Quantum Mechanical Study of Dealkylation Processes and the Reducing Trifluralin Herbicide

doi:10.21203/rs.3.rs-4870335/v1

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Quantum Mechanical Study of Dealkylation Processes and the Reducing Trifluralin Herbicide

https://doi.org/10.21203/rs.3.rs-4870335/v1

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Trifluralin, a widely used herbicide, undergoes complex degradation processes, including dealkylation and reduction, in environmental systems. In this study, we investigate these processes through advanced quantum mechanical calculations and computational analysis, employing density functional theory (DFT) in Gaussian 09 software. The calculations were conducted at two levels: B3LYP/6-311++(d,p) and 6-311G. Our findings indicate that both dealkylation and reduction reactions require energy input and are endothermic processes. However, compared with dealkylation, reduction tends to occur more spontaneously and with lower energy requirements, suggesting that it is a faster process. Additionally, we observed significant molecular changes in trifluralin during reduction, indicating a preference for this pathway, especially in soil conditions where chemical catalysts may enhance the process. In conclusion, investigating herbicide degradation via quantum mechanics elucidates questions regarding herbicide breakdown pathways in the environment and aids in examining their fate and transport more comprehensively through the lens of physical chemistry.

Trifluralin

water

herbicide

DFT

reduction

dealkylation

Trifluralin, a widely used herbicide since the 1960s, has undergone degradation processes in soil, including dealkylation and reduction (Fernandez et al., 2013; Eppi et al., 2018). Understanding the thermodynamic properties of these processes is crucial for predicting their behavior in environmental systems. Most herbicides degrade through either biotic (microbial degradation) or abiotic (hydrolysis and chemical breakdown) pathways in response to environmental conditions, the chemical structure of the herbicide, and its propensity toward a specific transformation pathway influencing the rates of both processes (Fenner et al., 2013). However, the relative role of nonbiological processes compared with biodegradation in the fate of trifluralin remains incompletely understood (Fernandez et al., 2013). Accurate descriptions of the molecular structure, modeling, and prediction of electronic properties, such as electronic energies and electron distributions, contribute to understanding molecular behavior, herbicidal mechanisms, and environmental effects (Islam et al., 2022; McQuarrie & Simon, 1999). Quantum mechanical calculations can also be used to examine intramolecular and intermolecular energy transfer, aiding in better understanding real-time chemical and physical processes and elucidating the mechanisms of herbicidal effects (Wang et al., 2021; Yang et al., 2019). The primary pathways for trifluralin decomposition include dealkylation, halogen elimination, decarboxylation, hydroxylation, side-chain oxidation, isomerization and ring formation, alkoxy bond cleavage, and the reduction of triple bonds to double bonds and nitro groups to amino groups (Nie et al., 2014). Among these processes, dealkylation and reduction have the most significant effects on reducing trifluralin residues in soil through chemical degradation, photodegradation, or microbial action (Coleman et al., 2020). To better explain the role of chemical processes versus biological processes in trifluralin degradation, quantum mechanical calculations were performed to simulate the reduction and dealkylation pathways of this herbicide.

The trifluralin herbicide molecules, specifically 2,2-dinitro-N,N-dipropyl-4-trifluoromethyllaniline, were optimized, and the vibrational frequencies were determined via density functional theory (DFT) in Gaussian 09 software (Biegler, 2001). This was done at two levels: B3LYP 6-311++(d,p) and 6-311G. Additionally, its thermodynamic properties were evaluated via the same computational method. This computational approach is commonly used for organic molecules because of its balance between accuracy and computational efficiency (Yu et al., 2017). Frequency calculations were conducted for each optimized structure to confirm stability and obtain vibrational frequencies.

Furthermore, the one-way reaction pathway of trifluralin was optimized in both the water and gas phases via DFT with the B3LYP/6-31G level of theory, specifically in water (Seinfeld & Atkinson, 1986). Thermodynamic properties, such as standard Gibbs free energy (ΔG°), enthalpy (ΔH), and entropy (ΔS), were computed at a temperature of 298.15 Kelvin and pressure of 1 atm (McQuarrie & Simon, 1999). The parameter ΔG‡ was determined via the Eyring method, incorporating calculated values of ΔG° and ΔH, along with the gas constant (R) and standard temperature (Seinfeld & Atkinson, 1986).

Figure 1 depicts the optimized three-dimensional (3D) structure of the trifluralin molecule, which has the molecular formula C13H16F3N3. The molecule is in the configuration known as 2,2-dinitro-N,N-dipropyl-4-trifluoromethyllaniline. This optimized structure visually represents the spatial arrangement of atoms, revealing the positions of carbon, hydrogen, fluorine, and nitrogen within the molecule. The figure offers valuable insights into the molecular geometry, including bond angles and bond lengths, thereby elucidating its structural properties.

Trifluralin demonstrates highly favorable thermodynamic conditions for dealkylation, as evidenced by its markedly negative ΔG° value of -33497.1 kcal/mol. Furthermore, the positive ΔS values observed across all the compounds (2,2-Dinitro-N,N-dipropyl-4-trifluoromethylaniline and 2,6-Dinitro-4-trifluoroaniline) suggest an increase in disorder during the dealkylation process. Trifluralin exhibits moderate polarity with a dipole moment (µ) value of 4.18 D. In summary, the thermochemical analysis results indicate that trifluralin undergoes dealkylation under highly favorable thermodynamic conditions, accompanied by an increase in molecular disorder. For further details, refer to Table 1 and Fig. 2. These results indicate that the microbial degradation process plays a more important role in dechlorination. Research also supports this notion. Therefore, biological degradation is more important than chemical degradation.

Table 1

Thermochemical data for the dealkylation of trifluralin and the dealkylation of trifluralin. The values are given for the standard Gibbs free energy change (ΔG°), entropy change (ΔS), and dipole moment (µ).
cientific Name	Chemical Formula	ΔG° (kcal/mol)	ΔS (cal/mol-kelvin)	µ (D)
2,2-Dinitro-N,N-dipropyl-4-trifluoromethyllaniline	C13H16F3N3O4	-33497.1	163.097	4.18
2,6-Dinitro-3-trifluoromethyl-N-propyl aniline	C10H10F3N3O4	-27265.1	143.356	4.48
2,6-Dinitro-4-trifluoroaniline	C7H4F3N3O4	-30389.8	124.989	2.24

Trifluralin, with a molecular formula of C13H16F3N3O4, undergoes reduction through successive stages denoted as 2-nitro-6-nitroso-4-trifluoroaniline, 2-hydroxyamino-6-nitro-4-trifluoroaniline, and 2-amino-6-nitro-4-trifluoroaniline (see Fig. 3). The negative values of ΔG° for all the reduction stages indicate that the reduction process is thermodynamically favorable. Trifluralin presented the highest negative ΔG° value, indicating the most favorable reduction process among the stages examined (refer to Table 2).

Table 2

Thermochemical data for trifluralin reduction and thermochemical data for the reduction of trifluralin. The values are given for the standard Gibbs free energy change (ΔG°), entropy change (ΔS), and dipole moment (µ).
Compound Name	Chemical Formula	ΔG° (kcal/mol)	ΔS (cal/mol-kelvin)	µ (D)
2,2-Dinitro-N,N-dipropyl-4-trifluoromethyllaniline	C13H16F3N3O4	-33497.1	163.097	4.181388
2-Nitro-6-nitroso-4-trifluoroaniline	C13H16F3N3O3	-31442.6	160.509	7.230247
2-Hydroxyamino-6-nitro-4-trifluoroaniline	C13H18F3N3O3	-31475.6	164.389	7.071261
2-Amino-6-nitro-4-trifluoroaniline	C13H18F3N2O2	-29511.9	159.071	6.912275

The positive ΔS values observed for all the reduction stages imply an increase in molecular disorder during the reduction process. This increase in entropy suggests a shift toward a state of greater randomness or disorder, which is a characteristic feature of chemical reactions progressing toward equilibrium. The dipole moment (µ) values gradually decrease from 2-nitro-6-nitroso-4-trifluoroaniline to 2-amino-6-nitro-4-trifluoroaniline, indicating a decrease in the polarity of the molecules as the reduction process advances. This decline in polarity is evident in the decreasing dipole moment values, indicating a reduction in the uneven distribution of charge within the molecules. Overall, the thermochemical data suggest that the reduction of trifluralin progresses through successive stages, each characterized by a thermodynamically favorable process accompanied by an increase in molecular disorder and a reduction in molecular polarity. These findings suggest that the reduction process in chemical degradation is more important than that in biological degradation for trifluralin; it may even occur spontaneously in the presence of hydrogen ions or iron (Coleman et al., 2020).

Investigating the reaction pathway of trifluralin in water and gas

These findings suggest that water affects the activation energy and ΔG differently for both the dealkylation and reduction processes. While water increases the activation energy and decreases ΔG for both processes, the reaction rates decrease in water compared with those in the gas phase, despite the reactions being more spontaneous. The reaction pathway of trifluralin in aqueous solution was compared with its reactions in the gaseous state. This pathway, comprising 11 different states, has an overall energy of 1552.44 kcal/mol in water and 1454.34 kcal/mol in gas. The energy difference is significantly greater in the water phase than in the gas phase. In aqueous environments, trifluralin does not completely dissolve and may form more stable complexes with water molecules through molecular interactions. However, significant structural changes are not observed along the reaction pathway (see Fig. 4).

Thermodynamically, the calculated ΔG for the dealkylation process is greater than that for reduction, indicating nonspontaneity and the requirement for external energy. Both reactions are endothermic, with dealkylation resulting in higher ΔH values than reduction. Additionally, the change in electric charge distribution in the product molecule is more asymmetric in terms of reduction, accompanied by a lower activation energy, suggesting more significant and dynamic changes in the molecule (see Table 3).

Table 3

Comparison of Dealkylation and Reduction Processes, Comparison of the thermodynamic parameters (ΔG°, ΔH, ΔS, µ, ΔG‡) for the dealkylation and reduction processes of trifluralin. These values provide insight into the energetics and feasibility of each process.
Parameter	Dealkylation	Reduction
ΔG° (kcal/mol)	295541	93576.44
ΔH (kJ/mol)	147.467	92.378
ΔS (cal/mol-K)	-1.374	-4.02
µ (D)	2.53099	5.114658
ΔG‡ (kJ/mol)	147.877 ≤ ΔG‡ ≤ 155.817	93.576 ≤ ΔG‡ ≤ 101.516

"In summary, trifluralin's reduction process outpaces dealkylation, owing to its lower activation energy and higher likelihood of spontaneous occurrence, leading to more significant molecular changes. Analysis of key parameters such as ΔG°, ΔH, ΔS, µ, and ΔG‡ revealed that both reactions are endothermic, with dealkylation resulting in higher values than reduction. Furthermore, the higher ΔG and ΔG‡ values for dealkylation indicate its lower stability and greater spontaneity than those of reduction. This reduction, with its more asymmetric charge distribution and lower activation energy, suggests more dynamic changes in the molecule than does dealkylation. The reduction process likely proceeds faster because of its lower activation energy, resulting in more extensive molecular transformations. Additionally, the chemical environment of the soil, combined with water, favors the reduction process, possibly with the aid of chemical catalysts (Coleman et al., 2020).

In conclusion, our study investigated the thermodynamic properties of the dealkylation and reduction processes of trifluralin, a commonly used herbicide, to understand their behavior in environmental systems. We observed that water affects these processes, although the reaction rate in water is lower than that in the gas phase. The gradient in the water phase was greater than that in the gas phase, indicating a more complex reaction pathway and potentially slower kinetics in the presence of water. The total energy in the water phase is lower at all reaction stages than that in the phase without interaction. This suggests that the reaction products are more stable in the presence of water (see Table 4).

Table 4

Gradient and total energy along the IRC for no interaction and water phases with structural intermediates
State	Gradient (1C/Å) along IRC - No Interaction	Gradient (1C/Å) along IRC - Water	Total Energy (kcal/mol) - No Interaction	Total Energy (kcal/mol) - Water
1	0.000821	0.004028	-5280.58	-5280.88
2	0.000785	0.003887	-5280.67	-5280.96
3	0.00044	0.001741	-5281.13	-5282.37
4	0.000332	0.000827	-5281.27	-5282.65
5	0.00016	0.000229	-5281.31	-5282.74
6	0.000053	0.000091	-5281.34	-5282.77
7	0.000017	0.000054	-5281.36	-5282.8
8	0.000015	0.000042	-5281.37	-5282.83
9	0.000011	0.000033	-5281.39	-5282.87
10	0.000006	0.000024	-5281.41	-5282.89
11	0.000002	0.000018	-5281.42	-5282.92

Dealkylation resulted in higher ΔG values, indicating that a less spontaneous process requires external energy, whereas both reactions were endothermic, with a decrease in ΔH values. Additionally, the reduction led to more significant changes in the molecular structure, as evidenced by its higher dipole moment (µ) and asymmetric charge distribution. A comparison between dealkylation and reduction highlighted the faster occurrence of reduction due to its lower activation energy, greater spontaneous occurrence tendency, and more significant molecular changes. The reduction process also showed greater potential for spontaneity and heat release, leading to a more dynamic transformation than dealkylation. Overall, our findings emphasize the importance of understanding herbicide degradation thermodynamics for effective environmental management and sustainable agricultural practices. According to the results obtained, despite trifluralin being nonpolar, water significantly influences the decomposition pathway of the trifluralin molecule in the soil environment through hydrogen bonding.

Conflict of interest:

The authors declare that they have no conflicts of interest.

Ethical Approval:

This study did not require ethical approval from a research ethics committee or similar body because it does not involve research on humans or animals.

Informed Consent:

This study does not involve any research on humans or animals; therefore, informed consent is not applicable.

Funding:

This study was not supported by any external funding sources.

Author Contribution

Author Contributions StatementM.B. conceptualized and designed the study, conducted the quantum mechanical calculations, and wrote the initial draft of the manuscript. S.S. assisted with the computational analysis, interpreted the results, and contributed to the manuscript's writing and revision. M.B. and S.S. both reviewed and approved the final version of the manuscript.

Data Availability

The data supporting the findings of this study are provided within the manuscript and its supplementary information files. For any additional requests or inquiries, please contact the corresponding author.

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No competing interests reported.

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Quantum Mechanical Study of Dealkylation Processes and the Reducing Trifluralin Herbicide

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Investigating the reaction pathway of trifluralin in water and gas

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