatic quantum yield.28
To further test the actual formation of OH, the time evolution
of phenol was monitored upon addition of 0.1 M 2-propanol.
The initial formation rate of phenol upon irradiation of 0.1 mM
−
(3.36 0.28)×109[NO2
]
F1•NN ≥ (1.7 0.3)×10−2
×
6.0×105 +(3.36 0.28)×109[NO2
]
−
NO
2
(4)
∑
Generation of ∑OH by irradiated 1NN
Brigante and coworkers15 have shown that excited 1NN could
∑
produce OH upon oxidation of water. Moreover, the authors
suggested that the reaction of 1NN∑- with oxygen leads to the
∑
formation of the superoxide radical anion (O2 -), following reac-
∑
-
tion (5). The radical O2 (pKa = 4.88) could undergo dismutation
to generate hydrogen peroxide (reaction (6)), with a second-order
rate constant of 9.7 ¥ 107 M-1 s-1 at pH 6.5.26 H2O2 could then be
∑
photolysed to OH under the irradiation conditions used in this
work (l ≥ 300 nm).
∑
1NN∑- + O2 → 1NN + O2
-
(5)
∑
∑
-
O2 + HO2 + H2O → O2 + H2O2 + OH-
(6)
A preliminary experiment was performed in order to support
this hypothesis. Terephthalic acid (TA) reacts with ∑OH leading to
the formation of 2-hydroxyterephthalic acid (TAOH), quantifiable
via the fluorescence technique.27 Therefore, TA was used as chem-
ical probe to assess the photoformation of ∑OH during irradiation
of 1NN. TA (4.0 ¥ 10-4 M) was irradiated in the presence of
1NN (3.5 ¥ 10-5 M) in aerated and argon-saturated solution. The
Fig. 7 Time evolution of phenol upon irradiation of 0.1 mM 1NN + 4 mM
benzene, and of 0.1 mM 1NN + 4 mM benzene + 0.1 M 2-propanol, under
the TL 01 lamp at pH 6.5 and in aerated solution.
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1NN + 4 mM benzene was RPhenol = (1.47 0.08) ¥ 10-10 M s-1. In
the presence of 2-propanol the rate decreased to (5.07 0.53) ¥
10-11 M s-1. Based on the reaction rate constants of benzene and 2-
propanol with ∑OH,29 competition for the hydroxyl radical between
4 mM benzene and 0.1 M 2-propanol should decrease the phenol
formation rate to (2.07 0.11) ¥ 10-11 M s-1. Therefore, there is a
residual RPh,2Pr = (3.00 0.64) ¥ 10-11 M s-1 that cannot be accounted
∑
for by reaction with OH. A possible explanation could be the
3
direct benzene oxidation by 1NN: a similar effect has already
been observed in the presence of anthraquinone-2-sulfonate under
irradiation, which also forms a reactive triplet state.30 Under this
hypothesis, in the presence of 1NN + benzene under irradiation the
formation rate of phenol that could be accounted for by reaction
between benzene and ∑OH would be R¢ = RPhenol - RPh,2Pr = (1.17
0.14) ¥ 10-10 M s-1.
∑
Note that the radical OH could also react with 1NN and the
Fig. 8 Initial transformation rate of 1NN and initial formation rates of
15DNN and 18DNN upon UVA irradiation in aerated solution of 0.1 mM
1NN and 1 mM NaNO2, as a function of pH, adjusted by addition of
HClO4.
rate constant is not reported. However, even in the case of a
diffusion-controlled reaction, the hydroxyl scavenging by 0.1 mM
1NN compared to 4 mM benzene would introduce a ~5% error
that is within the range of experimental incertitude. The reaction
∑
between benzene and OH yields phenol with a yield of around
K05 lamps. The pH was adjusted by addition of HClO4. Note
that 13DNN was not detected under the adopted irradiation
conditions. Some pH increase (up to around 7.5) was observed
upon irradiation of the samples at the natural pH (pH 6), possibly
because of H+ consumption in reaction (2). In contrast, the pH
variation upon irradiation of the samples acidified with HClO4
was negligible.
∑
95%.28 Therefore, the formation rate of OH by irradiated 1NN
can be expressed as R OH = R¢ (0.95)-1 = (1.23 0.15) ¥ 10-10 M s-1.
∑
ꢀ
1NN
The photon flux absorbed by 1NN is Pa
=
lp0(l) ¥ (1 -
10-A (l)) dl = 3.60 ¥ 10-7 einstein L-1 s-1, where p0(l) is the lamp
spectral photon flux density reaching the solution (see Fig. 1)
and A1NN(l) = e1NN(l)b[1NN], with b = 0.4 cm and [1NN] =
1.0 ¥ 10-4 M. Therefore, the polychromatic quantum yield of
1NN
Fig. 8 also shows that the nitration of 1NN into 15DNN and
18DNN takes place with low yield and is maximum around pH 3.5.
This is an unusual finding considering that, in most cases, the
photonitration processes closely follow the acid–base equilibrium
between nitrous acid and nitrite, with a flexus around pH 3.3
(the pKa of HNO2).25 Therefore, photonitration is usually more
effective under acidic conditions.11,32–34 Because of its unusual
features, the nitration pathway of 1NN was further studied.
∑
1NN
∑
OH photogeneration by 1NN under irradiation is U
=
OH
1NN -1
)
= (3.42 0.42) ¥ 10-4.
∑
R
OH(Pa
∑
Interestingly, in the presence of nitrite the radicals OH gen-
-
erated by 1NN under irradiation could react with NO2 and
contribute to the photoproduction of ∑NO2. Scheme 1 reports the
main processes involving 1NN, after radiation absorption, in the
presence of H2O, nitrite and oxygen.
∑
The addition of 0.1 M 2-propanol as OH scavenger was able
to inhibit significantly the formation of 18DNN and 15DNN at
pH 3.5 (see Fig. ESI2†).
Discussion
Photonitration of 1NN
-
The transformation rate of 1NN with HNO2/NO2 under irradi-
ation was higher at low pH. A similar trend was also observed
for the rate of the direct phototransformation of 1NN, although
the transformation of the substrate was faster in the presence of
-
HNO2/NO2 (see Fig. ESI3†). In the absence of nitrite it has
Scheme 1 Proposed reaction pathways and formation of reactive species
taking place after radiation absorption by 1NN.
3
been shown that the decay rate constant of 1NN is higher at
low pH, because of the formation of protonated 1NN-H+ that
3
undergoes faster decay compared to 1NN.15,35 In the presence
3
of nitrite/HNO2 (pKa,HNO ª 3.325), photolysis of these species
2
Photonitration of 1NN
∑
to yield OH could enhance the transformation of 1NN. Note
First of all, no nitration of 1NN was detected in the presence
of HNO2 in the dark or of (NH4)2Ce(NO3)6 + HNO3 under
irradiation, the latter yielding ∑NO3 + ∑NO2.18,31
Fig. 8 reports the pH trend of the initial transformation rate
of 0.1 mM 1NN and of the initial formation rates of 15DNN
and 18DNN, upon irradiation with 1 mM NaNO2 under the TL
that the photolysis of HNO2 (reaction (7)) is considerably more
efficient than that of nitrite,36 which could contribute to the faster
transformation of 1NN at pH 3 compared to pH 6.5. Another
process that would contribute to the transformation of 1NN is the
reaction between 31NN and nitrite/HNO2. This process would be
more important at higher pH (see Fig. 4).
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HNO2 + hn → NO + ∑OH [U7 = 0.35]
(7)
Generation of ∑NO2 by 1NN and nitrite/nitrous acid under
irradiation
∑
As far as the inhibition of 1NN photonitration by 2-propanol
It was shown before that the photonitration of 1NN would involve
∑
is concerned (Fig. ESI2†), the scavenging of OH by the alcohol
∑
∑NO2. The generation of NO2 in the studied system can take
∑
would inhibit the formation of the nitrating agent NO2 upon
place by the following processes (see Scheme 3): (1) oxidation
irradiation of nitrite/HNO2 (see reactions (2), (3), (7), (8)).18
∑
of nitrite/nitrous acid by the OH radicals photogenerated by
their photolysis (reactions (2), (3), (7), (8)); (2) oxidation of
∑
HNO2 + ∑OH → NO2 + H2O [k8 = 2.6 ¥ 109 M-1 s-1]
(8)
∑
3
nitrite/nitrous acid by OH photogenerated by 1NN; (3) direct
3
oxidation of nitrite/nitrous acid by 1NN (reaction (1)). Under
The effect of 2-propanol on the formation of the dinitronaph-
thalenes is compatible with NO2 being involved at some level
into the nitration of 1NN. Interestingly, no formation of the
dinitronaphthalenes was observed in the presence of g-MnOOH +
HNO2 in the dark. The Mn (hydr)oxide in acidic solution is able
to oxidise HNO2 to ∑NO2:37
∑
neutral conditions, nitrite and its (photo)chemistry would strongly
prevail over HNO2. The quantum yield of reaction (2) varies from
0.07 below 300 nm to 0.025 above 350 nm.39 In the studied systems
nitrite was irradiated in the presence of 1NN, which also absorbs
radiation and can yield ∑NO2 with a polychromatic quantum yield
1NN
∑
U
that is described by eqn (4). In the presence of 1 mM
NO
2
-
NO
2
g-MnOOH + HNO2 + 2 H+ → Mn2+ + ∑NO2 + 2 H2O
(9)
nitrite and 0.1 mM 1NN under the TL K05 lamp, it is Pa
=
3.76 ¥ 10-7 einstein L-1 s-1 and Pa
reasonable value for the polychromatic photolysis quantum yield
= 6.88 ¥ 10-6 einstein L-1 s-1. A
1NN
In contrast, the dinitronaphthalenes were detected when the sys-
tem 1NN + g-MnOOH + HNO2 was UVA irradiated, suggesting
that either (i) nitration involves excited rather than ground-state
1NN, or (ii) it is necessary that reactions (2) and (7) produce ∑OH
for 1NN to be nitrated.
of nitrite under the adopted lamp is 0.035,39 which gives R
- ª
NO
2
∑
OH
1.3 ¥ 10-8 M s-1. Nitrite is expected to be the main scavenger of ∑OH
-
NO
2
∑
in the system, thus it would also be R
ª 1.3 ¥ 10-8 M s-1.
NO
2
Interestingly, a significant decrease of the formation of both
18DNN and 15DNN was observed in deoxygenated solution (N2
atmosphere, see Fig. ESI4†). A nitration pathway that involves
∑
∑OH + NO2 would be inhibited by oxygen, which would shift
the reaction toward the formation of oxygenated/hydroxylated
compounds (see pathway (a) in Scheme 2). In contrast, a nitration
process directly involving ∑NO2 would require oxygen in the second
step to abstract a H-atom (see pathway (b) in Scheme 2).
∑
Scheme 3 Processes leading to NO2 formation in the studied system.
Numbers are referred to the ∑NO2 generation pathways as described in the
text.
1NN
1NN
This is to be compared with R
= U
1NNPa
2
≥ (1.0
∑
∑
NO
NO
2
-
0.4) ¥ 10-7 M s-1 (from eqn (4), with [NO2 ] = 10-3 M). Finally,
∑NO2 could also be produced upon oxidation of nitrite by OH,
∑
photogenerated by 31NN. Considering that nitrite would scavenge
∑
∑
almost all the photogenerated OH, the formation rate of NO2
∑
via this pathway would be equal to the formation rate of OH
3
1NN
1NN
∑
∑
by 1NN (R
= U 1NNPa
= (2.4 0.3) ¥ 10-9 M s-1). By
OH
OH
comparing the three pathways it can be seen that the oxidation of
nitrite by 31NN would play the main role toward the formation of
∑NO2. The photolysis of nitrite would be less important, while
Scheme 2 shows the nitration pathways of 1NN (by ∑OH + ∑NO2 or ∑NO2
alone) that would be compatible with the experimental data reported so
far.
∑
the contribution of OH generated by 31NN would be minor.
∑
-
Note that the photoproduction of NO2 by 31NN + NO2 would
not necessarily enhance 1NN photonitration. Indeed, if the latter
process involves 31NN + ∑NO2, the scavenging of 31NN by nitrite
would decrease the steady-state [31NN], which would compensate
for the parallel ∑NO2 generation.
Therefore, the inhibition of 1NN photonitration under N2
atmosphere is consistent with pathway (b), and the nitration of
∑
3
excited 1NN (probably 1NN) would involve NO2 alone. Inter-
estingly, a similar conclusion has been reached for the nitration of
the mononitrophenols.11 The second-order rate constant between
∑NO2 and the excited triplet states of the mononitrophenols has
been estimated, as 7.9 ¥ 107 M-1 s-1 for 2-nitrophenol and 5.9 ¥
107 M-1 s-1 for 4-nitrophenol.38
The use of polychromatic photolysis quantum yields leads to
unavoidable approximations. However, in this case the differences
between the estimated rates of the ∑NO2 generation processes are
equal to or higher than one order of magnitude. Therefore, the
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approximations in the rate estimates would not be able to bias the
conclusions concerning the rate comparison.
Under acidic conditions, in the presence of 0.1 mM 1NN +
foreseen by eqn (4), but a 4 mM nitrite level is well within the range
of fog waters42 and is also significant for rainwater in polluted
areas.41
1 mM HNO2 it would be Pa
= 7.6 ¥ 10-7 einstein L-1 s-1,
HNO
2
1NN
-1
HNO
2
∑
Pa
= 6.8 ¥ 10-6 einstein L-1 s , and U
= 0.35.39 Therefore,
OH
Conclusions
one obtains R
= 2.7 ¥ 10-7 M s-1. Nitrous acid would be
HNO
2
∑
OH
∑
HNO
The excited triplet state of 1NN (31NN) is able to oxidise nitrite to
2
∑
the main OH scavenger in the system, thus R
ª 2.7 ¥
NO
2
∑
∑
10-7 M s-1. Note that the photoproduction of NO2 by HNO2 is
much more efficient compared to that by nitrite. The generation
of ∑NO2 by 31NN is expected to decrease under acidic conditions
(see Fig. 4), thus HNO2 photolysis could be the main source of
∑NO2 at pH £ 3.
3
-
NO2. The second-order rate constant k
varies from (3.56
1NN,NO
2
0.11) ¥ 108 M-1 s-1 at pH 2.0 to (3.36 0.28) ¥ 109 M-1 s-1 at pH 6.5.
The polychromatic quantum yield of NO2 photogeneration by
1NN in neutral solution, U
∑
1NN, is described by eqn (4) and
∑
NO
2
is valid in the wavelength interval of 300–440 nm. In neutral
∑
3
Fig. 8 shows that the formation of the dinitronaphthalenes
solution, the oxidation of nitrite by 1NN is a competitive NO2
source compared to the photolysis of nitrite. Irradiated 1NN
is also able to produce ∑OH via oxidation of water and/or
via reactions (5) and (6) followed by the photolysis of H2O2,
∑
is maximum at pH 3.5. At higher pH, the formation of NO2
would be decreased because the photolysis of nitrite is less efficient
compared to that of HNO2. That would inhibit the photonitration
of 1NN. Moreover, the steady-state [31NN] is expected to decrease
with increasing pH, because the reaction rate constant between
31NN and nitrite increases with pH (Fig. 4). As far as 1NN
1NN
∑
with a polychromatic quantum yield U
= (3.42 0.42) ¥
OH
10-4 between 300 and 440 nm. The irradiation of 1NN in the
presence of nitrite yields the dinitronaphthalene isomers 15DNN
and 18DNN, and the photonitration pathway is likely to involve
reaction between excited 1NN (possibly 1NN) and NO2. The
photonitration of 1NN is maximum around pH 3.5. At higher pH
the formation rate of ∑NO2 would be lower because the photolysis
of nitrite is less efficient compared to that of HNO2. Moreover,
∑
3
photonitration (probably involving 1NN + NO2) is concerned,
the scavenging of 31NN by nitrite would compensate for the
generation of ∑NO2 by the same reaction.
∑
3
At low pH values, the nitration pathways might be modified
3
by the presence of the protonated triplet state, 1NN-H+.14,15 To
∑
3
account for the inhibition of 1NN photonitration below pH 3.5,
one has to consider that nitration is probably involving reaction
the production of NO2 by 1NN + nitrite (reaction (1)) could
not enhance photonitration because of the parallel scavenging of
31NN, which is likely involved into the nitration process. At lower
pH, the reaction between 31NN and ∑NO2 is probably replaced by
∑
between 31NN and NO2, and that both 31NN and 31NN-H+
would likely react with ∑NO2. Under the hypothesis that only the
∑
∑
3
other processes (e.g. reaction between 1NN-H+ and NO2) that
3
reaction of NO2 with 1NN produces the dinitronaphthalenes,
∑
3
3
if 1NN-H+ reacts with NO2 much faster than 1NN, depletion
do not yield the dinitronaphthalenes.
∑
3
of NO2 without production of 15DNN or 18DNN could be
operational in the presence of 1NN-H+. As a consequence, the
Overall, nitrite can be an important scavenger of 1NN at the
3
-
tens mM [NO2 ] levels that can be found in fog water in polluted
formation rate of the dinitronaphthalenes would be decreased.
areas
Atmospheric significance
Acknowledgements
The triplet state of 1NN is able to react with O2 (rate constant
(1.95 0.05)¥109 M-1 s-1),15 but also with dissolved anions such
as bromide ((7.5 0.2) ¥ 108 M-1 s-1)15 and nitrite ((3.36 0.28) ¥
109 M-1 s-1 at pH 6.5) (this work) (see Scheme 1). In aerated
solution the concentration of O2 can be around 0.3 mM, bromide
can reach up to 20 mM in sea-salt aerosol,40 nitrite up to 4 mM in
rain41 and up to 60 mM in fog42 (pH around 6–6.5 in both cases).
With 4 mM nitrite and the cited O2 and bromide levels, 95% of
31NN would be scavenged by O2 and 2–3% each by bromide and
nitrite. In contrast, 60 mM nitrite would scavenge around 25%
DV, VM and CM acknowledge financial support by PNRA-
Progetto Antartide and MIUR-PRIN 2007 (2007L8Y4NB, Area
02, project n◦ 36). The work of PRM in Torino was supported by
a Marie Curie International Incoming Fellowship (IIF), under the
FP7-PEOPLE programme (contract n◦ PIIF-GA-2008-219350,
project PHOTONIT). MB, TC and GM acknowledge the support
of the INSU-CNRS through the projects LEFE-CHAT and ORE
BEAM and Conseil Regional d’Auvergne for PhD grant provided
to TC.
3
of 1NN. These data suggest that nitrite could be an important
Notes and references
3
scavenger of 1NN in fog water in polluted areas. The reaction
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