ARTICLE pubs.acs.org/JPCA
The Role of Triplet State KetoEnol Tautomerism in the Photodeamination of Metamitron Sofia Kouras-Hadef,†,^ Pascal de Sainte-Claire,‡,§ Alexandra ter Halle,†,§ Amina Amine-Khodja,^ and Claire Richard†,§,* †
Laboratoire de Photochimie Moleculaire et Macromoleculaire (LPMM), Universite Blaise Pascal, Clermont Universite, BP 10448, 63000 Clermont-Ferrand, France ‡ LPMM, ENSCCF, Clermont Universite, BP 10448, F-63000 Clermont-Ferrand, France § LPMM, CNRS, UMR 6505, BP 80026, F-63171 Aubiere, France ^ Laboratoire des Techniques Innovantes de Protection de l’Environnement, Universite Mentouri, 25000, Constantine, Algeria
bS Supporting Information ABSTRACT: Substituted 4-amino-1,2,4-triazin-5-ones undergo photodeamination through cleavage of the NNH2 bond in the presence of oxygen and water. To elucidate the mechanism of this reaction, we investigated the photolysis of metamitron (4-amino-6-phenyl-3-methyl-1,2,4-triazin-5-one) by nanosecond laser flash photolysis, steady-state irradiation, and ab initio calculations. Upon pulsed laser excitation of deoxygenated aqueous metamitron, two transient species are clearly detected. The predictions of ab initio results are consistent with experimental results: (i) it is proposed here that the transient species are, respectively, the keto and diradical forms of the metamitron ketoenol tautomerism in the triplet state, and (ii) in water, the activation free energy barrier of enolization is drastically decreased. Thus, the formation of the diradical triplet is enabled in aqueous solvent. A detailed analysis of the intermediate structures that lead to the final products (HNO2 and deaminometamitron) is provided.
1. INTRODUCTION The physicochemical behavior of organic micropollutants released in the aquatic environment is thoroughly evaluated for environmental and safety reasons. Usually, the photochemical degradations in surface waters play a key role because the absorption of sunlight radiation is efficient. Photochemical studies of micropollutants or related compounds have given interesting results in term of mechanistic approaches.14 Pesticide structures belong to a very large variety of chemical families and functionalities. Interestingly, the triazine derivatives are encountered in three main categories: herbicides, fungicides, and insecticides. Triazines include a large variety of substituted 1,3,5-triazines with subtituents such as Cl, N-ethyl, N-propyl, N-tert-butyl, N-cyclopropyl, methylthio, or methoxy. These compounds exhibit absorption below 300 nm, and their photolysis is observed under irradiation conditions not relevant to natural conditions.5 Metamitron (4-amino-6-phenyl-3-methyl-1,2,4-triazin-5-one) and metribuzin (4-amino-6-tert-butyl-3-methylthio-1,2,4-triazin-5-one), on the other hand, contain a carbonyl function that imparts absorption above 300 nm, until 350 nm. Thus, these two molecules are susceptible to photolysis in solar light. Metamitron (MT) and metribuzin (MB) were reported to undergo photodeamination by cleavage of the NNH2 bond (see Scheme 1).611 r 2011 American Chemical Society
Interestingly, photodeamination of MT was only observed in water and in the presence of oxygen.9 In the case of MB, photodeamination was also reported in a large range of solvents, but this reaction follows desulfuration.11 The chemical structure of these asymmetric triazin-5(4H)ones suggests that the carbonyl function in the triplet excited state could react through an intra- or intermolecular hydrogen abstraction process.12,13 However, the detailed reaction mechanism leading to deamination was never understood, and these solvent effects were never rationalized. To address this question, we investigated the photochemical transformation of MT by means of steady-state irradiations, laser flash photolysis, and ab initio calculations.
2. EXPERIMENTAL SECTION MT (Riedel de H€aen; purity Pestanal 99.9%), 2,4,6-trimethylphenol (Aldrich, purity 99%), furfuryl alcohol (Aldrich, purity 99%), sodium azide (Aldrich, purity 99%), and methyl viologen (Aldrich, purity 98%) were used as received. Water was purified with a Milli-Q (Millipore) device. Phosphate buffer was prepared Received: September 15, 2011 Revised: November 2, 2011 Published: November 02, 2011 14397
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Scheme 1. Photodeamination of Metamitron (MT) and Phototransformation of Metribuzin (MB)a
a
Atom labels are provided for the diketometribuzin species.
from sodium hydrogenophosphate (Prolabo; purity 99.5%) and potassium dihydrogenophosphate (Prolabo; purity 99.5%). The experimental setup for the transient absorption experiments has been described elsewhere.14 Measurements were carried out using a frequency-quadrupled Nd:YAG laser (Quanta-Ray DCR-1, 266 nm, pulse duration 10 ns or Quanta-Ray GCR-130-1, 266 nm, pulse duration 9 ns). The ε Φ products were evaluated using potassium peroxodisulfate as a chemical actinometer. The concentrations of peroxodisulfate and MT were adjusted for the solutions to show the same absorbance of 0.30 ( 0.01 at 266 nm. The slopes (s1)375, (s1)560, and s2 of the linear dependence of the pulse end absorption on laser energy were obtained for the MT solution at 375 and 560 nm, and for the peroxodisulfate solution, at 450 nm, respectively. The ε Φ product at 375 nm was evaluated using the equation ε Φ = (ε Φ)peroxo (s1)375/s2 and ε Φ product at 560 nm using ε Φ = (ε Φ)peroxo (s1)560/s2. The value (ε Φ)peroxo was taken at 3000 ( 200 M1 cm1.15 LC-ESI-MS analyses were performed using a Hewlett-Packard HP1100-MSD system working under positive atmospheric pressure ionization equipped with a Varian Omnispher C18 100 mm 3.5 mm (3 mm) column. UVvisible spectra were recorded on a Cary 3 (Varian) spectrophotometer. Analytical HPLC analyses were carried out using a Waters apparatus equipped with a photodiode array detector, a conventional reverse phase 5 μm column, and a mixture of water and formic acid (0.1%)/ ACN (8020, v/v) as a mobile phase. Nitrite and nitrate ions were titrated by ionic chromatography (Dionex DX320 on a AS11 4 mm 250 mm Dionex column). MT was irradiated in a cuvette at 313 nm using a high pressure mercury lamp and a Schoeffel monochromator. Solutions were deoxygenated by argon bubbling for 20 min prior to irradiation and oxygen saturated by oxygen bubbling. The pH was adjusted using phosphate buffer (103 M). The quantum yield of MT photolysis was measured by monitoring the initial rate of MT photolysis. The irradiation times were adjusted to get losses of MT between 5% and 15%. Potassium ferrioxalate which was synthetized as previously described, was used as chemical actinometer.16 The quantum yield of 6-hydroxy-pyran-3-one formation was measured by adjusting the irradiation times to get losses of furfuryl alcohol between 5% and 20%. Density functional theory (DFT) calculations were performed with the Gaussian03 series of programs17 to characterize the
Figure 1. Transient absorption spectra of MT in deoxygenated phosphate buffer solution upon irradiation at 266 nm. (A) Absorbances at the laser pulse end (solid line) and 8 μs after the pulse end (dashed line). The time profile of the transient absorption monitored at 560 nm (a) is given in the inset. The decay of T1 and the formation and decay of T0 1 are obtained by kinetic modeling. Some of the atoms in S0 0 carry a large spin density. (B) Influence of methyl viologen (5.6 105 M) on transient absorbance at 400 nm; O: MT alone; b: MT with methyl viologen; Δ: methyl viologen alone. The inset gives the absorbance profile at 600 nm in the presence (b) and in the absence (O) of methyl viologen.
structures of the T1 and T0 1 species. The B3LYP exchange correlation functional18 and the 6-31G(d) basis set were used for geometry optimizations, while single point energies were obtained at the B3LYP/6-311++G(d,p) level. Unpolarized Hamiltonians were used for open-shell species. In addition, single point excited state energies were computed from time-dependent DFT calculations (TD-DFT) and the 6-311++G(d,p) basis set on optimized ground-state structures.
3. RESULTS AND DISCUSSION 3.1. Formation of the KetoEnol Tautomer Triplets (T1 / T0 1). Laser Flash Photolysis. Irradiation of a neutral deoxygenated
aqueous solution of MT (1.5 105 M) yielded at pulse end the transient shown in Figure 1A and named hereafter T1. The transient spectrum showed a maximum of absorption below 400 nm and a shoulder between 500 and 600 nm. The ratio of the absorbances at 375 and 560 nm was 10. Eight microseconds after the pulse end, the shape of the transient absorption spectrum changed. A maximum at 560 nm appeared, and the ratio of the absorbances at 375 and 560 nm decreased to 3.5. In addition, the 14398
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Table 1. Specific Bond Lengths (Å) and Dihedral Angles (deg) in Metamitron (see labels in Figure 2 ) and Diketometribuzin (labels in Scheme 1) S0 0
S0
T1 opt
T0 1 opt
Metamitron R1 (O(8)Ha)a
2.167
1.006
2.261
0.989
R2 (N(7)Ha)a C(5)N(4)N(7)Ha
1.023 23.8
1.749 0.8
1.023 29.2
1.870 5.4 176.0
C(5)N(4)N(7)Hb
86.9
178.5
82.6
O(8)C(5)N(4)N(7)
0.7
0.6
0.4
6.5
C(5)C(6)N(1)N(2)
0.5
0.2
39.2
32.5
δ
3.2
18.0
3.2
2.1
0.985/2.245
Diketometribuzin 0
R1/R 1
a
a
2.195/2.331
1.000/2.263
2.087/2.277
R2/R0 2a
1.020/1.019
1.819/1.021
1.024/1.018
1.948/1.028
C(1)NNHa C(2)NNHb
28.3 36.6
∼0 ∼0
7.4 12.0
0.1 0.2
OC(1)NN
7.0
∼0
4.7
0.3
OC(2)NN
7.0
∼0
1.9
0.2
C(1)CNN
0.3
∼0
5.4
∼0
R1, R2, R0 1, and R0 2 are shown in Figure 4a (metamitron) and 4e (diketometribuzin).
absorbance decay can be described by a biexponential function. Both results are evidence for the presence of a species different from that detected at pulse end. The long-lived species named T0 1 shows a first-order decay rate constant of 1 105 s1, and T1 a first-order decay rate constant higher than 3 105 s1. In oxygen-saturated aqueous medium, T1 readily disappeared. The bimolecular rate constant of reaction with oxygen was found to be 1.3 ( 0.2 109 M1 s1. T0 1 was no longer detected. The pulsed laser excitation of MT (1.5 105 M) in deoxygenated acetonitrile yielded at pulse end a transient spectrum similar to that of T1, with a decay first-order rate constant of 3.5 105 s1. Under these experimental conditions, T0 1 was not detected. Either T1 and T0 1 are produced through parallel pathways or they are in a daughtermother relationship. The fact that T0 1 is not observed in aerated water solution and in deoxygenated acetonitrile supports the second hypothesis. However, it was not possible to observe the formation of T0 1, because T1 and T0 1 absorb in the same spectral region. If we assume that T1 gives T0 1, then the decay absorbance profile measured in deoxygenated medium is the overlapping of T1 decay and T0 1 formation. One can thus model the absorbance profile of the two species separately assuming that the sum of the two profiles is equal to the experimental decay curve (curve a in Figure 1A). Assuming the rates of T1 decay and T0 1 formation are equal and decay exponentially, an accurate fit of the experimental absorbance decay is obtained with kd = 5.0 105 s1 and k0 d = 1.0 105 s1, where kd is the apparent first-order rate constant of the T1 decay and T0 1 formation, and k0 d is the apparent first-order decay rate constant of T0 1. Results are shown in the inset of Figure 1A. Using potassium peroxodisulfate as a chemical actinometer, the ε Φ product was 52 900 ( 4000 M1cm1 at 375 nm and pulse end, and 1900 ( 130 M1cm1 at 560 nm 8 μs after the pulse end. Attempts to trap one of these species using methyl viologen (MV) as an electron acceptor were successful. Photolyzing MT in the presence of MV (5.6 105 M) in deoxygenated medium produced a new species (Figure 1B) showing a maximum around 400 and 600 nm as described for the reduced form of methyl
Figure 2. The structure of metamitron in the keto form (S0) and the diradical enol structure (S0 0) at the B3LYP/6-31G(d) level in the singlet ground state. Bond lengths are in angstr€ oms.
viologen MV•+.19 The absorbance decay changed after 2 μs, showing that MV reacted with T0 1 but not with T1. The formation of MV•+ via photoreduction of excited MV is ruled out considering the very short lifetime of excited MV and the low concentration of MT as a possible reductant.19 In the inset of Figure 1B, the formation of MV•+ is better observed; the firstorder rate constant of formation is equal to 2.0 105 s1 instead of 1.0 105 s1 in the absence of MV. The rate constant of reaction between T0 1 and MV is therefore equal to 1.0 105 s1, and the bimolecular rate constant of reaction around 2 109 M1 s1 in good agreement with literature data.20 Calculations. Ground-State Structures. Important bond lengths and dihedral angles are given in Table 1 for the different electronic states of the molecules investigated below. The structures are given as Supporting Information. MT is stabilized in the ground state (S0) by intramolecular hydrogen interactions between the primary amine and the carbonyl group. The O 3 3 3 H bond length is 2.167 Å in S0 (see labels in Figure 2). In Figure 2 (S0), the phenyl ring, triazine ring, and the N(7)N(4)C(5)dO 3 3 3 H structure are nearly coplanar. The twist angle δ between the rings is 3, while the OC(5) N(4)N(7) and C(5)N(4)N(7)Ha dihedral angles are 0.7 and 23.8, respectively. In this state, there is competition between the repulsive interaction of the N(4) and N(7) adjacent nitrogen lone pairs and the attractive long-range O 3 3 3 H hydrogen bonding. The dihedral angle between the nitrogen lone pairs 14399
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Scheme 2. KetoEnol Equilibrium for Metamitron in the Ground State (S0) and the Triplet State (T1)a
a
Hydrogen atoms are shown explicitly. Some of the atoms in these structures carry a large spin density (see Table 4).
Scheme 3. Structure of Aminofluorenones
Figure 3. The B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) excited state energies (singlet and triplet states) of MT and the energy levels of the diradical species in Figure 2. Energies (kcal/mol) are given with respect to S0. The oscillator strength (f) is given for the singlet states. Calculations were performed on the optimized geometry of the S0 state. The energy level of the fully optimized structure of T1 (named here T1 opt) is also shown on this figure. Note the energy levels of T0 1opt (9H2O) and the respective TS are given with respect to T1opt (9H2O), i.e., a structure where the solvent is modeled with nine water molecules.
in MT (defined as the angle between the plane that bisects the CNC angle and the plane that bisects the hydrogen atoms in NH2) is 61.2. In comparison, in the hydrazine molecule (NH2 NH2), the dihedral angle between the two bisectors of the HNH angles is 91 ( 2,21 owing exclusively to the repulsive nitrogen lone pair interactions. In this work, two transient species T1 and T0 1 were detected during the laser flash photolysis of MT. While T1 is very likely the lowest triplet state of MT in the keto form, the nature of T0 1 needs further investigation. Examination of the MT structure indicates that ketoenol tautomerism might play a key role in the formation of T0 1. Two different mechanisms that lead to the T0 1 species were investigated. First, T0 1 may arise from excitation and intersystem crossing of metamitron in its diradical form, provided that the ketoenol equilibrium is significantly driven toward the diradical structure in the ground state (S0 0 in Scheme 2 below). Note the keto tautomer to the very right of Scheme 2 has not been taken into account in our calculations because its energy is largely above that of the respective enol counterpart (25.2 kcal/mol above S0 0).The second mechanism assumes that T0 1 arises from enolization of metamitron in the triplet state (T1). The ketoenol tautomerism represented in Scheme 2 was first investigated on the singlet surface (S0 / S0 0) at the B3LYP/ 6-311++G(d,p)//B3LYP/6-31G(d) level of theory. The electronic energy of S0 0 is 21.8 kcal/mol above S0 (Figure 3), and the
respective equilibrium constant exp(ΔGr/RT) is 1016 at room temperature. This result rules out the presence of S0 0 in the medium prior to irradiation. Excited States. The MT electronic excited states were obtained with the TD-DFT method at the B3LYP/6-311++G(d, p)//B3LYP/6-31G(d) level of theory. Respective energy levels are shown in Figure 3. The first electronic state S1 at λ = 377 nm arises from the transition of triazine ring valence σ electrons to the π* LUMO of MT. Thus, the oscillator strength is weak. However, the transition is slightly allowed because (1) the rings are slightly twisted with respect to one another (δ = 3 deg) and (2) the LUMO in MT is delocalized among both rings. The second singlet excited state S2 at λ = 329 nm resulted from a ππ * transition which is strongly allowed. Last, among the four MT triplet states shown in Figure 3, only T2 has strong σ radical character. Provided S2 f S1 internal conversion is faster than S2 f T3, T4 intersystem crossing (isc), the first transient species T1 is obtained through S1 f T1 isc. The nature of the lowest excited states in metamitron plays a key role in understanding the deamination mechanism, and it is important to see whether S1 and T1 are charge transfer states. For example, in aminofluorenone (Scheme 3), S1 and T1 are intramolecular charge transfer (ICT) states with distinct charge transfer property from the amino group to the fluorenone moiety.22,23 In aminofluorenones, the dipole moment of the charge transfer states changes significantly from that of the ground state upon photoexcitation. Thus, the quenching of these excited states depends strongly on solvent polarity. This molecule is similar to MT since both species carry neighboring primary aromatic amine and aromatic carbonyl groups. In addition, it was shown that intersystem crossing (isc) deactivation of CT states was enhanced by strong intramolecular hydrogen bonding,23 while the deactivation of species with no intramolecular hydrogen bonding as in dimethylaminofluorenone (R = CH3 in Scheme 3) proceeds exclusively through internal conversion between the S1 and S0 states. However, examination of the electronic density of the T1 state in MT shows that it is not a CT state. The dipole moment in the S0 and T1 states in MT are, respectively, 2.89 and 1.40 D, and the intramolecular O 3 3 3 H bond distance increases from 2.167 Å to 2.261 Å. Contrarily, in aminofluorenone (R = H in Scheme 3), the dipole moment increases significantly from 2.35 to 4.00 D, 14400
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Table 2. O 3 3 3 H and N 3 3 3 H Long-Range Bond Lengths (Å) for the Singlet and Triplet Transition States Shown in Figures 4ae and 5 Metamitron 0
T1 f T0 1
S0 f S 0 0(H2O) Figure 4a
1(H2O) Figure 4b
2(H2O) Figure 4c
(2 wire)(H2O) Figure 4d
0(H2O) Figure 4a
1(H2O) Figure 4b
2(H2O) Figure 4c
(2 wire)(H2O) Figure 4d
9(H2O) Figure 5
R1 (COH)
1.120
1.130
1.223
1.249
1.190
1.266
1.076
1.382
1.225
R2 (NH)
1.441
1.466
1.657
1.457
1.342
1.335
1.281
1.434
1.882
R3 (H2OH)
1.330
1.221
1.179
1.155
1.423
1.095
1.195
R4 (H2OH)
1.114
1.052
1.103
1.159
1.224
1.111
0.991
R5 (H2OH)
1.105
1.180
R6 (H2OH) R (H2OH)
1.360
1.254 1.027
R0 (H2OH)
1.579 Diketometribuzin 0
S0 f S 0 0(H2O) Figure 4e
T1 f T0 1 0(H2O) Figure 4e
R1 (COH)
1.137
1.346
R2 (NH) R0 1 (COH)
1.421 2.473
1.218 2.502
R0 2 (NH)
1.016
1.018
and the O 3 3 3 H bond distance is simultaneously reduced, in agreement with the charge transfer character of the low triplet excited state.24 These results indicate that the respective excited state T1 in metamitron is not a CT state. Thus, the correlation between intramolecular H-bond strengths in CT states and isc rate enhancement may not hold true for MT. In our work, two transient triplet species were detected. One of these (T1) was obtained through isc and is not a CT state. It is shown later in this work how solvent interactions ease the conversion between T1 and the second transient species, T0 1. Second, the nature of the lowest singlet excited state in MT and aminofluorenone differ significantly. In aminofluorenone, S1 involves π-electron excitations between molecular orbitals that include the resonant nitrogen lone pair, while in metamitron, S1 arises from σ-electron excitations. This difference is mainly due to the specific nature of the NN bond in MT. As mentioned above, the competitive interactions between the nitrogen lone pairs and the intramolecular O 3 3 3 H bond is at the origin of the 61.2 twist of the NN bond in MT (see above for the definition of this angle). Contrarily, the NH2 group in aminofluorenone lies in the plane of the ring structure (no twist), and as a consequence, the excited states with significant π character are stabilized. This effect is not as strong in MT. It was shown above that intramolecular hydrogen transfer between the primary amine and the carbonyl group was disfavored on the singlet ground-state adiabatic potential energy surface. This reaction is now investigated on the triplet state surface (T1 / T0 1, see Figure 3). Geometries of the reactant, transition states, and product species were obtained at the B3LYP/6-311+ +G(d,p)//B3LYP/6-31G(d) level of theory. The respective free energy of activation for this reaction is 23.3 kcal/mol at 298 K,25 and the forward unimolecular kinetic rate constant is 7.2 105 s1 at room temperature. This value is much smaller than the experimental T1 decay rate constant (∼105 s1), and thus the T1 / T0 1
reaction described in this section cannot account for the detection of the second transient species T0 1. To be competitive with the overall decay rate constant of the first transient species T1, the forward T1 f T0 1 rate constant should not be smaller than ∼104 s1, i.e., the respective free energy of activation should not exceed ∼12 kcal/mol. It is shown below that T0 1 is rapidly trapped by triplet oxygen in radical/radical recombination reactions, thus driving the ketoenol equilibrium toward the enol species. The successful trapping of T0 1 by methyl viologen confirms its diradicalar nature.20 Influence of Water. Because T0 1 was detected in aqueous media only, we investigated the influence of water on its formation. The solvent was first treated with the polarizable continuum model (PCM) where the solute is placed in spherical cavities within the solvent reaction field. The Pauling atomic radii were used, and hydrogen atoms were treated explicitly.26 However, the activation energy barrier for intramolecular hydrogen transfer did not change significantly, and it was decided to pursue the investigation by treating the solvent explicitly. In the following, the ketoenol equilibrium is investigated through cooperative assistance of one or more water molecules. Models of different sizes were studied. The respective geometries and electronic activation barriers at the transition states are presented in Table 2 and Figure 4, and Table 3, respectively. In the simplest model, hydrogen transfer is assisted by a single water molecule (see Figure 4b). In Figure 4c, the influence of solvent interactions with both hydrogen atoms of the primary amino group is studied. In Figure 4d, hydrogen transfer through a wire of two water molecules (Grotthuss-like mechanism)27 is investigated. However, in none of these models did the activation barrier drop significantly.28 In the last model (Figure 5 and Tables 2 and 3), solvent water molecules cover the entire region above the triazine ring, and in the triplet state, the unpaired radical electron on the N(1) 14401
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Figure 4. B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) singlet and triplet state geometries of the transition states for nonassisted (a) and water-assisted (bd) intramolecular hydrogen transfer in MT. The models include one H2O, 2 H2O, and 2 H2O wire-like molecules in Figure 4b, 4c, and 4d, respectively. The transition states for the non-water-assisted reaction of MB are shown in e.
nitrogen center (see Figure 2) is solvated as well. The geometry and spin densities of T1, T0 1 and the transition states are given in Table 4. When the solvent is not included in the model, the structure and spin density at N(7) in the TS is intermediate between that of the reactant and the product species. However, in our large model with nine H2O molecules, the properties of the TS and T0 1 are similar and the nitrogen atom N(7) (see Figure 2)
bears strong radical character in the transition state. Thus, the activation barrier is significantly stabilized by solvent water molecules. Thus, in the presence of water molecules, the activation energy barrier drops significantly. The free energy of activation at 298 K is 11.5 kcal/mol for this reaction (the respective electronic energy is 12.6 kcal/mol in Table 3) and the rate constant is 2.3 104 s1. Thus, the intramolecular hydrogen transfer in the 14402
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Table 3. B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) Electronic Activation Energy for Non-Water-Assisted (n(H2O) = 0) and Water-Assisted KetoEnol Tautomerism of Metamitron and Diketometribuzin in the Singlet and Triplet Statesa metamitron
a
diketometribuzin
n(H2O)
S0 f S0 0
T1 f T0 1
S0 f S0 0
T1 f T0 1
0 (Figures 4a, 4e)
23.9
23.3
28.7
6.7
1 (Figure 4b)
33.1
26.3
2 (Figure 4c)
39.6
35.2
2, wire (Figure 4d)
31.0
29.7
9 (Figure 5)
12.6
The respective transition state geometries are shown in Figures 4 and 5.
triplet state can compete with other quenching reactions involving the T1 species. In addition, our results may explain why water plays a key role as a solvent in the formation of T0 1. This is in agreement with experimental data, because it was shown that the triplet enol structure was not detected when the reactions were conducted in acetonitrile.9 3.2. Mechanism of Photodeamination. Steady-State Irradiation. The irradiation of MT (1.5 105 M) at 313 nm in airsaturated and pH 6.5 buffered aqueous solution resulted in the absorbance changes shown in Figure 6. HPLC-MS analyses confirmed the formation of deaminometamitron as a main photoproduct.69 Nitrite ions were produced in the course of the irradiation. In air-saturated solution, the quantum yield of MT photolysis was equal to 0.020 ( 0.002 whatever the MT concentration within the range 105104 M, and that of nitrite ions was 0.014 ( 0.002. In argon-saturated solution, the quantum yield of MT photolysis was less than 0.001, while in oxygensaturated medium, it was equal to 0.0061 ( 0.0006, i.e., 3.5-fold less than that in air-saturated solution. The quantum yield of singlet oxygen production (ΦSO) through energy transfer between T1 and O2 was measured using furfuryl alcohol (FFA) as a scavenger (kFFA+SO = 1.2 108 M1 s1). We first checked that FFA did not react with T1 by comparing the absorbance decay at 375 nm in the absence and in the presence of FFA (105 to 2 104 M). Then we followed the reaction between singlet oxygen and FFA by monitoring the formation of 6-hydroxypyran-3-one (P, main photoproduct, chemical yield of 85%).29 The quantum yield of P formation (ΦP) was measured upon steady-state irradiation of MT (105 M) and FFA (5 105 to 104 M) in air-saturated medium. Under the assumption that singlet oxygen either deactivates or reacts with furfuryl alcohol, (ΦP) can be expressed as: ΦP ¼ 0:85 Φso
kFFA ½FFA kd þ kFFA ½FFA
ð1Þ
where kd is the rate constant of singlet oxygen deactivation in water (2.5 105 s1).30 The plot of 1/ΦP against 1/[FFA] was linear (R2 = 0.990). From the slope, one gets ΦSO = 0.50 ( 0.05. Last, FFA was added at a high concentration (103 M) to trap a significant part of singlet oxygen (32%). This addition did not affect the quantum yield of MT photolysis, indicating that singlet oxygen does not take part in any of the reactions leading to deamination. Involvement of T0 1 in the Deamination. The photodeamination was not observed in the absence of oxygen, thus an indication that the key step involves the reaction of oxygen with one of the
Figure 5. Large water cluster model (n = 9 H2O)-B3LYP/6-311++G(d, p)//B3LYP/6-31G(d) triplet-state geometry of the transition state for water-assisted ketoenol tautomerism in MT (a: top view; b: side view).
Table 4. Mulliken Electronic Spin Density for Specific Atoms of Triplet-State Metamitron (see Figure 2 for labels)a N(1)
N(2)
C(3)
N(4)
C(5)
T1
0.69
0.08
0.21
0.08
TS
0.72
0.23
0.06
0.23
T0 1
0.66
0.09
0.10
0.18
0.16
δ0
C(6)
N(7)
0.05
0.50
0.01
82.6
0.02
0.18
0.43
146.9
0.02
0.77
176.0
Not Solvated
Large Solvent Model
a
T1
0.54
0.14
0.13
0.01
0.43
0.04
107.5
TS
0.43
∼0
0.07
0.10
0.21
0.26
0.06
0.83
149.8
T0 1
0.47
∼0
0.09
0.18
0.32
0.01
0.87
147.7
δ0 is the dihedral angle C(5)N(4)N(7)H(b) in degrees.
two triplets. Several results provide evidence that T0 1, rather than T1, is this reactive species. First, photodeamination did not take place in solvents where T0 1 formation was not observed. The second argument is linked to oxygen effects. T1 is quenched by oxygen with a bimolecular rate constant of 1.3 ( 0.2 109 M1 s1. Thus, 41% of T1 is trapped by oxygen in air-saturated medium, while it is 77% in oxygen-saturated solution. If T1 was the key intermediate in the route to deamination, one would expect deamination to be about twice as efficient in oxygen-saturated medium than it was in air-saturated solutions. The quantum yield measurements invalidate this hypothesis. In contrast, the involvement of T0 1 is consistent with the experimental data because its formation is competitive with the scavenging of T1 by oxygen. Considering the quantum yield of singlet oxygen (0.50), the rate constant of T1 deactivation (5 105 s1) and that of T1 reaction with oxygen (1.3 109 M1 s1), the quantum yield of intersystem crossing is close to unity. Given this result and the rate constant for enolization (ke = 2.3 104 s1), a quantum yield of 0.027 is obtained for T0 1 formation, in agreement with the measured quantum yield of MT loss. Last, MB was shown10 to undergo photodeamination in nonaqueous solvents, contrary to that for metamitron. However, this reaction was observed with simultaneous diketometribuzin formation (see Scheme 1).11 In ref 10, the photodegradation of MB was shown to proceed competitively through sulfoxidation and deamination. Thus, the T1 f T0 1 activation barrier was investigated here for the diketometribuzin species, because it might be the precursor for deamination. In diketometribuzin, the 14403
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The Journal of Physical Chemistry A transition state is strongly stabilized from both neighboring carbonyl groups (the amino group is planar in this species; see Table 1) and the activation electronic energy barrier is only 6.7 kcal/mol (Table 3). Thus, in MB, sulfoxidation leads to a diketo species that readily undergo enolization in the triplet state (see Scheme 4). This result may explain why MB photodeamination is observed in nonaqueous solvents. Contrarily, in MT, the amino group is stabilized by one carbonyl group only, and the T1 f T0 1 activation barrier is very high in non aqueous solvents. The O 3 3 3 H bond distance for T1 is 2.087 Å in diketometribuzin, while it is larger in MT (2.261 Å). In addition, the C(5)N(4)N(7)Ha dihedral angle is 29.2 in MT (see Table 1), while these atoms are nearly planar in diketometribuzin (7.4). In Table 2, bond distances are given for the transition states. At the TS, in diketometribuzin, one of the O 3 3 3 H bond distances is thus larger than the other. In addition, excitation of the diketo structure in Scheme 4 gives rise to singlet and triplet excited states that bear some charge transfer character because the amino nitrogen lone pair is conjugated with the π system of the ring structure. Examination of the dipole moments and O 3 3 3 H hydrogen bond distances confirms this analysis, because the former increased from 2.16 to 2.82 D between S0 and T1, respectively, while the intramolecular hydrogen bond distances were simultaneously shortened from 2.195 Å and 2.331 Å to 2.087 Å and 2.277 Å (see Table 1). However, this effect is much smaller than that in aminofluorenones, and further investigation of these discrepancies will be of great interest to understand the quenching and reactivity of these species. Calculations. Photodeamination of the T0 1 State. The photodeamination mechanism is now investigated, performing theoretical calculations. When MT is irradiated above 300 nm,
Figure 6. UV spectrum of neutral MT (solid line) and absorbance variation during the course of irradiation. MT concentration is 1.5 105 M.
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the T1 and T0 1 states may behave as oxygen quenchers or react with triplet O2. While the quintet PES is nonreactive, the reaction on the triplet and singlet adiabatic surfaces needs further investigation. However, T0 1 does not react with 3O2 on the triplet PES (Figure 7a). Thus, the reaction of oxygen with T0 1 was investigated on the singlet adiabatic PES. Two series of reactions were characterized at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level of theory. These results are shown in Figures 7b and 8. On the singlet surface, the reactant structure shown in Figure 7b can possibly transform with no barrier into a hydroperoxide (structure I in Figure 8). This species cannot be observed experimentally because it is formed in too small amounts (maximum quantum yield of 0.027) for detection in transient spectroscopy. Once the hydroperoxide is obtained, it cannot return to T0 1 because the reverse pathway faces a high activation barrier and thus undergoes degradation. First, provided this species is obtained before intramolecular vibrational relaxation (IVR) is complete, it may be produced with excess energy, even in solution.3034 The statistical distribution of the energy is enhanced when one species faces a high entrance channel activation barrier (T0 1 + O2 f I) or a high activation barrier toward products (I f II,V). In our work, there is no electronic energy barrier in the entrance channel, and the transition states of the I f II,V reactions lie below the energy level of T0 1 + O2. Thus, we expect that a fraction of the excess energy is available to species I to overcome the second energy barrier toward products II and V. As the reactants move to the final products (deamino MT + HNO2), there is less available excess energy (dissipation through the solvent bath) and intermediate structures are likely to be thermalized prior to reaction. However, the other activation barriers are small and easily overcome (time scales are smaller than the microsecond). Second, under our continuous irradiation conditions, hydroperoxide I may undergo photolytic dissociation and release a hydroxyl radical. The fast in-cage abstraction reaction of the neighboring hydrogen atom (∼1012 cm3/mol/s) yields species III, a precursor of the deaminated product. Last, the degradation of hydroperoxide I may also be enhanced by the continuous absorption of the triazine ring. In one reaction path, species III is obtained from II through the release of a water molecule. Further rearrangement (IV) yields metamitron and nitrous acid. Another reaction pathway was
Figure 7. The reactivity of T0 1 with triplet oxygen on the singlet and triplet adiabatic potential energy surfaces.
Scheme 4. Photodeamination of Metribuzina
a
Hydrogen atoms are shown explicitly. Some of the atoms in T0 1 carry a large spin density. 14404
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The Journal of Physical Chemistry A
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Figure 8. Metamitron photodeamination mechanism (bond lengths in angstroms). Energy levels are shown in the inset (energies in kcal/mol). The reference energy level is that of the reactants. The bold and dashed lines represent local minima and transition states, respectively.
Scheme 5. Ring-Opening Concerted Reaction of Triplet Oxygen and Triplet Metamitron (enol structure) on the Lowest Singlet PESa
a
Hydrogen atoms are shown explicitly.
found. It involves the thermal dissociation of the long-range NN bond in structure V (the respective bond length is 2.729 Å in species V). An electronic transition state could not be located between species V and the separated products. The minimum energy path was followed for fixed interfragment distances between the triazine ring and the HNOO moiety. Electronic energy increased monotonically toward separated products. In that case, the activation free energy for dissociation may be found from canonical variational transition state theory calculations.35 However, it is not expected to differ significantly from the reaction energy. Last, it was found that an intramolecular rearrangement of structure V yielded structure VII, i.e., a long-range complex that involved the HN(O)O fragment. However, the activation energy is high and is not competitive with the other reactive paths aforementioned.
Scheme 6. Metamitron and Methylidene Equilibrium Structures
Other Reactions. The concerted reaction of triplet oxygen with T0 1 was also investigated. In T0 1, the spin density is large on N(7), N(1), N(4), and C(5) (see labels in Figure 2 and Table 4). The reaction that involved a bridged dioxygen structure like that shown in Scheme 5 resulted in the spontaneous opening of the triazine ring. However, this open-ring species was not detected in 14405
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The Journal of Physical Chemistry A our laboratory. This might be due to the necessary electronic spin constraints required in concerted reactions of two openshell species, or steric constraints induced by strong solvent interactions. Last, the interaction of one σ bond in CH3 and the triazinic π system, known as hyperconjugation, may lead to a species with a methylidene structure. In Scheme 6, structure A is 27.7 kcal/mol above MT, at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level. Thus, this reaction is not expected under the conditions of the experimental setup described here. A is indeed strongly destabilized with respect to MT because the phenyl and the triazine rings are not coplanar in this structure.
4. CONCLUSION It is proposed in this work that metamitron undergoes photodeamination through an original mechanism. The key step is the enolization of the ketonic triplet excited state into an excited diradical structure with large spin density on the pendant amino group. Interestingly, in aqueous solution, the activation barrier of this ketoenol tautomerism is significantly reduced by solvation. This may explain why photodeamination is not observed in nonaqueous solvents. Contrarily, while the keto triplet excited state reacts with oxygen by energy transfer and formation of singlet oxygen, the triplet diradical reacts with oxygen in an addition reaction that leads to an hydroperoxide. Intramolecular rearrangement of this species finally leads to deamination with nitrite anion formation. ’ ASSOCIATED CONTENT
bS
Supporting Information. B3LYP/6-31G(d) geometries of the species investigated in this work (equilibrium and transition state structures). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*
[email protected].
’ ACKNOWLEDGMENT S.K.H. thanks the French-Algerian PROFAS program for the 18-month grant. ’ REFERENCES (1) Cosa, G.; Martínez, L. J.; Scaiano, J. C. Phys. Chem. Chem. Phys. 1999, 1, 3533. (2) Freccero, M.; Fagnoni, M.; Albini, A. J. Am. Chem. Soc. 2003, 125, 13182. (3) Gil, M.; Douhal, A. J. Phys. Chem. A 2008, 112, 8231. (4) Korang, J.; Grither, W. R.; McCulla, R. D. J. Am. Chem. Soc. 2010, 132, 4466. (5) Canle, L. M.; Fernandez, M. I.; Santaballa, J. A. J. Phys. Org. Chem. 2004, 18, 148. (6) Bartl, P.; Korte, F. Chemosphere 1975, 4, 169. (7) Parlar, H.; Pletsch, B. Chemosphere 1988, 17, 2043. (8) Cox, L.; Hermosín, M. C.; Cornejo, J.; Mansour, M. Chemosphere 1996, 33, 2057. (9) Palm, W. -U.; Millet, M.; Zetzsch, C. Chemosphere 1997, 35, 1117. (10) Pape, B. E.; Zabik, M. J. J. Agric. Food Chem. 1972, 20, 72. (11) Raschke, U.; Werner, G.; Wilde, H.; Stottmeister, U. J. Photochem. Photobiol. A 1998, 115, 191.
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