Computational Study of the Structure and Degradation Products of

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Computational Study of the Structure and Degradation Products of Alloxydim Herbicide Juan José Villaverde, Pilar Sandin-España, Jose luis Alonso-Prados, Al Mokhtar Lamsabhi, and Manuel Alcami J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00865 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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The Journal of Physical Chemistry

Computational Study of The Structure And Degradation Products of Alloxydim Herbicide

Juan J Villaverde,a Pilar Sandín-España,a José L. Alonso-Prados,a Al Mokhtar Lamsabhi,*,b,c and Manuel Alcamí,b,c,d

a

Plant Protection Products Unit, DTEVPF, INIA, Crta. La Coruña, Km. 7.5, 28040 Madrid,

Spain b

Departamento de Química, Facultad de Ciencias, Módulo 13, Universidad Autónoma de

Madrid, 28049 Madrid, Spain c

Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de

Madrid, 28049 Madrid, Spain. d

Instituto Madrileño de Estudios Avanzados en Nanociencias (IMDEA-Nanociencia),

28049 Madrid, Spain

*

Corresponding author. Phone: +34914975017 Fax: +34914975238

E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Density Functional Theory calculations allowed to study alloxydim herbicide and to identify the most stable conformers, the factors that governs their stability as well as the interconversion mechanisms among the most relevant conformers. The degradation chain involves, as a first step, the cleavage of the N–O bond and the formation of a stable intermediate difficult to characterize experimentally. The study performed also allowed to identify the properties of this elusive intermediate and to determine that the dominant fragmentation process in the gas phase is the homolytic fragmentation. Stability of alloxydim conformers and homolytic fragments were also assessed in water phase. Computed IR spectra were consistent with those observed experimentally.


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1. INTRODUCTION Pesticides are subjected to degradative processes from the moment they are synthetized. This phenomenon is increased when they are applied in the field or in the place for which is intended for use. Inorganic end products, such as H2O, CO2, and NH4+ would be obtained after their full mineralization. However, the mineralization process is not always performed until completion or it arises slowly in the environment. During this process, diverse intermediates, generally referred as pesticide degradation products (DPs), are formed. These new compounds have different physicochemical properties that may greatly affect to their environmental behavior.1,

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For

instance, several studies have documented that some of these DPs persist for longer periods than the parent active compound, show higher mobility values and/or higher toxicity.3-6 In particular, cyclohexanedione oxime (CHD) herbicides are a family of compounds that show large mobility in environmental compartments, as a consequence of their polar character and relatively high solubility in water.7 Moreover, CHD are easily degradable and thermally labile, conferring them a great capacity to form DPs.8-12 This work is focused on alloxydim, a selective systemic herbicide from the CHD class and developed for post-emergence control of grass weeds and volunteer cereals in sugar beet, vegetables and broad-leaved crops. This family of herbicides is important, since they easily degrade in the environment, in order to avoid soil and groundwater contamination and to fulfil the environmental requirements of new international regulations.13 Several studies showed that DPs from alloxydim exhibit higher toxicity than the parent active compound.9 Therefore, a suitable knowledge of the alloxydim DPs is indispensable to prevent the appearance of undesirable effects by an inappropriate use of pesticides. However, most researches have usually focused on the active substance and not in their DPs, as a consequence of the difficulties for their identification, isolation and lack of analytical standards. Besides, in the environment DPs


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concentration is very low (range of µg L-1). Therefore, the current experimental results usually offer only a partial and very limited overview of the problem. Moreover, in many cases degradation mechanisms are not well stablished and as a consequence, intermediate products are difficult to characterize. In this respect, computational studies of pesticides offer a large potential to identify the most relevant stable DPs and their physicochemical properties. Indeed, theoretical simulations are waking up a special interest within the pesticide legislative frameworks worldwide to be implemented in the toxicological risk assessment of pesticides. The reason for this reality lies in the ability of these computational simulations to overcome the challenges imposed by the modern legislations, as minimizing animal testing. Despite of this, systematic studies of pesticides and their DPs are still scarce in the literature. The present study provides an exploration of the degradative process of alloxydim by means of DFT calculations. The main aim is to identify, not only the structure and properties of the parent compounds, but also the most relevant DPs, i.e. those formed after the breaking of the most labile N–O bond and the loss of the oxime ether group.

2. Materials and Methods 2.1. Computational Details. Density functional theory (DFT) calculations were performed by using the B3LYP functional, which combines the Becke’s three-parameter non-local hybrid exchange potential14 with the non-local correlation function of Lee, Yang, and Parr.15-17 Geometries were fully optimized by combining this functional with the 6-311G(d,p) basis set. The harmonic vibrational frequencies of the stationary points of the alloxydim potential energy surface (PES) were obtained at the same level of theory to identify stationary points as local minima and transition states (TS), as well as to calculate the zero-point vibrational energy corrections (ZPE). Final energies were obtained by single point calculations over the optimized


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geometries, using the same B3LYP functional and a 6-311+G(2df,2p) extended basis set. In order to test the use of diffuse functions and different functionals in geometry optimization, geometries of the most stable conformers were re-optimized at B3LYP/6-311+G(d,p) and using the M06-2X functional

proposed by Truhlar.18 In both cases final energies were obtained using the 6-

311+G(2df,2p) basis set. Results are given in Table 2S of the SI. Adding a diffuse function in the geometry optimization, lead to results that differ in less than 0.1 kJ/mol in relative energies, the differences in vibrational frequencies lead to changes in ∆G below 2 kJ/mol. Therefore, in this system, the use of diffuse functions in geometry optimization does not compensate the computational cost. Changing the functional has a slightly larger effect but relative energies vary in less than 5 kJ/mol in the worse case. Therefore the results obtained confirm the validity of B3LYP/6-311+G(2df,2p)//B3LYP/6-311G(d,p) level of computation. Connectivity among the transition structures and their adjacent minima were determined by using the intrinsic reaction coordinate (IRC) procedure.19 The evaluation of the solvent effect in many cases was also achieved by means of the Polar Continuum Model (PCM) taking water as a solvent.20 B3LYP has shown to provide a rather accurate description of the properties of the electronic ground state for the kind of organic compound used as pesticides21, 22 and also offers a good compromise between computational cost and accuracy in the study of binding energies and infrared spectra. All calculations were performed using the Gaussian-09 series of programs.23 The bonding characteristics were performed by means of the quantum theory of atoms in molecules (QTAIM)24, 25 and a Natural Bond Orbital Analysis (NBO).26 The corresponding density energy plots were obtained at both population analysis by means of AIMALL27 and NBO-6.028 series of programs.


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2.2. Reagents.

Alloxydim

(methyl

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(E)-(RS)-3-[1-(allyloxyimino)butyl]-4-hydroxy-6,6-

dimethyl-2-oxocyclohex-3-enecarboxylate) was gently supplied by Dow AgroSciences (98% purity, > 1 kg L−1 solubility in water, CAS Number: 55634-91-8) as a white powder. Potassium bromide (for IR spectroscopy Uvasol®) was purchased from Sigma Chemicals Co. (Madrid, Spain). 2.3. Experimental Infrared Spectroscopy. Infrared alloxydim spectra were obtained on a 1310 infrared spectrophotometer (Perkin Elmer, Norwald, CT, USA) using a KBr disk. Spectra were recorded in the 300-3300 cm-1 range with 20 scans at a resolution of 2.0 cm-1.

3. RESULTS AND DISCUSSION In order to study the structure and properties of the most stable alloxydim isomers and their potential DPs, theoretical calculations were performed to systematically explore a) the possible alloxydim isomers and their properties, b) the structure of the DPs when oxime ether group (HO– CH2–CH=CH2) is lost leaving a neutral, positively or negatively charged fragment. These results are presented in the following three subsections and will be compared with the experimental findings accomplished during the alloxydim degradative process in environmental matrices.8, 9, 2931

3.1. Structure of initial alloxydim isomers. One of the main difficulties in the theoretical study of medium/large size organic molecules is the existence of a large number of possible conformers. In the case of alloxydim, two main isomers (E- and Z-) regarding the oxime ether double bond (C7–N) are possible (see Figure 1). Sandin-España et al.9 and Falb et al.32 observed that CHD are usually marketed as E-isomers but some of them may equilibrate with the Z-isomer in polar media such as water. In this sense, it is expected that alloxydim may exist as E- and Zlow energy forms (see Figure 1). Several works have confirmed that the oxime E-, Z-


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isomerization can be induced by effect of UV-Vis light, temperature, a solvent and acid and/or basic medium.33 As shown in Figure 2 in both E- and Z- isomers several tautomeric forms can be envisaged: a) two diketo forms, denoted as E’-A(N7) and E’-A(C3); b) two enolic forms, denoted as E-A(O2) and E-A(O4). In the case of E-A(O2), an important difference exists if the OH on position 2 forms a hydrogen bond with the carbonyl, the ether oxygen or the nitrogen. These possible structures are named as E-A(O2a), E-A(O2b) and E-A(O2c), respectively. Presumably, in the diketo forms the double bond is located between the C3 and C7 (see numbering in Figure 1). Therefore the Z- or E- character refers to this bond and both were named as Z’- and E’-, respectively (see Figure 1). In the E’- forms the oxyimino moiety is directed towards O4, while in the Z’- forms it is oriented towards O2. In both cases, two conformers can be envisaged depending on the orientation of the NH–OR of the oxyimino with respect the ketone groups of the ring. A hydrogen bond is formed in the forms denoted as N7a, while the two oxygens are confronted in those forms denoted as N7b. Therefore, the fourteen main initial structures depicted in Figure 2 were considered for exploration of the alloxydim PES. On top of this, several conformations can be obtained by rotating the different single bonds of the systems, i.e. different spatial dispositions of the oxyimino moiety, the aliphatic chain of the imine, the ester group, as well as the hydrogen orientation in the hydroxyl group. Several conformations of these four groups were considered in exploration of the PES. The ester group locates perpendicular to the plane of the six-membered ring in the optimized geometries, except when forming hydrogen bonds that locates in the plane of the ring. On the other hand, the spatial orientation of the aliphatic and oxyimino chains has a significant impact in the final stability of the systems. For the sake of clarity, the discussion refers to the most stable conformers depicted in Figure 2. Their relative energies are listed in Table 1. The values of relative energy for the remaining conformers


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explored in the present study are given as supporting information in Table S1. All structures explored are available in the supporting information. Table 1 shows that, in the gas phase, the most stable tautomeric forms correspond to those having a hydrogen bond between oxygen and nitrogen atoms, either an O–H····N bond (i.e. EA(O4) and E-A(O2c)) or a O····H–N bond (i.e. E’-A(N7a) and Z’-A(N7a)). The lowest energy is obtained for the E’-A(N7a) conformer. However, the difference with E-A(O4) is very small and the inclusion of entropic effects favors the latter conformer. All other structures lie high enough in energy, as not be relevant in the gas phase if an equilibrium is established under ambient temperature conditions. A complete study of alloxydim isomerization PES would imply considering not only all possible conformers for each isomer, but also the different TS connecting them, which is out of the scope of the present article. Alternatively, Figure 3 summarizes the different kind of TS needed to connect all the structures of Figure 2, allowing to estimate the energy barrier associated with the most relevant processes. The highest barriers, around 100 kJ mol-1, correspond to the rotation of the oxyimino groups (for instance from E’-A(N7a) to E’-A(N7b)). Rotation of the hydroxyl group (for instance from E-A(O2c) to E-A(O2b)) requires 70 kJ mol-1. In both cases, the hydrogen bonds are broken. The rotation of the ester group or the aliphatic chain of the imine typically requires lower barriers, i.e. 40-50 kJ mol-1 for the former and 30-45 kJ mol-1 for the latter (Figure 3). The most stable isomers are connected by H-Transfers (from E-A(O2c) to Z’A(N7a) and from E-A(O4) to E’-A(N7a) and imply practically negligible energy barriers (less than 2 kJ mol-1 or 6.2 kJ mol-1 for the reverse processes) (Figure 3). Direct isomerization between Z- and E- forms would require much higher energy barriers because this process involves breaking the C7–N double bond. Nevertheless, there is an alternative pathway that may connect Z- and E- forms through the H-transfer. In these cases, it is


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important to note that in the keto-enolic forms (i.e. E-, Z-A(O4)) the C7–N and C3–C4 bonds present a double character. The H-transfer to get the diketo forms implies an electronic reorganization in the involved bonds, which consequently lead to a reinforcement of the C3–C7 bond and a weakening of the C7–N and C3–C4 bonds (see Figure 1). This fact was verified by performing a topological analysis of the electron density,24, 25 as shown in Figure 4. As it was discussed above, evolution from E-A(O4) to E’-A(N7a) is a one-step mechanism that involves only the H-transfer. Once E’-A(N7a) is formed, rotation over the C7–N becomes possible although it involves a high energy barrier over 100 kJ mol-1 (Figure 3d). This process would lead to E’-A(N7b) that has the Z-orientation of the –N–O–CH2–CH=CH2 group. It worth noting that H-transfer in the gas phase cannot explain the connection with other Z- structures, as further evolution from E’-A(N7b) to Z’-A(N7b) or Z’-A(N7a) (from which Z-A(O2c) is easily accessible) would involve the rotation around the C3–C7 double bond. Nevertheless, the easy exchange of -NH and -OH hydrogens with the media (e.g. water) would open other possibilities and strongly decrease the energy needed for E- to Z- conversion. An example of this mechanism could be observed if, once E’-A(N7b) is formed, deprotonation of the NH is followed by a protonation at O4, leading directly to Z-A(O4). This would be in agreement with the experimental finding that E-, Z- isomerization can be induced by acid and/or basic medium.33 A further analysis of the electronic density (Figure 4) and the strength of the hydrogen bonds (Figure 5) can help to understand the different factors governing the relative stability of the most stable conformers. In the most stable conformations, the carbonyl groups not involved in the hydrogen bond and the oxygen of the oxyimino group are close enough to the alkyl groups as to interact weakly with them. This fact is reflected in the analysis of the electronic density by a localization of bond critical point between the oxygen and one or two hydrogens of the alkyl groups (Figure 4). Although these bonds are very weak, they explain why the most stable


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conformers for each tautomer are those represented in Figure 2. In some other conformations, the position of the alkyl groups does not allow the interaction with the oxygens (Table S1 of the SI). The four most stable forms for alloxydim have in common the existence of a hydrogen bond. In the case of the E’-A(N7a) and Z’-A(N7a) isomers, the bonds strength of the six-membered ring is very similar, as a result of the carbonyl groups in positions 2 and 4. However, the strength of the hydrogen bond is higher in the E’- form than in the Z’- one, as revealed by a larger electronic density (Figure 4) and a larger value of the second order perturbation interaction between the occupied and vacant molecular orbitals involved in the hydrogen bond (Figure 5). As a consequence, the E’- form is 2.5 kJ mol-1 more stable than the Z’- one. On the other hand, the bonds strengths of the six-membered ring are very different when comparing the keto-enolic forms E-A(O4) and E-A(O2c): C3–C4 and C3–C2 have different single or double character imposed by the relative position of the carbonyl group and C4–C5 and C1–C2 bonds present quite different characteristics as a consequence that in positions 2 and 4 exists a carbonyl or a hydroxyl group. Therefore, E-A(O4) is 7.9 kJ mol-1 more stable than E-A(O2c), mainly as a consequence of the more favorable distribution of the functional group on the sixmembered ring of E-A(O4) and not due to the different strength of the hydrogen bond. Indeed, in this case the electronic density and NBO analysis reveal that the hydrogen bond is slightly stronger in E-A(O2c). It is worth noting that all the discussion refers to the stability of the different species in the gas phase. An inspection of the dipole moments of the different alloxydim conformers in the gas phase shows quite large range from 2.30 to 7.5 D (Table 2), pointing out that the interaction of these conformers with polar solvents could be signiﬁcantly different. This fact suggests that solvent polarity influences significantly in the interaction with the conformers and therefore also on their relative abundance. For this reason, the relative stabilities were re-evaluated using water


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as solvent. In this regards, Table 1 shows that changes are not too significant. It is noteworthy that the energy differences between structures showing an O–H····N or O····H–N bond increases in 5 kJ mol-1, with respect to the values in gas phase. As a consequence, E’-A(N7a) is predicted now to be the most stable isomer when considering both energies or Gibss free energies. Moreover, the predicted abundances are dominated by the E’-A(N7a) and Z’-A(N7a) isomers, both showing a O····H–N hydrogen bond. In order to confirm these structural aspects and to have a direct comparison with the experimental data, Figure 6 shows the experimental IR spectra of alloxydim in KBr compared with the calculated gas phase IR spectra using a scalar factor 0.9806 proposed by Radom et al.34 for the three most stable forms of alloxydim. In general, increases of ṽ around 100-200 cm-1 are observed in the theoretical signals with respect to the experimental ones. The largest difference between the keto-enolic and diketo tautomers is observed in the 2800-3100 cm-1 region, where the vibrations corresponding to the O–H and N–H bonds appear nearest to 2800 cm-1and 3100 cm-1, respectively. Additionally, in the experimental spectra range between ṽ = 1350-1750 cm-1, four groups of signals appear with decreasing intensities, i.e. a) 1350-1450 cm-1; b) 1450-1540 cm-1; c) 1540-1630 cm-1; d) 1680-1750 cm-1. According to the theoretical study, the bands a) and b) are associated with the N–H stretching mode. These features clearly fit better with the calculated spectra of the diketo forms E’-A(N7a) and Z’-(N7a). This fact indicates that under the experimental conditions in which the spectra were recorded only these two forms are present. The bands c) and d) provide less information because correspond to the vibration of carbonyl group in the ring and ester, respectively. Both are common groups in both keto-enolic and diketo tautomers. The alloxydim experimental IR spectrum also shows bands between ṽ = 830 and 960 cm-1, corresponding to the rocking mode of N–O and C–O in oxime group.


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Another relevant aspect is how the N–O bond changes among the different conformers. This is the weakest alloxydim bond and the first that is broken in the environmental degradation.8,

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Table 2 shows a significant difference between keto-enolic and diketo forms. In the latter cases, the bond is significantly stronger, which points out that these forms should be more stable. This issue will be further analyzed in the next section. 3.2. Heterolytic and homolytic fragmentations. As indicated in previous works,8 one of the major fragments in the MS/MS spectrum of E- and Z-alloxydim corresponds to the formation of the [MH]+ - [HO–CH2–CH=CH2] specie at m/z 266.1. This fact is associated with the elimination of a m/z 58.0 neutral allyl alcohol and evidences the easiness of the oxime N–O bond to break and to form the imine molecule, detected experimentally as the main DP.8 In the current study, it was considered that environmental bond cleavage may occur through homolytic or heterolytic mechanisms. For this reason, the following discussion considers those cases in which a neutral or a charged (either positive or negative) fragments are formed. Figure 7 shows all fragments obtained as stable species in the calculation for the neutral, anionic and cationic forms in the gas phase. Only the most stable conformers are shown. For the sake of simplicity, less stable conformers arising from the movements of the functional groups (i.e. allylic and ester groups) are not discussed. The corresponding relative energies are summarized in Table 3. As in the case of alloxydim, the key factor is the possibility to have a keto-enolic tautomerism and the formation of hydrogen bonds. In the case of the neutral fragments (homolytic fragmentation), the most stable forms are similar to those described for alloxydim. Keto-enolic forms (i.e. (F(O4) and F(O2c)) and diketo forms (i.e. F(N7a) and F(N7b)) are possible and have similar energies (Table 3). The formation of hydrogen bonds explains the enhanced stability of these four conformers. In these cases, the conformers presenting a O–H····N bond (i.e. F(O4) and F(O2c)) are more stable in the gas phase


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than those presenting a O····H–N bond (i.e. F(N7a) and F(N7b)). Similarly to alloxydim, in water the O····H–N bond is favored and as a consequence F(N7a) becomes the most stable isomer, but only slightly more stable than F(O4) whereas F(N7b) and F(O2c) become practically degenerated (Table 3). In the case of a heterolytic fragmentation, two different cases can be envisaged: when the fragmentation product is negatively charged, the forms issued from the hydroxyl hydrogen atom migration to the nitrogen one to give the diketo forms (i.e. F(N7a) or F(N7b)) are dominating in gas phase and in water media. This trend is expected, since the negative charge is more stabilized in the carbonyl oxygens and that corresponding O····H–N hydrogen bond is also reinforced with a higher negative charge on the oxygen. On the contrary, when the fragmentation product is positively charged, the most stable form becomes the F(C3) conformer in both phases. The charge redistribution due to the loss of one electron avoids evidently the formation a hydrogen bond but instead new type of bonding between atoms takes place. In fact, the nitrogen presents a positive charge with a sp hybridization compared to the other conformer where this atom offers a sp2 hybridization. The dissociation energies for different fragmentation processes show that the most favorable breaking mechanism in the gas phase is the homolytic one (Table 4). Dissociation energies are between 174 and 227 kJ mol-1 for the homolytic fragmentation, while the heterolytic ones (either when the fragment is positively or negatively charged) are above 444 kJ mol-1. This is a consequence of the large instability of the charged fragments. However, it is noteworthy that in solution the situation can be quite different: the ionic species are stabilized by the solvent and the CH2–CH–CH2–O fragment considered in the calculation would present a hydroxyl terminal group instead of oxygen atom. A general conclusion from Table 4 is that the N–O bond is stronger in the neutrals, i.e. presents higher dissociation energy. This feature is especially


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remarkable in the case of the diketo forms, as already expected from the analysis of the charge density and bond distances presented before. We performed different scans enlarging the N-O distance and the results show that the fragmentation is barrierless. CONCLUSIONS Density functional theory calculations have shown that, in the case of alloxydim, there are four conformeric forms that present high stability in comparison with other possible conformeric forms in gas phase. These four structures are characterized by presenting a strong intramolecular hydrogen bond. Two of them have a keto-enolic structure and the other two correspond to diketo tautomers. H-Transfers between the two Z- tautomers or between the corresponding E- tautomers present a lower energy barrier. Interconversion between E- and Z-forms implies much higher energy barriers in the gas phase. However, in polar liquid phases, the H-transfers between oxygen and nitrogen atoms should favor the E- to Z- conversion by exchanging protons with the environment. Homolytic fragmentation of the N–O bond is the most favorable mechanism to form stable intermediate species in gas phase. Stability of the resultant fragments is controlled by the existence of intramolecular hydrogen bonds. The most stable species identified could play an important role in the environment as long-live by-products in the whole degradation mechanism of alloxydim herbicide. This theoretical study can serve as new strategy for initial estimation of the pesticides degradation pathway and by-products formation, in order to prioritize experimental investigations for risk assessment.

SUPPORTING INFORMATION The values of relative energy for the remaining conformers explored in the present study are given as supporting information in Table S1. All structures explored are available in the supporting information.


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ACKNOWLEDGMENTS This work has been supported by the DGI Projects no. CTQ2015-63997-C2, CTQ2016-76061-P (MINECO) by the Project MADRISOLAR2, Ref.: S2009PPQ/1533 of the Comunidad de Madrid, and by Consolider on Molecular Nanoscience CSC2007-00010 and RTA2014-00044-0000. A generous allocation of computing time at the Centro de Computación Científica of the UAM is also acknowledged.

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E-alloxydim

Z-alloxydim

Page 20 of 31

E’-alloxydim

Z’-alloxydim

Figure 1. Chemical structure of alloxydim isomers.


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Figure 2. Most stable alloxydim conformers in the gas phase.


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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 c) 25 26 27 28 29 30 31 32 33 9.4 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

b)

a)

45.7#

30.5#

3.5

3.9 E -A(N7a) Conf2

'

Z-A(O4) Conf3

0.5 Z-A(O4)

d)

0.0

E'-A(N7a)

e)

72.3#

8.7#

E-A(O2c)

50.0 E-A(O2b)

2.6# 2.5

Z -A(N7a)

'

1.5

E-A(O4) 9.4

0.0

E-A(O2c)

E -A(N7a)

'

Figure 3. Examples of different isomerization barriers for alloxydim in the gas phase obtained at B3LYP/6-311+G(2df,2p) level of theory. a) Rotation of ester group from Z-A(O2a) to ZA(OA2b) (left) and from E’-A(N7a) Conf2 to E’-A(N7a) Conf3 (right). b) Rotation of the alkyl group from Z-A(O4) to Z-A(O4) Conf3 (left) and from E’-A(N7a) to E’-A(N7a) Conf2 (right). c) H-Transfer from E-A(O2c) to Z’-A(N7a’) (left) and from E-A(O4) to E’-A(N7a) (right). d) Rotation of the oximino group from E’-A(N7a) to E’-A(N7b). e) Rotation of the hydroxyl group from E-A(O2c) to E-A(O2b).


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Figure 4. Molecular graphs of some selected alloxydim conformers. Green and Red dots represent bond and ring critical points, respectively. Electron densities are in a.u.


23


Page 24 of 31

Figure 5. NBO molecular orbital interactions between occupied and empty orbitals in the most important conformers. Energy values are in kJ mol-1.


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Z'-A(N7a)

E'-A(N7a)

E-A(O4)

Experimental

500

1000

1500

2000

2500

3000

-1

λ (cm )

Figure 6. Theoretical IR spectra in the gas phase of the three most stable conformers of alloxydim versus experimental IR spectra of alloxydim in KBr.


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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 26 of 31

Figure 7. The main initial fragments obtained as stable species in the calculation for the neutral, anionic and cationic form in the gas phase.


26

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Table 1. Relative energies (∆0E), free energies (∆298Gº) and conformer abundances (%) using a Boltzmann distribution of the most relevant alloxydim conformers, in both gas and water phases. Energies were obtained at B3LYP/6-311+G(2df,2p) level of theory, including ZPE and thermal corrections evaluated at B3LYP/6-311G(d,p) level. Conformer E-A(O2a) E-A(O2b) E-A(O2c) E-A(O4) E’-A(N7a) E’-A(N7b) E-A(C3) Z-A(O2a) Z-A(O2b) Z-A(O2c) Z-A(O4) Z’-A(N7a) Z’-A(N7b) Z-A(C3)

∆0E (kJ mol-1) 39.9 50.0 9.4 1.5 0.0 61.4 48.1 34.7 45.6 31.5 35.2 2.5 71.9 42.5

Gas phase ∆298Go (kJ mol-1) 41.0 51.1 10.7 0.0 2.0 62.1 43.3 32.2 44.6 33.1 35.1 3.4 71.4 39.7

% 0.0 0.0 0.8 58.1 26.2 0.0 0.0 0.0 0.0 0.0 0.0 15.0 0.0 0.0

Water phase ∆0E (kJ mol-1) ∆298Go (kJ mol-1) 33.8 34.5 41.5 38.9 15.9 15.6 6.4 5.6 0.0 0.0 43.3 41.5 43.1 39.1 28.5 28.5 37.3 35.2 34.1 35.5 22.7 21.6 1.7 0.6 50.4 51.4 40.1 37.5


% 0.0 0.0 0.1 5.5 53.2 0.0 0.0 0.0 0.0 0.0 0.0 41.3 0.0 0.0

27


Page 28 of 31

Table 2. N–O bond lengths and dipole moments of the most relevant alloxydim conformers, in both gas and water phases. Both properties were obtained at B3LYP/6-311+G(2df,2p) level of theory after geometry optimization at B3LYP/6-311G(d,p) level. Conformer E-A(O2a) E-A(O2b) E-A(O2c) E-A(O4) E’-A(N7a) E’-A(N7b) E-A(C3) Z-A(O2a) Z-A(O2b) Z-A(O2c) Z-A(O4) Z’-A(N7a) Z’-A(N7b) Z-A(C3)

Gas phase N–O bond length (Å) Dipole moment (D) 1.404 6.78 1.404 4.82 1.394 2.30 1.394 3.41 1.388 3.03 1.389 3.47 1.394 2.77 1.404 7.05 1.404 5.20 1.424 2.70 1.402 5.21 1.388 3.88 1.378 2.54 1.404 2.71

Water phase N–O bond length (Å) Dipole moment (D) 1.408 9.52 1.407 6.84 1.392 3.04 1.393 4.99 1.384 3.89 1.377 6.10 1.398 5.46 1.406 9.98 1.406 7.47 1.420 3.48 1.403 8.91 1.384 4.77 1.377 4.23 1.404 3.53


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Table 3. Relative energies (∆0E), free energies (∆298Gº) and conformer abundance (%) using a Boltzmann distribution of the most relevant conformers of alloxydim imine fragment, for the neutral, -1 and +1 charge states, in both gas and water phases. Energies were obtained at B3LYP/6-311+G(2df,2p) level of theory, including ZPE and thermal corrections evaluated at B3LYP/6-311G(d,p) level. Neutral Gas phase Conformer ∆0E ∆298Go (kJ mol-1) (kJ mol-1) F(O2a) 32.7 30.6 F(O2b) 43.8 42.2 F(O2c) 1.3 0.6 F(O4) 0.0 0.0 F(N7a) 3.6 2.1 F(N7b) 8.8 7.6 F(C3) 41.6 39.3

% 0.0 0.0 35.2 43.9 18.8 2.0 0.0

Water phase ∆298Go ∆0E (kJ mol-1) (kJ mol-1) 15.0 15.4 25.5 25.0 2.9 3.4 0.2 1.1 0.0 0.0 4.2 3.7 30.2 29.9

Anion % 0.1 0.0 12.1 30.0 47.3 10.5 0.0

Gas phase ∆0E ∆298Go (kJ mol-1) (kJ mol-1) 269.7 267.9 271.0 269.6 0.0 0.0 4.6 4.9 254.5 251.2

% 0.0 0.0 87.9 12.1 0.0

Water phase ∆298Go ∆0E (kJ mol-1) (kJ mol-1) 213.1 213.6 206.5 205.6 0.0 0.0 1.0 0.3 182.5 180.6


% 0.0 0.0 52.8 47.2 0.0

Cation Gas phase Water phase ∆0E ∆298Go ∆298Go ∆0E % % (kJ mol-1) (kJ mol-1) (kJ mol-1) (kJ mol-1) 61.9 66.5 0.0 68.9 72.9 0.0 90.1 93.6 0.0 90.3 94.0 0.0 104.5 106.2 0.0 96.1 101.3 0.0 118.9 120.7 0.0 99.5 100.0 0.0 0.0 0.0 100.0 0.0 0.0 100.0

29


Page 30 of 31

Table 4 Dissociation energies (∆ED) from both the most stable alloxydim conformer E'-A(N7a) and the alloxydim tautomers included in Figure 2 that present a similar structure that the fragment reported, in gas phase. AllylO refers to the allyl alcohol–H fragment formed in the dissociation process. Energies were obtained at the B3LYP/6-311+G(2df,2p) level of theory, including ZPE and thermal corrections evaluated at B3LYP/6-311G(d,p) level. Dissociation Homolytic (neutral) E’-A(N7a) / E-A(O2a) / Z-A(O2a) F(O2a) + AllylO E’-A(N7a) / E-A(O2b) / Z-A(O2b) F(O2b) + AllylO E’-A(N7a) / E-A(O2c) F(O2c) + AllylO E’-A(N7a) / E-A(O4) F(O4) + AllylO E’-A(N7a) F(N7a) + AllylO E’-A(N7a) / Z’-A(N7a) F(N7b) + AllylO E’-A(N7a) / E-A(C3) / Z-A(C3) F(C3) + AllylO Heterolytic (anion) E’-A(N7a) / E-A(O2a) / Z-A(O2a) F(O2a)- + AllylO+ E’-A(N7a) / E-A(O4) F(O4)- + AllylO+ E’-A(N7a) F(N7a)- + AllylO+ E’-A(N7a) / Z’-A(N7a) F(N7b)- + AllylO+ E’-A(N7a) / E-A(C3) / Z-A(C3) F(C3)- + AllylO+ Heterolytic (cation) E’-A(N7a) / E-A(O2a) / Z-A(O2a) F(O2a)+ + AllylOE’-A(N7a) / E-A(O2b) / Z-A(O2b) F(O2b)+ + AllylOE’-A(N7a) / E-A(O4) F(O4)+ + AllylOE’-A(N7a) F(N7a)+ + AllylOE’-A(N7a) / E-A(C3) / Z-A(C3) F(C3)+ + AllylO-


∆ED (kJ mol-1) -215.0 / -175.1 / -180.3 -226.1 / -176.1 / -180.5 -183.7 / -174.2 -182.3 / -180.8 -185.9 -191.2 / -188.7 -224.0 / -175.9 / -181.5 -713.6 / -673.7 / -678.9 -714.9 / -713.4 -443.9 -448.5 / -446.0 -698.4 / -650.3 / -655.9 -674.8 / -635.0 / -640.2 -703.0 / -653.0 / -657.4 -717.4 / -715.9 -731.8 -612.9 / -564.8 / -570.5

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Computational Study of the Structure and Degradation Products of

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