Phototransformation of azoxystrobin fungicide in organic solvents. Photoisomerization vs. photodegradation

Article abstract of DOI:10.1039/c3pp50241d

Azoxystrobin is a systemic fungicide that has a tendency to accumulate at the surface of crop leaves or inside their cuticle where it undergoes photodegradation. Its photochemistry was investigated in n-heptane and isopropanol to mimic the polarity of wax leaves. Using analytical and kinetic data, we demonstrate that azoxystrobin (isomer E) undergoes efficient photoisomerization into the isomer Z with a quantum yield of 0.75 ± 0.08. This value is 30-fold higher than that reported in aqueous solution. The photoisomerization of isomer Z into azoxystrobin is more efficient with a chemical yield of 0.95 ± 0.1. In addition, a pseudo photostationary equilibrium is reached when the ratio [azoxystrobin]/[isomer Z] is 2.0 ± 0.1. Photodegradation also takes place from azoxystrobin (quantum yield = 0.073 ± 0.008). Photoproducts mainly arise from bond cleavage between rings and from demethylation of the ether with or without saturation of the acrylate double bond. Theoretical calculations were undertaken to investigate the photoisomerization mechanism and the solvent effect. These data show that the photochemical reactivity of azoxystrobin is enhanced when the solvent polarity decreases and thus should be significant in leaf waxes.

Full text of DOI:10.1039/c3pp50241d

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Phototransformation of azoxystrobin fungicide in organic solvents. Photoisomerization
vs photodegradation.
J. Chastain1,3, A. ter Halle1,3, P. de Sainte Claire1,2, G. Voyard1,3, M. Traikïa4, C. Richard1,3
*
(1) Clermont Université, Université Blaise Pascal, Institut de Chimie de ClermontꢀFerrand,
Equipe Photochimie, BP 10448, Fꢀ63000 CLERMONTꢀFERRAND
(2) Clermont Université, ENSCCF, Institut de Chimie de ClermontꢀFerrand, BP 10448, Fꢀ
63000 CLERMONTꢀFERRAND
(3) CNRS, UMR 6296, ICCF, Fꢀ63171 AUBIERE
(4) Clermont Université, Université Blaise Pascal, Institut de Chimie de ClermontꢀFerrand,
Equipe SEESIB, BP 10448, Fꢀ63000 CLERMONTꢀFERRAND
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* Corresponding author : Claire Richard
Claire.richard@univꢀbpclermont.fr
Tel: +33 (0)473407142
Abstract
Azoxystrobin is a systemic fungicide that has a tendency to accumulate at the surface of crop
leaves or inside their cuticle where it undergoes photodegradation. Its photochemistry was
investigated in nꢀheptane and isopropanol to mimick the polarity of wax leaves. Using
analytical and kinetic data, we demonstrate that azoxystrobin (isomer E) undergoes efficient
photoisomerization into the isomer Z with a quantum yield of 0.75±0.08. This value is 30ꢀfold
higher than that reported in aqueous solution. The photoisomerization of isomer Z into
azoxystrobin is more efficient with a chemical yield of 0.95±0.1. In addition, a pseudo
photostationary equilibrium is reached when the ratio [azoxystrobin]/[isomer Z] is 2.0±0.1.
Photodegradation also takes place from azoxystrobin (quantum yield = 0.073±0.008).
Photoproducts mainly arise from bond cleavage between rings and from demethylation of the
ether with or without saturation of the acrylate double bond. Theoretical calculations were
undertaken to investigate the photoisomerization mechanism and the solvent effect. These
data show that the photochemical reactivity of azoxystrobin is enhanced when solvent polarity
decreases and thus should be significant in leaf waxes.
Keywords: Fungicide, azoxystrobin, photoisomerization, photodegradation, solvent effect,
theoretical calculations
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1. Introduction
Strobilurins are one of the most important classes of fungicides since their commercial
introduction in 1996. Presently, over ten strobilurin fungicides are available. The huge impact
of this new class of pesticides in agriculture is well illustrated by the rapid success of
azoxystrobin (AZX). Four years after it was launched on the market, azoxystrobin was
registered for use on over 400 crop/diseases systems, making it the world’s bestꢀselling
fungicide.1 The huge success of strobilurin fungicides has prompted in recent years many
research investigations about their fate in food crops and food products.2ꢀ6 However,
photolysis of azoxystrobin has been much less studied and investigations only concern the
aquatic medium.7ꢀ8 In water, photolysis is a slow process, due to a low quantum yield (<5×10ꢀ
3
in simulated solar light) and a low absorptivity of the compound. Azoxystrobin mainly
undergoes photoisomerization (isomer E ꢀ isomer Z) (Scheme 1) and, at a lesser extent,
photohydrolysis of the methyl ester and of the nitrile group, cleavage of the acrylate double
7ꢀ8
bond and cleavage between the aromatic rings.
The photolysis of azoxystrobin is
interesting to study in water to predict its fate in surface waters. However, the understanding
of its photochemical dissipation on leaves after the crop treatment requires also to investigate
the reaction in a medium that is more representative of the natural environment.
Scheme 1
Here, we investigated the phototransformation of azoxystrobin in nꢀheptane and isopropanol.
The goal was to vary the polarity of the medium and to approach medium polarity similar to
that of the leaf cuticle in which azoxystrobin accumulates. Indeed, epicuticular wax contain
apolar long chain alkanes, but also more polar constituents offering to the substrates polar
environments. We focused on the photoisomerization vs photodegradation pathways in these
two solvents. The main photoproducts were characterized, and the quantum yields of
photoisomerization and photodegradation for both isomers were measured.
Photoisomerization is an important class of reactions frequently observed in the degradation
mechanisms of micropollutants.9ꢀ13 In this work, the photoisomerization of AZX is
investigated by means of experimental and theoretical approaches.
2. Materials and Methods
2.1 Chemicals and reagents
2

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Azoxystrobin
(methyl
(2E)ꢀ2ꢀ(2ꢀ{[6ꢀ(2ꢀcyanophenoxy)pyrimidinꢀ4ꢀyl]oxy}phenyl)ꢀ3ꢀ
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methoxyacrylate, 99.9%) was purchased from Fluka (SaintꢀQuentin Fallavier, France). Water
was purified by reverse osmosis RIOS 5 and Synergy (Millipore) (resistivity 18 Mꢁ cm,
DOC < 0.1 mg Lꢀ1). Spectroscopic grade (99%) nꢀheptane and isopropanol were purchased
from Aldrich (SaintꢀQuentin Fallavier, France). Methanol and acetonitrile (99%, HPLC
grade) were provided by Riedel de Haën (SaintꢀQuentin Fallavier, France). All the solvents
and chemicals were used as received.
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2.2 Irradiation protocols
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Parallel beam irradiations were performed using a xenonꢀmercury arc lamp (250 W, Lotꢀ
Oriel, France) equipped with a Schoeffel monochromator. Solutions of azoxystrobin (2 ×10ꢀ5
M) were irradiated in a quartz cuvette (1 cm optical length). The wavelength was set at 280 ±
5 nm. This wavelength falls outside the range of energies of the solar spectrum received on
earth surface (wavelengths larger than 295 nm). However, according to Kasha's rule,
photoreactivity is expected to take place from the lowest excited state because internal
conversion is fast compared to the other photophysical and photochemical processes. It is thus
expected to be independent of the excitation wavelength.14 Nevertheless, we performed
preliminary tests to ensure that the irradiations at 280 nm did not change the nature and
proportion of the photoproducts in comparison to those performed at 300 nm, but merely
changed the timescale of the experiment. The photon fluence rate was measured by chemical
actinometry. At 280 nm it was 5.5×1013 photons cmꢀ2 sꢀ1. Samples were taken at regular
irradiation times and quantified by HPLC. Three samples of 500 ꢀL were made per cuvette.
Samples for preparative HPLC were irradiated as follows. Fifty mg of azoxystrobin were
deposited on glass dishes, left in the dark until complete solvent evaporation and irradiated in
a Suntest (CPS Atlas, filtered xenon lamp, λ > 290 nm) for 4 h. Remaining azoxystrobin and
photoproducts were recovered using acetonitrile as a solvent and separated by preparative
HPLC. Laser flash photolysis experiments were also undertaken. The apparatus was
previously described.15 Azoxystrobin solubilized in nꢀheptane was irradiated at 266 nm using
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ꢀ
a NdꢀYAG laser. Peroxodisulfate was used as a chemical actinometer. The formation of SO4
radical was monitored at 450 nm (the product of the quantum yield of radical formation and
its molar absorption coefficient was set at 1900 Mꢀ1 cmꢀ1.15
2.3 Analyses
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Analysis of azoxystrobin and its photoproducts in nꢀheptane or isopropanol was performed by
normal phase HPLC on a MerckꢀHitachi apparatus consisting of a pump (Lꢀ6200A), an
autosampler (ASꢀ2000) and a UV/Vis detector (Lꢀ4250). Data were collected and processed
by the chromatographic software ClassꢀVP (Shimadzu SA, Japan). A silica Sunfire column
(250×4.6 mm i.d., 5 ꢂm particle size ꢀ Waters SAS ꢀ France) was used for these separations.
Sample injection volumes were 20 ꢂL and the UV detector set at 250 nm. The isocratic
mobile phase consisted of 20% isopropanol and 80% nꢀheptane (flow rate of 1 mL.minꢀ1).
Retention times were 9.8 min for azoxystrobin and 14.4 min for the isomer Z. The
azoxystrobin detection limit was approximately 10ꢀ6 M, which was well below the
concentration range used in the study. Semi preparative HPLC was done on a Varian
apparatus consisting of Prestar 218 pumps, a Prestar 410 injector, a Prestar 701 collector and a
DAD 335 detector. The column was a Varian microsorb 8 ꢀm, 250×41 mm and the eluent
consisted of a mixture water/acetonitrile 50%/50% at a flux of 24 mL/min. The detection
wavelength was set at 250 nm.
The mass spectrometer has been previously described.16 A LC/QTOF equipped with an
orthogonal geometry Zꢀspray ion source was used (Waters/Micromass, Manchester, UK). UV
detection under MS heading was a Waters Alliance 2695 photodiode array detector system.
The elution conditions were the same as for analytical experiments. The optimum voltages
found for the probe and ion source components (to produce maximum intensity) were 2.1 kV
for the stainlessꢀsteel capillary, 35 V for the sample cone, and 2 V for the extractor cone. In
the negative mode voltages were switched. Scanning was performed from m/z 100 to 1000.
Tandem mass spectrometric (MS/MS) experiments were conducted in a collision cell with an
argon pressure of 1 bar. A collision energy gradient (15–35 eV) was used for preliminary
fingerprints and specific energies of 15–45 eV were used to unambiguously assign the
MS/MS fragments. Scanning was performed from m/z 50–300 in the “high resolution” mode.
For mixtures obtained by irradiation in nꢀheptane, it was necessary to evaporate nꢀheptane and
dissolve remaining azoxystrobin, isomer Z and photoproducts in methanol.
UVꢀVisible absorption spectra were recorded using a Cary 3 spectrophotometer. NMR
analyses were done on dried samples reꢀsuspended in 600 ꢀL of CDCl3. CDCl3 was used for
locking and shimming. Sodium tetra deuterated trimethylsilyl propionate (TSPd4, Eurisotop)
constituted a reference for chemical shifts (0 ppm). The instrument used was a Bruker Avance
1
500 MHz equipped with a 5 mm inverseꢀtriple tuned (TXI) H/13C/15N with zꢀgradient coil
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probe. For 1D 1HꢀSpectra, 128 scans were collected with an impulsion time of 7.5 ꢂs, a
relaxation delay of 5 s, an acquisition time of 4.67 s, a spectral window of 5000 Hz and 64K
data points zeroꢀfilled to 128K before Fourier transformation with 0.3 Hz line broadening.
The 2D homonuclear (1H/1H COSY and TOCSY) and heteronuclear (1H/13C HSQC and
HMBC) experiments were performed with quadrature phase detection in both dimensions,
using stateꢀTPPI or QF detection mode in the indirect one. For each 512 (homonuclear
experiments) or 256 (heteronuclear experiments) increments in the indirect dimension
(spectral width of 30000Hz), 2K data points were collected (5000 Hz) and 32 or 64 transients
were accumulated in the direct dimension. 13C decoupling (GARP) was performed during
acquisition time for heteronuclear experiments. A π/2 shifted square sineꢀbell function was
applied in the indirect dimension before Fourier transformation. Spectra were treated with
Topspin version 2.0. All NMR spectra were recorded at 25°C. The NMR attributions of
azoxystrobin and isomer Z are the following:
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Azoxystrobin
1H chemical shifts ppm in CDCl3 : 3.62 (3H, s, O=COMe), , 3.73 (3H, s, OMe) , 6.42 (1H, s,
Hꢀ5 pyrim) , 7.20 (1H, d, 8Hz, Hꢀ3 Ph) , 7.28 (1H, d, 7.6 Hz, Hꢀ5 Ph) , 7.30 (1H, d, 7.5 Hz,
Hꢀ6 PhCN) , 7.32 (1H, td, 7.7 and 2 Hz, Hꢀ4 PhCN) , 7.34 (1H, td, 7.6 and 0.8 Hz, Hꢀ6 Ph) ,
7.39 (1H, 7.5 and 1.8 Hz, Hꢀ4 Ph) , 7.47 (1H, s, C=CꢀHꢀ3) , 7.64 (1H, td, 7.9 and 1.6 Hz, Hꢀ5
PhCN) , 7.69 (1H, dd, 7.8 and 1.5 Hz, Hꢀ3 PhCN) , 8.40 (1H, s, Hꢀ2 pyrim).
13C chemical shifts ppm in CDCl3 : 51.82 (O=COMe) , 62.23 (OMe) , 92.66 (Cꢀ5 pyrim) ,
107.24 (Cꢀ2) , 107.58 (Cꢀ2 PhCN) , 115.43 (CN) , 122.28 (Cꢀ3 Ph) , 123.3 (Cꢀ5 Ph) , 126.1
(Cꢀ1 Ph) , 126.3 (Cꢀ6 PhCN) , 126.31 (Cꢀ4 PhCN) , 129.38 (Cꢀ4 Ph) , 132.99 (Cꢀ6 Ph) ,
133.80 (Cꢀ3 PhCN) , 134.42 (Cꢀ5 PhCN) , 150.42 (Cꢀ2 Ph) , 154.38 (Cꢀ1 Ph CN) , 158.19 (Cꢀ
1 pyrim) , 160.95 (Cꢀ3) , 167.66 (Cꢀ1) , 170.32 (Cꢀ4 pyrim and Cꢀ6 pyrim).
Isomer Z
1H chemical shifts ppm in CDCl3 : 3.59 (3H, s, O=COMe), , 3.85 (3H, s, OMe) , 6.44 (1H, s,
Hꢀ5 pyrim) , 6.60 (1H, s, C=CꢀHꢀ3) , 7.17 (1H, d, 8.1Hz, Hꢀ3 Ph) , 7.27 (1H, d, 7.4 Hz, Hꢀ6
PhCN) , 7.30 (1H, d, 7.2 Hz, Hꢀ6 Ph) , 7.31 (1H, td, 7.8 and 2 Hz, Hꢀ5 PhCN) , 7.35 (1H, td,
7.6 and 0.8 Hz, Hꢀ4 PhCN) , 7.39 (1H, 7.5 and 1.8 Hz, Hꢀ4 Ph) , 7.66 (1H, td, 7.9 and 1.6 Hz,
Hꢀ5 PhCN) , 7.70 (1H, dd, 7.8 and 1.4 Hz, Hꢀ3 PhCN) , 8.36 (1H, s, Hꢀ2 pyrim).
13C chemical shifts ppm in CDCl3 : 51.56 (O=COMe) , 62.70 (OMe) , 92.94 (Cꢀ5 pyrim) ,
107.10 (Cꢀ2) , 107.4 (Cꢀ2 PhCN) , 115.43 (CN) , 122.42 (Cꢀ3 Ph) , 123.3 (Cꢀ6 PhCN) , 123.3
(Cꢀ5 Ph) ,126.1 (Cꢀ1 Ph) , 126.2 (Cꢀ4 PhCN) , 129.16 (Cꢀ4 Ph) , 132.06 (Cꢀ6 Ph) , 133.3 (Cꢀ3
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PhCN) , 134.3 (Cꢀ5 PhCN) , 150.5 (Cꢀ2 Ph) , 154.1 (Cꢀ1 Ph CN) , 158.01 (Cꢀ1 pyrim) ,
159.55 (Cꢀ3) , 165.47 (Cꢀ1) , 170.8 (Cꢀ4 pyrim and Cꢀ6 pyrim).
2.4 Theoretical calculations
The isomerization reaction was also investigated from theoretical calculations at the
B3LYP/6ꢀ31G(d) level with the Gaussian03 suite of programs.17 Due to computer efficiency
requirements, a model compound was used in place of azoxystrobin. It was assumed that the
substituted diazine ring had no significant effect on the electronic properties of the
isomerization process. Internal rotation about one of the two adjacent bonds labeled δ1 and δ2
in Scheme 2 may lead to 4 different conformers, named hereafter Etrans, Ecis, Ztrans and Zcis.
The E/Z notation is consistent with rotation about δ1. The procedure was as follows: several
minima and transition state structures were found on the S0 ground state Potential Energy
Surface (PES) with the DFT method at the B3LYP/6ꢀ31G(d) level of theory. The connection
between these structures was established from Intrinsic Reaction Coordinate calculations
followed by full geometry optimizations. In addition, Minimum Energy Path (MEP)
computations were performed at the same level of calculation by freezing selectively one
torsion angle (δ1 or δ2) while all other coordinates were allowed to relax.
Scheme 2
The potential energy of the first singlet excited state S1 was obtained by performing
B3LYP/6ꢀ31G(d) TimeꢀDependent Density Functional Theory (TDꢀDFT) calculations. This
procedure yielded excitation energies at the MEP geometries.
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3. Results and discussion
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3.1 Analytical study
Azoxystrobin absorption spectrum in nꢀheptane is shown in Figure 1. It shows a regular
decrease in absorbance from 200 to 300 nm with shoulders at 240 nm and 280 nm. This latter
wavelength was chosen for irradiation. A typical HPLC chromatogram of irradiated
azoxystrobin is shown in Figure 2. Structural data of photoproducts obtained from mass
spectrometry and NMR are given in the experimental section and in Table 1. The main
photoproduct is the isomer Z. Its structure was confirmed both by HPLC/MS with
determination of the high resolution m/z value and by proton and carbonꢀ13 NMR analysis of
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the fraction obtained by semiꢀpreparative HPLC. NMR results for azoxystrobin and isomer Z
are in accordance with those found in the literature.18 The difference between the two isomers
is observed for the ethylenic proton with chemical shifts at 7.47 ppm and 6.60 ppm for
azoxystrobin and Z isomer, respectively. UV spectra of azoxystrobin and isomer Z have
similar shape (see Figure 1).
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Several minor photodegradation products were also detected (Scheme 3). The main one that
eluted before azoxystrobin is AZꢀPyOH. This compound, which is most probably produced
by CꢀO cleavage between two aromatic rings, was also observed in photolysis experiments
that were conducted in water (Chovelon et al, 2007). Two other very minor photoproducts
were observed. The first one, AZꢀOH (m/z = 392 with fragment at m/z = 374 in ES+)
probably corresponded to the replacement of OMe by OH. Its formation was also reported in
water (Chovelon et al, 2007). The second one is more surprising. The high resolution mass
analysis revealed an elemental composition of C23N3O6H21 that corresponded to
azoxystrobin+CH4O. A mass/mass experiment gave two fragments at m/z = 404 and 372, in
line with the decomposition of C23N3O6H21 into azoxystrobin (m/z = 404) and its main
fragment (m/z = 372). Last, a semiꢀpreparative HPLC fraction enriched in this compound was
analyzed by NMR. This analysis confirmed the diene saturation (the signal of the ethylenic
proton was not present on the spectrum). This product was detected even when the whole set
of experiments was performed in the absence of MeOH as a solvent. Thus, it may be obtained
in the addition reaction of a MeO radical on the acrylate double bond, the latter being released
upon irradiation of another azoxystrobin molecule. These analyses demonstrate the radicalar
character of the reactions.
Scheme 3
3.2 Transients study
Nanosecond laser flash photolysis experiments were undertaken to get information on the
transient species photogenerated. Excitation of azoxystrobin at 266 nm induced an increase of
the absorbance between 280ꢀ305 nm. This absorbance increase was observed at the end of the
pulse, showing that corresponding species were produced during the pulse, i.e. extremely
rapidly. Moreover, the absorbance of the solution after the flash was stable over several
hundred of microseconds, indicating that the produced species are already photoproducts
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(isomer Z and minor photodegradation products). By actinometry, we determined the ε×Φ
product at 290 nm and found 300 ± 50 Mꢀ1cmꢀ1.
3.3 Kinetic study
The disappearance of azoxystrobin and the formation of isomer Z upon irradiation were
monitored by HPLC. To quantify the concentrations of isomer Z, it is necessary to know its
absorption coefficient at the detection wavelength (250 nm). As HPLC data indicate that
isomer Z is clearly the main photoproduct at the beginning of the reaction, we postulated the
full conversion of azoxystrobin into isomer Z. By subtracting the contribution of azoxystrobin
from the UV absorbance of irradiated solutions, one gets the UV absorption spectrum of
isomer Z. It is found that the absorption coefficients of azoxystrobin and isomer Z are similar
within the wavelength range 220ꢀ250 nm. This hypothesis is fully consistent with other data
reported later in the manuscript.
The evolution of the concentrations of azoxystrobin and isomer Z in irradiated nꢀheptane are
shown in Figure 3. In the first part of the reaction, there is a rapid loss of azoxystrobin and
formation of isomer Z. A stabilization occurs after 50 min when the ratio [AZX]/[Z] of the
isomer concentrations is equal to 2.0±0.1 in favor of azoxystrobin. In the second part, a slow
photodegradation of both compounds takes place. Deoxygenation of the solutions did not
affect these kinetic data. To analyze the kinetic data, it was postulated that photoreaction took
place as shown in Scheme 3. Based on the low absorbance of azoxystrobin and isomer Z at
280 nm in the chosen experimental conditions, one can assume that the rate of photon
absorption by azoxystrobin and Z are:
Equation 1
Equation 2
where I0 is the photon fluence rate, l is the path length, ꢀꢁꢄꢂꢅꢃꢆ and ꢀꢁꢅꢂꢃ are the absorption
coefficients of AZX and Z, respectively, at 280 nm. In these conditions, the rate of
azoxystrobin isomer Z concentration evolution are given by:
Equation 3
Equation 4
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where Φꢇꢈꢉ→ꢈ and Φꢈ→ꢇꢈꢉ are the quantum yields of photoisomerization of AZX and isomer
Z. In eqs. 3ꢀ4, ΦdegꢀAZX and ΦdegꢀZ are the quantum yields of photodegradation of AZX and
isomer Z. If ꢊꢄꢅꢆ→ꢈ and ꢊꢈ→ꢇꢈꢉ are the firstꢀorder rate constants of photoisomerization for
azoxystrobin and isomer Z, one can deduce from eqs 1ꢀ4 that:
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286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
Equation 5
Equation 6
In scheme 3, kdeg-AZX and kdeg-Z are the firstꢀorder rate constants of photodegradation for
azoxystrobin and Z isomer, respectively. The rate constants were calculated by fitting the
solution of the respective set of differential equation for AZX and Z by a oneꢀstep order
solver based on an explicit RungeꢀKutta formula. Note the set of differential equations was
not stiff in this work. The results are given in Table 2 and Figure 3. Isomer Z undergoes faster
photoisomerization than azoxystrobin, and photodegradation arises mainly from azoxystrobin.
Quantum yields are deduced from these rate constants and the experimental absorption
ꢀ1
ꢀ1
ꢄꢅꢆ
ꢁꢂꢃ
coefficients at 280 nm given in eqs. 5 and 6. For azoxystrobin, using ꢀ
=2800 M cm , one
gets Φꢇꢈꢉ→ꢈ = 0.75 and ΦdegꢀAZX = 0.073. ꢀꢁꢅꢂꢃ can be obtained using our different data. First,
considering that the values of molar absorption coefficients are identical for AZX and isomer
Z at 250 nm, one can obtain ꢀꢁꢅꢂꢃ = 3900 Mꢀ1cmꢀ1. Second, from the ε×Φ product
ꢄꢅꢆ
ꢄꢅꢆ
+
ꢁꢋꢃ
measurement at 290 nm that corresponds to Φꢇꢈꢉ→ꢈ×(ꢀꢁꢅꢋꢃ−ꢀ
), we obtain ꢀꢁꢅꢋꢃ = ꢀ
ꢁꢋꢃ
390 = 1100 Mꢀ1cmꢀ1. This latter value confirms the aboveꢀmentioned ꢀꢁꢅꢂꢃ based on the UV
spectrum. Finally, one gets Φꢈ→ꢇꢈꢉ = 0.95. Photoisomerization is thus a very efficient process
in alkane. Photodegradation mainly arises from azoxystrobin.
The phototransformation of azoxystrobin and the formation of isomer Z in isopropanol are
shown in Figure 4. Rate constants and quantum yields are reported in Table 2. It appears that
reactivity is affected by the solvent changing. For AZX, the quantum yield of
photoisomerization is reduced by 2.1 and the quantum yield of photodegradation by 1.3. The
quantum yield of isomer Z photoisomerisation is reduced by a factor 1.7. In isopropanol, the
concentrations stabilization occurs after 115 min instead of 50 min in nꢀheptane and the ratio
[AZX]/[Z] is equal to 2.3 instead of 2.
3.4 Theoretical calculations
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307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
The isomerization mechanism can be understood by considering the two torsion angles δ1 and
δ2 shown in Scheme 2. The ground state Minimum Energy Path for E/Z isomerization and the
corresponding TDꢀDFT energies are shown in Figure 5. Four minimum structures were found
in the electronic ground state S0. The Ecis/Etrans and Zcis/Ztrans pairs of conformers have similar
energies (0.0/0.6 kcal/mol and 4.0/3.5 kcal/mol) and are separated by low electronic energy
activation barriers (6.7 and 2.1 kcal molꢀ1, respectively). Thus, ground state AZX or Z species
are actually a mixture of equal amounts of the respective two conformers. In addition, large
activation barriers (above 45 kcal molꢀ1) prevent isomerization in the ground state
(Etrans→Ztrans or Ecis→Zcis). These calculations show that isomerism between E and Z is not
possible without irradiation, this point is confirmed experimentally.
The computation of the excited state potential energy surface S1 shows the presence of two
avoided crossings. In the region of the first avoided crossing (AC1), the S0/S1 energy gap is
small, thus the non adiabatic transition probability is large. Note that higher level
multideterminant CASSCF calculations would be necessary to see whether AC1 is actually a
conical intersection. Considering that the reactive medium is a mixture of Ecis(S0) and
Etrans(S0), our calculations show that, upon irradiation, isomerization is favored for the Etrans
species with respect to Ecis: the barrier from Etrans(S1) to AC1 is smaller than that on the other
side of the potential (11.7 kcal/mol). This reactive path leads to Etrans and Ztrans, i.e. E→Z
isomerization. On the other hand, Ecis(S1) faces a large barrier (10.9 kcal/mol) towards
Etrans(S1). Ecis (S1) may also react through the second avoided crossing (AC2). However, at
AC2, the energy gap is large and the transition probability is small for isomerization via the
cis conformers (Ecis →Zcis). Thus, our calculations show that photoisomerization is expected
to be kinetically favored between the trans conformers.
Second, the dipole moments were also investigated to understand the effect of solvent
polarity. It is shown in the previous sections that the rates of photoisomerization (ꢊꢄꢅꢆ→ꢈ and
ꢊꢈ→ꢇꢈꢉ) are smaller by a factor of two in isopropanol in comparison to those that were
obtained in heptane (see Table 2). The theoretical molecular dipole moments were obtained
from the electronic density of the ground state and the first electronic state wavefunctions.
These values are shown in Figure 5. In polar solvents, stabilization is expected to be
significant for species with large dipole moments. At AC1, the ground state dipole moment is
larger (8.3 D) than that of the excited state (1.8 D). Thus at AC1, in polar solvents, the excited
state is less stabilized than the ground state potential energy. This effect leads to a larger S0/S1
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340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
gap in isopropanol. Consequently the transition probability is expected to be smaller in
isopropanol than in heptane, and so are the rates of photoisomerization. This is in agreement
with our experimental results (see Table 2).
4. Conclusions
In this study one got important information on the photoreactivities of azoxystrobin and its
photoisomer (isomer Z). In apolar medium (nꢀheptane), azoxystrobin undergoes efficient
photoisomerization and photodegradation while its photoisomer gives back azoxystrobin with
a quantum yield close to unity. Theoretical calculations made using a model molecule
revealed that four conformers may participate in the isomerization mechanism (Ecis, Etrans, Zcis
and Ztrans). However, investigations of the potential energy surfaces in the ground state and
singlet first excited state showed that isomerization is favored between Etrans and Ztrans. In a
more polar medium (isopropanol), photoreactions are comparatively slower. This is
rationalized in terms of stabilization of ground and excited states at the avoided crossing. This
medium effect may have an impact on the persistence of azoxystrobin, which should depend
on the constitution of leaves epicuticular wax or on the nature of formulation agents. Last,
several photoproducts were detected. Their nature suggests that photoreactions are radicalar.
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359
References
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361
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363
364
365
366
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368
369
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379
380
381
382
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15. F. Bonnichon and C. Richard, Phototransformation of 3ꢀhydroxybenzonitrile in water,
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and nicosulfuron on their photochemical reactivity on cuticular wax film, Chemosphere 2010,
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17. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima,
Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V.
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433
434
435
436
437
438
439
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441
Martin, D. J. Fox, T. Keith, M. A. AlꢀLaham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
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18. J. J. Parra, J. V. Mercader, C. C. Agulló, A. A. AbadꢀSomovilla and A. A. AbadꢀFuentes,
Synthesis of azoxystrobin transformation products and selection of monoclonal antibodies for
immunoassay development. Toxicol. Lett. 2012, 210, 240ꢀ7.
442
443
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444
445
N
N
N
N
hν
hν
O
O
O
O
CN
MeO
OMe
CN
MeO
O
OMe
O
Isomer Z
Azoxystrobin = Isomer E
446
447
448
Scheme 1: Photoisomerization of azoxystrobin.
449
450
451
452
453
Ecis
Zcis
Ztrans
Etrans
MeO
OMe MeO
O
O
OMe
1
2δ
δ
O
O
OMe
OMe OMe
OMe
454
455
456
457
458
459
460
Scheme 2: The model molecule used in this work for azoxystrobin. Rotation about torsion
angles δ1 and δ2 leads to four different conformers.
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kAZXꢀ>Z
Azoxystrobin
isomer Z
kdegꢀZ
kZꢀ>AZX
kdegꢀAZX
N
N
N
N
N
N
+
+
O
O
O
O
O
OH
CN
MeO
OMe
OMe
CN
MeO
OH
CN
O
O
AZꢀOMe
AZꢀPyꢀOH
AZꢀOH
461
462
463
464
Scheme 3: Phototransformation paths proposed for azoxystrobin.
465
466
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467
Figures
468
469
470
471
472
473
474
475
476
477
478
479
480
481
0,4
0,3
0,2
0,1
0,0
200
250
300
350
482
483
484
Wavelength/nm
485
486
487
488
489
490
491
Figure 1: Absorption spectra of azoxystrobin (full line) and isomer Z (dotted line) in nꢀ
heptane.
492
493
Figure 2 : HPLC chromatogram of azoxystrobin irradiated in nꢀheptane, analysis by reverse
phase analysis after nꢀheptane evaporation, conversion extent of 15%, detection at 250 nm.
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494
495
Figure 3: Evolution of the concentrations of azoxystrobin and isomer Z in heptane with
irradiation time. These results are given in perꢀcent of the initial concentration of AZX.
Experimental data (■,□) and theoretical model (solid lines) obtained from the rate constants in
Table 2.
496
497
498
499
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500
501
Figure 4: Evolution of the concentrations of azoxystrobin and isomer Z in isopropanol with
irradiation time. These results are given in perꢀcent of the initial concentration of AZX.
Experimental data (■,□) and theoretical model (solid lines) obtained from the rate constants in
Table 2.
502
503
504
505
506
507
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150
140
130
120
110
100
90
9.8D
7.4D
4.8D
7.8D
4.6D
7.4 D
7.4D
4.0D
S1
S0
3.2D
80
70
1.8D
8.3D
60
50
40
30
E
Z
20
Z
cis
1.6D
E
trans
cis
trans
4.0D
2.3D
10
4.3D
0
-124.8
179.7
-90.2
2.8
-4.2
179.7
3.6
δ1
δ2
-99.8
179.5
-116.5
-89.1
3.6
-6.1
79.2
-172.9
-179.2
Angle (50°/div)
508
509
Figure 5. Ground state MEP potential energy surface (S0) calculated at the B3LYP/6ꢀ31G(d)
level. The first excited state energies (S1) were obtained from TDꢀDFT calculations at the
B3LYP/6ꢀ31G(d) level on S0 geometries. The magnitudes of the dipole moments (D units) are
indicated for important structures on S0 and S1.
510
511
512
513
514
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516
517
Table 1 : Mass data (ES+) for azoxystrobin, isomer Z and photoproducts (see structures in
Scheme 3)
m/z (main
fragments)
in ES+
High resolution m/z
value (Da) in ES+
Calculated mass
(Da) in ES+
Mass error
(mDa)
Compound
Azoxystrobin
Isomer Z
AZꢀPyOH
AZꢀOH
404.1230
404.1237
214.0612
404.1246
404.1246
214.0617
1.6
0.9
0.5
404 (372)
404 (372)
214
392 (374)
436 (404, 372)
AZꢀOMe
436.1539
436.1509
2.9
518
519
520
521
522
523
524
525
Table 2 : First order rate constants fitted from the experimental data and calculated quantum
yields for photoisomerization and photodegradation in nꢀheptane and isopropanol.
Uncertainties on data are of 10%.
kiso/sꢀ1
kdeg/sꢀ1
Φiso
Φdeg
Isopro
Isopro
Isopro
panol
0.36
0.57
Isopro
panol
n-heptane
n-heptane
n-heptane
n-heptane
panol
2.1×10ꢀ4
panol
3.3×10ꢀ5
<10ꢀ8
4.4×10-4
7.8×10-4
4.3×10-5
AZX
Isomer Z
0.056
0.75
0.95
0.073
<10-5
<10-8
<10-5
4.7×10ꢀ4
526
527
528
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529
530
531
532
Equation 1 to 6
ꢒꢓꢔ
ꢕ
ꢖ
ꢄꢅꢆꢕ
ꢖ
IaAZXꢌ I0 ꢍ1-10-ꢎ
AZX l ꢗ ~ lnꢘ10ꢙꢚI0 ꢀꢁꢂꢃ AZX l
(1)
(2)
ꢏꢐꢑ
533
ꢓ
ꢕ ꢖ
ꢏꢐꢑ
IaZꢌ I0 ꢍ1-10-ꢎ Z l ꢗ ~ lnꢘ10ꢙꢚI0ꢀꢁꢅꢂꢃ Z l
ꢕ ꢖ
534
535
536
537
ꢛ ꢜꢕꢇꢈꢉꢖ ꢌ IꢞꢇꢈꢉΦꢇꢈꢉ→ꢈ ꢛ IꢞꢈΦꢈ→ꢇꢈꢉ ꢟ IꢞꢇꢈꢉΦꢜꢠꢡꢢꢇꢈꢉ
(3)
(4)
ꢜꢝ
ꢜꢕꢈꢖ ꢌ IꢞꢇꢈꢉΦꢇꢈꢉ→ꢈ ꢛ IꢞꢈΦꢈ→ꢇꢈꢉ ꢛ IꢞꢈΦꢜꢠꢡꢢꢈ
ꢜꢝ
538
539
ꢊꢄꢅꢆ→ꢈꢌlnꢘ10ꢙꢚI0 ꢀꢁꢄꢂꢅꢃꢆ
ꢊꢈ→ꢇꢈꢉꢌlnꢘ10ꢙꢚI0 ꢀꢁꢅꢂꢃl Φꢈ→ꢇꢈꢉ
l
Φꢇꢈꢉ→ꢈ
(5)
(6)
540
541
542
23