A density functional theory and laser flash photolysis investigation of carbofuran photodegradation in aqueous medium

Article abstract of DOI:10.1016/j.jphotochem.2012.02.018

Density functional theory (DFT) approach was used to study the photodegradation of Carbofuran in aqueous medium. This computational method enables us to assign the electronic transitions and interpret the dissociative behavior upon irradiation based on a thermodynamical analysis of the bond dissociation energies (BDE) of Carbofuran. According to these calculations, phenoxy C-O bond appears weaker than the C-N bonds. Hence, it was predicted that the photodegradation of Carbofuran should occur with an initial homolytic dissociation of the C-O bond of the carbamate moiety. Laser Flash Photolysis (LFP) results clearly indicate the formation of the phenoxyl radical, which support the outcome of this theoretical approach.

Full text of DOI:10.1016/j.jphotochem.2012.02.018

Journal of Photochemistry and Photobiology A: Chemistry 235 (2012) 1–6
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A density functional theory and laser flash photolysis investigation
of carbofuran photodegradation in aqueous medium
A. Atifia,b, M. Talipova, H. Mountacerb, M.D. Ryana, M. Sarakhac,∗
a Chemistry department, Marquette University, P,O. Box 1881, Milwaukee, Wisconsin 53201, USA
b Laboratoire des Sciences de l’Environnement et du Developpement, Equipe de Chimie Ecologique, FST Settat, Morocco
c Institut de Chimie de Clermont-Ferrand (ICCF) UMR6296, équipe de Photochimie, Université Blaise Pascal, 24, Avenue des, Landais, BP80026, 63171 Aubière, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 August 2011
Received in revised form 14 February 2012
Accepted 17 February 2012
Available online 8 March 2012
Density functional theory (DFT) approach was used to study the photodegradation of Carbofuran in aque-
ous medium. This computational method enables us to assign the electronic transitions and interpret the
dissociative behavior upon irradiation based on a thermodynamical analysis of the bond dissociation
energies (BDE) of Carbofuran. According to these calculations, phenoxy C O bond appears weaker than
the C N bonds. Hence, it was predicted that the photodegradation of Carbofuran should occur with an ini-
tial homolytic dissociation of the C O bond of the carbamate moiety. Laser Flash Photolysis (LFP) results
clearly indicate the formation of the phenoxyl radical, which support the outcome of this theoretical
approach.
Keywords:
DFT
Photodegradation
Carbofuran
BDE
© 2012 Elsevier B.V. All rights reserved.
Laser flash photolysis
Homolysis
1. Introduction
The initially formed radical pair may be converted to the more sta-
ble (in polar solvents) ion pair by electron transfer (ET). Finally, both
Carbofuran (2,3-dihydro-2,2-dimethyl benzofuran-7-yl methyl
carbamate) is broad spectrum insecticide, nematicide and
radical pairs and ion pairs may lead to formation of the starting sub-
strate, namely carbofuran, within a cage recombination process:
Radical Combination (RCom) or Ion Combination (ICom).
a
miticide [1]. Residual fate of Carbofuran has been extensively inves-
tigated in water, soil and in different experimental conditions
[2–5]. In fact, several pathways were suggested for Carbofuran pho-
todegradation, and the major observed photoproduct was phenol
derivative [6]. Chiron et al. have published a report on Carbofu-
ran photodegradation where two decomposition products were
identified by LC–MS, 3-hydroxy-7-carbofuranphenol as a product
of hydrolysis and 2-hydroxy-3-(2-methylprop-1-enyl)-phenyl-N-
methylcarbamate arising from a rearrangement product [7]. John
and Howard have proposed a mechanism for the first steps of
the Carbofuran photodegradation in water. According to these
authors, the carbamate group undergoes a cleavage process via
On the other hand, quantum chemical computations have
recently been considered as an effective tool for the analysis of
pesticide molecules [10–15]. Arul Dhas et al. have performed the
density functional theory (DFT) computations to interpret elec-
tronic and vibrational spectra, and intramolecular charge transfer
responsible for biological activity of Chlorothalonil [16]. Aaron
and al. have applied a theoretical, gradient-corrected Hartree–Fock
density functional theory (HF-DFT) approach to determine the bond
dissociation energy (BDE) for the photodegradation processes of
the 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide in the gas
phase and in aqueous medium [17].
C
O heterolysis, leading to the formation of carbamic acid and 2,3-
In this context, the present work examines the mechanistic
aspect of the first step of Carbofuran photochemical degradation
in water, based on a theoretical approach using the DFT methods.
In addition, our experimental data given by laser flash photolysis
permit to corroborate this theoretical approach.
dihydro-2,2dimethylbenzofuran-7-ol [8]. However, the knowledge
of the main photochemical reaction remains incomplete, and the
mechanistic data are still needed. A mechanism of similar system is
shown in Fig. 1 [9]. Upon irradiation, the generated excited singlet
may undergo intersystem crossing (ISC) from S1 to T1, homolytic
cleavage to form radical pair or heterolytic cleavage to form ion pair.
2. Materials and methods
2.1. Experimental
∗
The compound Carbofuran (98) was purchased from
Sigma–Aldrich. UV–vis spectrum was taken for aqueous
Corresponding author.
E-mail address: mohamed.sarakha@univ-bpclermont.fr (M. Sarakha).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2012.02.018

2
A. Atifi et al. / Journal of Photochemistry and Photobiology A: Chemistry 235 (2012) 1–6
ArOR
hv
by certain further improvements over the other DFT functional
methods [20]. Indeed, generally the later methods are constructed
with the main aim of improving the well-known deficiency in
their long-range behavior. Both of the employed computational
methodologies can be considered as hybrid HF-DFT (i.e. they
include an admixture of HF exchange energy), in contrast to the
pure DFT ones. The standard Pople-type 6-311++G(d,p) basis set
was employed for orbital expansion, solving the Kohn-Sham (KS)
SCF equations iteratively for each particular purpose of this study.
Electronic transitions for the molecule were calculated from
excited state calculations using the time dependent-density func-
tional theory (TD-DFT) method. This method is frequently found
to be a powerful and accurate approach for describing low-lying
excited states of conjugated molecules and has consequently been
applied to solve many chemical and physical problems [21–24].
The influence of the solvent on the energetics of bond dissocia-
tion processes for the species under study was accounted for in the
framework of the self- consistent reaction field (SCRF) methodol-
ogy, employing the Polarized Continuum Model (PCM) for water,
the later medium being treated as a continuous dielectric with a
relative dielectric constant (=78.39) [24–26]. In general, the PCM
calculations were based on the gas-phase structures optimized at
the same theoretical level. Such method is used in many other
works and is known to be consistent to study similar systems
[27,28].
Subsequently, after locating the stationary points on the poten-
tial energy surfaces (PESs) under study, their character was tested
by computing the harmonic vibrational frequencies. The absence of
imaginary frequencies (namely negative eigenvalues of the second-
derivative matrix) was used as a criterion that a particular point
on the PES corresponded to a minimum energy structure (instead
of being a saddle point). Thermochemistry of the bond disso-
ciation processes was analyzed in the following way, described
below.
The energies for each of the involved species were corrected
by the zero-point vibrational energies, computed on the basis of
harmonic vibrational analysis. Subsequently, the rotational, vibra-
tional and translational contributions were added to the obtained
E(0) values, in order to get the E(T) ones (for T = 298.15 K). All of
these contributions were calculated on the basis of standard statis-
tical mechanics expressions for an ideal gas constituted of harmonic
oscillators within a canonical ensemble [29]. The corresponding
enthalpies H(T) and free enthalpies G(T) were calculated from Eqs.
(1) and (2):
ICom
RCom
ISC
S1
T1
Hetero
ET
Homo
ArO
R
ArO
R
R
ArO
Ion-derived products
Radical-derived products
Fig. 1. General mechanism of photodissociation.
sample of Carbofuran (0.1 mM), using the HP-8452A photo-diode
array spectrometer in the region 190–700 nm.
Laser flash photolysis (LFP): Transient absorption experiments
in the 20 ns to 500 ␮s time scale were carried out on a nanosec-
ond laser flash photolysis spectrometer from Applied Photophysics
(LKS.60). Excitation (ꢀ = 266 nm) was from the fourth harmonic of
a Quanta Ray GCR 130-01 Nd:YAG laser (pulse width 5 ns), and was
used in a right-angle geometry with respect to the monitoring light
beam. A 3 cm3 volume of an argon-saturated solution was used in
a quartz cell, and was stirred after each flash irradiation. Individual
cell samples were used for a maximum of 10 consecutive experi-
ments. The laser energy was within the range 1–15 mJ. The obtained
signal was an average of about 10 shots. The transient absorbance at
preselected wavelength was monitored by a detection system con-
sistingof a pulsedxenonlamp(150 W), monochromator, anda 1P28
photomultiplier. A spectrometer control unit was used for synchro-
nizing the pulsed light source and programmable shutters with the
laser output. This also housed the high-voltage power supply for
the photomultiplier. The signal from the photomultiplier was dig-
itized by a programmable digital oscilloscope (HP54522A). A 32
bits RISC-processor kinetic spectrometer workstation was used to
analyze the digitized signal.
LC/MS studies were carried out with a Waters (Alliance 2695)
high performance liquid chromatography system coupled to a
Quattro LC triple quadrupole mass spectrometer (Micromass,
Manchester, UK) equipped with a pneumatically assisted elec-
trospray ionization source (ESI) and a Waters photodiode array
detector. Data acquisition and processing were performed by Mass-
Lynx NT 3.5 system. Chromatography was run using a Nucleodur
column100-5 C8 ec (250 × 4.6 mm, 5 ␮m) and a 60/40 (v/v) mix-
ture of acetonitrile and water with 0.2% acetic acid as mobile phase
at 1 mL min−1. The electrospray source parameters were:capillary
voltage 3.5 kV (or 3 kV in the negative mode), cone voltage 15 V,
source block temperature 120 ◦C, desolvatation gas temperature
400 ◦C. Argon was used for collisional activated dissociation (CAD)
at a pressure of 1.5 × 10−3 torr and 10–50 eV collision energy.
The irradiations at 254 nm were obtained with PHILIPS TUV 6 W
lamp delivering a parallel beam.
H(T) = E(T) + RT
G(T) = H(T) − TS
(1)
(2)
Finally, the bond dissociation reaction energies E and enthalpies
H were calculated from the standard expressions for the corre-
sponding quantities, given in Eqs. (3)–(5):
ꢀ
ꢀ
ꢁE0
ꢁH0
ꢁG0
=
=
=
Ep −
Er
(3)
(4)
(5)
p
r
ꢀ
ꢀ
Hp −
Hr
2.2. Computational details
p
r
ꢀ
ꢀ
The computation was performed using two density functional
theory (DFT) based methodologies. The first methodology is based
on the combination of the Beckes three-parameter adiabatic
connection exchange functional (B3) with the Lee–Yang–Parr
(LYP) correlation functional (B3LYP method) [18,19]. The second
DFT methodology, based on PBE1 exchange functional combined
with the PBE correlation one10 (PBE0 method) is characterized
Gp −
Gr
p
r
where the indices p and r represent, respectively, the products and
the reactants of the processes under study.
All quantum chemical calculations for the purpose of the present
study were carried out with the Gaussian 09 series.

A. Atifi et al. / Journal of Photochemistry and Photobiology A: Chemistry 235 (2012) 1–6
3
analyzed the involved molecular orbitals. It appears that all
observed bands are ␲–␲* transitions.
3. Results and discussion
3.1. Geometry optimization
Optimized structure of Carbofuran, using the atomic numbering,
is shown in Fig. 2. The relevant geometry parameters have been
calculated for Carbofuran and the relevant species arising from the
dissociation pathways under study, which correspond to minimas
on the B3LYP and PBE0/6-311++G(d,p) PESs in water environment.
Cartesian coordinates of all optimized structures are included in
the supporting material.
3.2. TD-DFT and UV–vis spectra
Electronic transitions are usually classified according to the
orbitals engaged or to specific parts of the molecule involved.
Common types of electronic transitions in organic compounds
are ␲–␲* and n–␲*. In order to understand those transitions in
Carbofuran molecule, TD-DFT calculation on electronic absorption
spectrum was performed. The UV absorption bands in Carbofuran
are observed at 203, 218 and 274 nm. The TD-DFT calculations
show three electronic transitions with significant oscillators
strength, which are in good agreement with experimental values
(see Table 1). The similarity of the spectra permitted to corroborate
the used of the adopted method/basis set for this study. In attempt
to characterize the electronic character of those transitions, we
Fig. 2. Optimized structure and atomic numbering of Carbofuran at DFT level cal-
culation.
Table 1
TD-DFT calculations of Carbofuran.
6-311++G(d,p)
Excited state
E1
B3LYP
PBE0
Exp
MOsa
ꢀ (nm)
fb
ꢀ (nm)
fb
ꢀ (nm)
58(␲)–62(␲
59(␲)–60(␲
∗
∗
257
0.0811
251
0.0825
274
)
)
59(␲)–61(␲
59(␲)–63(␲
57(␲)–60(␲
58(␲)–60(␲
∗
∗
∗
∗
225
196
0.0462
0.092
220
197
0.0439
0.3509
218
203
E3
)
)
)
)
E7
58(␲)–61(␲
58(␲)–62(␲
59(␲)–62(␲
∗
∗
∗
)
)
)
a
Molecular orbitals.
Oscillator strength.
b

4
A. Atifi et al. / Journal of Photochemistry and Photobiology A: Chemistry 235 (2012) 1–6
Table 2
Computed thermochemical parameters of Carbofuran in aqueous medium. Parameters are given in kJ/mol.
6-311++G(d,p)
Cleavage
B3LYP
Bond
PBE0
ꢁE
ꢁTE
ꢁH
ꢁG
ꢁE
ꢁTE
ꢁH
ꢁG
O(20) C(21)
C(21) N(23)
N(23) C(24)
226.3
385.6
359.4
225.6
386.3
362.8
228
388.8
365.3
173.2
335.8
314
255.1
410.4
378.8
254.4
411.2
382.4
256.9
413.7
384.9
202.3
360.5
332.9
Homolytic
O(20) C(21)
C(21) N(23)
N(23) C(24)
269.6
560.1
498.5
268.2
561.1
500.1
270.7
563.6
502.5
222
512.1
458.4
295.4
597.7
531.1
294.1
598.5
532.9
296.6
601
535.4
247.1
550.7
488
Heterolytic
3.3. Thermodynamical analysis
3.5. Products analysis
The characterization of some byproducts of
Based on a HF-DFT approach we have calculated by the B3LYP
and the PBE0 methods the bond dissociation reaction energies
(BDE) and enthalpies of the dissociation asymptotes correspond-
ing to the homolytic and heterolytic photocleavage processes
for Carbofuran in water. These thermodynamical analysis deals
with ground state of short-lived intermediates. The computed
thermochemical parameters characterizing the bond dissociation
processes of Carbofuran are presented in Table 2.
These theoretical, thermochemical parameters imply the fol-
lowing interpretation for the occurrence of the photodegradation
mechanism of Carbofuran in an aqueous medium.
In fact, the homolytic bond dissociation energies are lower
than heterolytic ones. Therefore, homolytic photocleavage of the
different bonds should be promoted than heterolytic processes.
Moreover, the homolytic BDE of the C O bond is lower than the
a
solution
3.0 × 10−4 mol L−1 of carbofuran irradiated at 254 nm and pH = 5.4
was achieved by HPLC/ESI/MS technique. As described in the
experimental section, the mass spectrometry experiments were
performed in both positive and negative ion modes. The positive
0.025
500 ns
1 µs
2 µs
0.020
0.015
0.010
0.005
0.000
C
N BDEs ones. This important thermodynamical argument would
favor the C O dissociation asymptote over the C N ones. These
preferential processes are also in agreement with other works
[30–32]. As reported for photolysis of carbamates in water, the pro-
cess should occur with an initial homolytic cleavage of the phenoxy
bond.
3.4. Laser flash photolysis
300
350
400
450
500
550
Wavelength, nm
In order to detect the transient species generated under light
excitation we performed laser flash experiment using the fourth
harmonic of a Nd:YAG laser (266 nm). The laser excitation of oxy-
gen free aqueous solution of carbofuran at the concentration of
5.0 × 10−3 mol L−1 leads the detection of transient species that
absorbs within the wavelength regions of 290 nm and 450 nm
(Fig. 3). The decay kinetics was the same within the whole wave-
length region suggesting the presence of a single intermediate
species [33,34]. It exhibits well-defined absorptions at 380 nm and
420 nm and was assigned unequivocally to the phenoxyl radical
derivative based and the similarity with the spectra reported for
this type of phenoxyl intermediates. The decay kinetics of the
observed species was monitored at 420 nm and shows that the
generated phenoxyl radical presents a half-life of about 1.5 ␮s in
oxygen free solutions (Fig. 3, inset). Such half-life appears to be
too short for a phenoxyl radical therefore suggesting that its decay
involves reactions with other radicals generated in the system.
Moreover, it is important to note that the absorbance of this tran-
sient species increased linearly with the laser energy within the
range 2–10 mJ indicating its formation through a monophotonic
process (Fig. 4, inset). A non-negligible oxygen effect was clearly
observed on the decay of phenoxyl radical. The pseudo first order
rate constant under oxygen saturated aqueous solution was eval-
uated to 4.0 × 106 s−1 (Fig. 4). It is worth noting that under our
experimental conditions, no significant absorption was observed
within the region 600–800 nm suggesting that no solvated electron
is formed through a photoejection process.
Fig. 3. Transient spectra showing phenoxyl radicals after laser flash photolysis
(266 nm) of Carbofuran in aqueous solutions, monitored 500 ns, 1 s and 2 s after laser
excitation. Inset: phenoxyl transient decay trace recorded after laser flash photolysis
(266 nm), monitored at 400 nm.
0.010
0.02
0.008
0.01
0.00
0.006
0.004
0.002
0.000
a
b
0
5
10
Laser energy, mJ
c
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
0.0
Time, µs
Fig. 4. Oxygen effect on the phenoxyl radical decay at ꢀ = 400 nm (a) oxygen free
solution; (b) aerated solution; and (c) oxygen saturated solution. The insert shows
the dependence of the absorbance at ꢀ = 380 nm with laser energy.

A. Atifi et al. / Journal of Photochemistry and Photobiology A: Chemistry 235 (2012) 1–6
5
O
O
Solvent cage
NH
NH
O
O
O
O
O2
O
O
hν
cage escape
(1)
NH
NH
O
O
homolytic cleavage
Solvent cage
O
O
hν
O
O
homolytic cleavage
O
O
O
O2
(2)
(3)
RH
OH
H
O
OH
O
H
O
O
O
O
O
O
O2
NH
+
2
H
O
Fig. 5. Mechanism for the first step of Carbofuran photodegradation in water.
H
O
O
O
O
mode was found to be more suitable method for the parent
compound and also for the of photogenerated products. An irra-
diated solution presenting 20% conversion showed the presence
of remaining carbofuran with [M+1] = 222 and several byproducts.
At least one isomer presenting the same molecular ion mass that
corresponds more likely to a structure with a different position
of the group function CO NH CH3. The presence of the phenolic
derivative was also detected at shorter retention time with a
molecular ion of [M+1] = 165. Moreover, trace concentration of
a hydroquinone form was also detected with [M+1] = 181. The
irradiation within the pH range 3–7 did not show any changes
concerning the nature of the generated byproducts.
O
O
(4)
H
O
O
Fig. 6. Proposed mechanism for the role of oxygen in the formation of quinone
derivative.
The action of molecular oxygen on the solvent cage will lead first
to a cage escape process (Fig. 6, Eq. (1)) that is in favor of an efficient
production of the phenoxyl radical and second to the formation of
the quinone derivative by its reaction on the para position of the
OH
O
hv
+
+
NH
O
O
O
O
NH
OH
OH
OH
O
[M+1] = 165
[M+1] = 181
[M+1] = 222
[M+1] = 222
aromatic moiety (Fig. 6, Eqs. (2) and (3)). The mechanism may be
proposed as shown in Fig. 6.
3.6. Mechanism
4. Conclusion
As already demonstrated, the BDE calculations imply an ini-
tial homolytic photocleavage of the Phenoxy bond of Carbofuran
molecule. This prediction is in complete agreement with laser flash
photolysis findings. These clearly show that under direct irradia-
tion of Carbofuran in aqueous solution the degradation involves
the formation of the phenoxyl radical.
It should be noted that, although photolysis involves the chem-
istry of excited state which requires a high level of theoretical
calculations, our simple approach taking into account only the
chemistry of ground state has allowed us to predict the photochem-
ical behavior of Carbofuran in water.
Considering the outcome of this investigation, we propose a
mechanism Fig. 5 for the first step of Carbofuran photodegradation
in water. This mechanism consists on an initial homolytic cleav-
age of the carbamate moiety, after light illumination, Carbofuran
singlet excited state permits the formation of a radical pair in the
solvent cage, which then undergoes intermolecular coupling or side
reactions.
The photo-Fries rearrangement mechanism is well documented
to occur in aryl esters, amides and carbamates [35]. It is also
reported in other studies on photodegradation of Carbofuran and
similar molecule [36], finally, to form carbofuran phenol and car-
bamic acid, we believe that these radicals must undergo side
reactions.
This investigation into the photochemistry of pesticides sug-
gests that the photodecomposition pathways of Carbofuran, in
aqueous medium, could be predicted by the density functional
theory approach. The thermodynamical analysis, based on Bond
Dissociation Energies comparison, implies an initial homolytic pho-
todissociation of Phenoxy bond of Carbofuran molecule. Further, it
is interesting to emphasize that these theoretical thermodynamic
calculations are well corroborated by our experimental, Laser flash
photolysis, results which reveals the formation of phenoxyl radical
upon direct irradiation of Carbofuran in water.
Acknowledgments
The authors would like to thank the North Atlantic treaty Orga-
nization (NATO)for the financial support through the Collaborative
LInkage Grant (CBP.MD.982508). The authors are grateful to FUL-
BRIGHT Organization for their support and to Marquette University
for using the Pere Cluster. The authors thank Dr. Q. Timerghazin for
his collaboration.
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