Unveiling the Influence of Water Molecules for NF3 Removal by the Reaction of NF3 with OH: A DFT Study

The removal of nitrogen trifluoride (NF3) is of significant importance in atmospheric chemistry, as NF3 is an important anthropogenic greenhouse gas. However, the radical species OH and O(1D) in atmospheric conditions are nonreactive towards NF3. It is necessary to explore possible ways to remove NF3 in atmosphere. Therefore, the participation of water molecules in the reaction of NF3 with OH was discussed, as water is abundant in the atmosphere and can form very stable complexes due to its ability to act as both a hydrogen bond donor and acceptor. Systemic DFT calculations carried out at the CBS-QB3 and ωB97XD/aug-cc-pVTZ level of theory suggest that water molecules could affect the NF3 + OH reaction as well. The energy barrier of the SN2 mechanism was decreased by 8.52 kcal/mol and 10.58 kcal/mol with the assistance of H2O and (H2O)2, respectively. Moreover, the presence of (H2O)2 not only reduced the energy barrier of the reaction, but also changed the product channels, i.e., formation of NF2O + (H2O)2-HF instead of NF2OH + (H2O)2-F. Therefore, the removal of NF3 by reaction with OH is possible in the presence of water molecules. The results presented in this study should provide useful information on the atmospheric chemistry of NF3.


Introduction
As the most extensively used perfluoro compound, nitrogen trifluoride (NF 3 ) has attracted great interest in recent years.NF 3 is commonly used in the semiconductor industry [1][2][3] and as a fluorine-supplying source in the electronic industry [4,5].The industrial use of NF 3 had been considered safe for a long time, as it did not produce carbon contamination residues.Hence, the production of NF 3 as a substitute for other perfluorinated gases such as CF 4 and C 2 F 6 increased dramatically in recent years [6], resulting in a very large amount of NF 3 atmospheric emissions.Unfortunately, recent studies have warned that there is a clear risk in using NF 3 [7].Firstly, NF 3 is considered a new greenhouse gas, although it is not included in the list of greenhouse gases in the Kyoto protocol [8,9].In fact, NF 3 has a global warming potential (GWP) of 17,200, which is 10,800 times greater than that of CO 2 when compared over a 100-year period [10][11][12].Furthermore, NF 3 and its decomposition products have been proposed to be toxic and pose a health risk [8].Because of these concerns, great interest has focused on developing new processes to destroy or remove unreacted effluent NF 3 .
To date, various methods have been reported for the adsorption and decomposition of NF 3 [13][14][15][16][17][18].However, these methods are designed to deal with the tail gas in the semiconductor industry and electronic industry.On the other hand, there is a significant shortage of research on removal and decomposition processes for NF 3 in the atmosphere.Gargano et al. [19] studied two important reactions involved in the decomposition of NF 3 , i.e., NF 3 + F and N 2 + F. Later, Cunha and coworkers [20] investigated other reactions involved in the decomposition of NF 3 employing theoretical calculations at the CCSD(T)/cc-pVTZ level of theory, for example, NF 2 + N, NF 3 + NF, and the dissociation of N 2 F 4 and N 2 F 3 .These pioneer studies provide fundamental insight into the mechanism of NF 3 decomposition.
Several studies have also focused on the removal of NF 3 in the atmosphere through the reaction with atmospheric oxidants.Wine and coworkers [21] studied the reaction of NF 3 with O( 1 D), measured the rate coefficient to be k(T) = 2.0 × 10 −11 exp(52/T) cm 3 molecule −1 s −1 , and suggested that the reaction with O( 1 D) is an important atmospheric sink for NF 3 .Baasandorj and coworkers [22] also measured the rate coefficient of O( 1 D) with NF 3 , which is in good agreement with the results of Wine and coworkers.However, the reaction of reactive OH radical with NF 3 was not mentioned.Dillon and coworkers [23] explored the possibility of removing NF 3 by reactions with the atmospheric oxidants O( 1 D), OH and O 3 , and the results showed that the reaction rate of NF 3 + OH is as slow as 2.0 × 10 −29 cm 3 molecule −1 s −1 ; thus, they concluded that OH could not play an important role in atmospheric NF 3 degradation.Although the reaction of OH with NF 3 is extremely slow, the possibility of removing NF 3 by reaction with OH should not be excluded because water molecules in the atmosphere have been shown to have a significant chemical catalytic effect on certain atmospheric reactions.Buszek and coworkers [24] reviewed the effect of water molecules on various atmospheric reactions, including radical-molecule, radicalradical, molecule-molecule and unimolecular reactions.It is surprising that, to our best knowledge, the influence of water molecules on the reaction of OH + NF 3 has not been explored yet, though the reaction of OH with various molecules, such as HCOOH [25], HNO 3 [26],CH 3 CHO [27][28][29], fluoroalcohols [30], HOCl [31], glyoxal [32,33], CH 4 [34], DMSO [35], CH 3 OH [36][37][38], etc., have been studied extensively.Could additional water molecules accelerate the reaction of OH + NF 3 ?If the answer is positive, how do the additional water molecules affect the reaction?Here, we decided to study the reaction of OH + NF 3 with the participation of water molecules by using computational methods.These questions are critical for exploring the processes of removing NF 3 in the atmosphere.

Results and Discussion
In principle, it is better to carry out benchmark calculations aiming to assess the accuracy of the DFT methods.Fortunately, several references have proved that the ωB97XD functional including dispersion was capable of treating various reactions [39,40].As a result, all the discussions are based on the data obtained from CBS-QB3//ωB97XD/aug-cc-pVTZ methods.Moreover, as the reactions discussed here are in the gas phase, the electronic energy with zero-point energy was employed to discuss the thermodynamics [41,42].

The Reaction of NF 3 + OH
According to a previous work [22], the reaction of NF 3 + OH can be carried out through three distinct processes, i.e., the S N 2 mechanism, F abstraction and H addition to the N center.However, the energy barrier is too high for H addition to N to be of consideration.As a result, only the S N 2 mechanism and F abstraction were discussed here.As for the S N 2 mechanism, an OH radical attacks the N, while one N-F bond is broken simultaneously, forming NF 2 OH and F. As shown in Figure 1, the corresponding transition state is TS1 with an energy barrier of 16.04 kcal/mol.Unfortunately, the product is endothermic by 2.50 kcal/mol, indicating this process is unfavorable thermodynamically, especially in atmospheric conditions.In the case of the F abstraction mechanism, an OH radical abstracts F from NF 3 directly, leading to the production of HFO and NF 2 .The transition state for this process is TS2, in which the OH interacts with the leaving F. As can be seen in Figure 1, the energy barrier of TS2 (32.72 kcal/mol) is much higher than that of TS1.Moreover, the product is endothermic by as much as 10.02 kcal/mol.These results indicate the F abstraction process is unfeasible.In a word, although the S N 2 mechanism is predominant in comparison to the F abstraction mechanism, the reaction of NF 3 + OH is difficult to accomplish in view of thermodynamics.This is in good accordance with the extremely slow reaction rate measured experimentally [23].As a result, the removal of NF 3 through the gas-phase reaction with OH radial is of minor importance in atmospheric conditions.These results are in accordance with a previous report [23].
Molecules 2024, 29, x FOR PEER REVIEW 3 of 10 atmospheric conditions.In the case of the F abstraction mechanism, an OH radical abstracts F from NF3 directly, leading to the production of HFO and NF2.The transition state for this process is TS2, in which the OH interacts with the leaving F. As can be seen in Figure 1, the energy barrier of TS2 (32.72 kcal/mol) is much higher than that of TS1.Moreover, the product is endothermic by as much as 10.02 kcal/mol.These results indicate the F abstraction process is unfeasible.In a word, although the SN2 mechanism is predominant in comparison to the F abstraction mechanism, the reaction of NF3 + OH is difficult to accomplish in view of thermodynamics.This is in good accordance with the extremely slow reaction rate measured experimentally [23].As a result, the removal of NF3 through the gas-phase reaction with OH radial is of minor importance in atmospheric conditions.
These results are in accordance with a previous report [23].

The Influence of Water Molecules on the Reaction of NF3 + OH
Inspired by the fact that the participation of water molecules could affect various atmospheric reactions in the gas phase [43][44][45][46][47][48][49], the influence of water molecules on the reaction of NF3 + OH is discussed here.In the condition where one H2O participates in the reaction, the NF3 + OH reaction takes place through two distinct process similar to the naked reaction shown in Figure 2. The corresponding geometry structures of all intermediates and transition states are available in Figure S1.
In contrast to the naked reaction, a pre-reactive complex formed in the entrance of the reaction due to the existence of a hydrogen bond between H2O and the reactants.For example, W1-RC1 and W1-RC2 are the pre-reactive complexes for the SN2 mechanism and F abstraction mechanism, respectively, as shown in

The Influence of Water Molecules on the Reaction of NF 3 + OH
Inspired by the fact that the participation of water molecules could affect various atmospheric reactions in the gas phase [43][44][45][46][47][48][49], the influence of water molecules on the reaction of NF 3 + OH is discussed here.In the condition where one H 2 O participates in the reaction, the NF 3 + OH reaction takes place through two distinct process similar to the naked reaction shown in Figure 2. The corresponding geometry structures of all intermediates and transition states are available in Figure S1.
In contrast to the naked reaction, a pre-reactive complex formed in the entrance of the reaction due to the existence of a hydrogen bond between H 2 O and the reactants.For example, W1-RC1 and W1-RC2 are the pre-reactive complexes for the S N 2 mechanism and F abstraction mechanism, respectively, as shown in Figure 2. Attributed to the formation of a hydrogen bond, W1-RC1 and W1-RC2 are 7.12 kcal/mol and 7.32 kcal/mol more stable than the reactants, respectively.Starting from W1-RC1, the reaction takes place via the S N 2 mechanism.The corresponding transition state is W1-TS1.It should be noted that the structure of W1-TS1 is similar to that of TS1, except the breaking F atom bonded to the H 2 O due to the formation of an F•••H•••O hydrogen bond.The energy barrier of W1-TS1 is 14.64 kcal/mol, which is only 1.40 kcal/mol lower than that of TS1.Therefore, the influence of one H 2 O molecule on the S N 2 mechanism is negligible in view of the kinetics.On the other hand, although the product complex W1-PC1 was located 18.84 kcal/mol below the reactants owing to the formation of a hydrogen bond, the final product, NF 2 OH + H 2 O-F, is endothermic by 1.90 kcal/mol, which is similar to that of PC1.It is reasonable to conclude that an additional H 2 O molecule is of no influence on the S N 2 mechanism in view of the thermodynamics as well.As for the F abstraction mechanism, the corresponding transition state is W1-TS2, with a geometry structure similar to that of TS2.Unfortunately, the relative energy of W1-TS2 is as high as 26.30 kcal/mol.As a result, the reaction should overcome the energy barrier of 33.62 kcal/mol, which is even about 1 kcal/mol larger than that of F abstraction in the absence of H 2 O (TS2).Thus the participation of one H 2 O molecule is unable to accelerate the F abstraction process.In a word, when one additional H 2 O molecule takes part in the reaction of NF 3 + OH, neither the kinetics nor thermodynamics are affected.This can be explained by the structure of the transition states.As can be seen in Figure S1, the hydrogen transfer process is not involved in either the S N 2 mechanism or F abstraction mechanism.The additional H 2 O molecule only connects OH and F with the formation of a hydrogen bond rather than assisting the hydrogen transfer.Thus an additional, single H 2 O molecule only acts as a spectator rather than catalyst in the reaction of NF 3 + OH.It is not unexpected that the effects of H 2 O molecules on various atmospheric reactions reported in the references do not appear here.
is endothermic by 1.90 kcal/mol, which is similar to that of PC1.It is reasonable to conclude that an additional H2O molecule is of no influence on the SN2 mechanism in view of the thermodynamics as well.As for the F abstraction mechanism, the corresponding transition state is W1-TS2, with a geometry structure similar to that of TS2.Unfortunately, the relative energy of W1-TS2 is as high as 26.30 kcal/mol.As a result, the reaction should overcome the energy barrier of 33.62 kcal/mol, which is even about 1 kcal/mol larger than that of F abstraction in the absence of H2O (TS2).Thus the participation of one H2O molecule is unable to accelerate the F abstraction process.In a word, when one additional H2O molecule takes part in the reaction of NF3 + OH, neither the kinetics nor thermodynamics are affected.This can be explained by the structure of the transition states.As can be seen in Figure S1, the hydrogen transfer process is not involved in either the SN2 mechanism or F abstraction mechanism.The additional H2O molecule only connects OH and F with the formation of a hydrogen bond rather than assisting the hydrogen transfer.Thus an additional, single H2O molecule only acts as a spectator rather than catalyst in the reaction of NF3 + OH.It is not unexpected that the effects of H2O molecules on various atmospheric reactions reported in the references do not appear here.

The Influence of an Additional Two H2O Molecules on the NF3 + OH Reaction
The structures of various pre-reactive complexes and transition states for the NF3 + OH reaction with an additional two H2O are shown in Figure 3.In contrast to the reaction with one participating H2O, there are three possible processes, as can be seen from the corresponding potential energy profiles (see Figure 4).
For the SN2 mechanism, the structure of the pre-reactive complex W2-RC1 is extremely similar to that of W1-RC1.However, the F-H bond in W2-RC1 is 0.3 Å shorter than that of W1-RC1, and the distance of N-O is reduced by about 0.2 Å.This could be attributable to the stabilization energy provided by the hydrogen bond of two H2O molecules, and could be proved by the energy of W2-RC1, which is 8 kcal/mol more stable than that of W1-RC1.The transition state corresponding to the broken N-F bond and N-O bond formation is W2-TS1, with an energy barrier of 12.17 kcal/mol.Moreover, W2-TS1 is located 3.06 kcal/mol below the initial reactants; thus, this transformation is accessible kinetically.It should be attributed to the direct participation of (H2O)2 in the reaction as a proton shuttle.As shown in Figure 3, the F-H in the W2-TS1 bond has shrunk to 1.89 Å,

The Influence of an Additional Two H 2 O Molecules on the NF 3 + OH Reaction
The structures of various pre-reactive complexes and transition states for the NF 3 + OH reaction with an additional two H 2 O are shown in Figure 3.In contrast to the reaction with one participating H 2 O, there are three possible processes, as can be seen from the corresponding potential energy profiles (see Figure 4).
For the S N 2 mechanism, the structure of the pre-reactive complex W2-RC1 is extremely similar to that of W1-RC1.However, the F-H bond in W2-RC1 is 0.3 Å shorter than that of W1-RC1, and the distance of N-O is reduced by about 0.2 Å.This could be attributable to the stabilization energy provided by the hydrogen bond of two H 2 O molecules, and could be proved by the energy of W2-RC1, which is 8 kcal/mol more stable than that of W1-RC1.The transition state corresponding to the broken N-F bond and N-O bond formation is W2-TS1, with an energy barrier of 12.17 kcal/mol.Moreover, W2-TS1 is located 3.06 kcal/mol below the initial reactants; thus, this transformation is accessible kinetically.It should be attributed to the direct participation of (H 2 O) 2 in the reaction as a proton shuttle.As shown in Figure 3, the F-H in the W2-TS1 bond has shrunk to 1.89 Å, which is 0.32 Å shorter than that of W1-TS1, indicating the eliminated F has connected to the H 2 O molecules.This is verified by the intrinsic reaction coordinate (IRC) [50] calculation (see Figure S2).which is 0.32 Å shorter than that of W1-TS1, indicating the eliminated F has connected to 163 the H2O molecules.This is verified by the intrinsic reaction coordinate (IRC) [50] calcula-164 tion (see Figure S2).Moreover, the O-H bond in the OH radical is intended to break in the 165 product direction of the IRC calculation, suggesting the products should change com-166 pared with the naked reaction and the reactions with one additional participating H2O.In  It is well-known that the concentrations of larger complexes involving more than two molecules are very low in the troposphere [37]; as a result, only an additional one or two H2O molecules were taken into account.In summary, it is obvious that the participation of additional H2O molecules influences the reaction of NF3 with OH dramatically.Taking the SN2 mechanism, for example (see Figure 5), without the assistance of additional H2O molecules, the reaction is difficult to accomplish, as the energy barrier is high, and the Considering the F abstraction mechanism, a pre-reactive complex, W2-RC2, was confirmed as well.Due to the hydrogen bond, W2-RC2 is 17.70 kcal/mol lower than the reactants, which is of marginal difference with the naked reaction and the reaction with one participating H 2 O molecule.The F abstraction was accomplished through W2-TS2, which is similar to W1-TS2 as well.However, the energy barrier of W2-TS2 is almost the same as that of TS2 and W1-TS2, inferring that (H 2 O) 2 has marginal influence on the F abstraction mechanism.This result is not unexpected because the (H 2 O) 2 plays the role of spectator, as can be seen from the structure of W2-TS2.
Apart from the S N 2 mechanism and F abstraction mechanism, there is a new reaction process in the case of (H 2 O) 2 participating in the NF 3 + OH reaction, as depicted in Figure 4.This process initiates by the formation of W2-RC3, which is a pre-reactive complex formed by the contact between (H 2 O) 2 -OH and NF 3 .Starting from W2-RC3, the reaction proceeds via the transition state of W2-TS3, in which the change in O-N and F-N bonds is similar to that in W2-TS1.It is interesting that the IRC calculation of W2-TS3 bears evidence of the interaction between the substituted F and (H 2 O) 2 , leading to the formation of complex NF 2 O-(H 2 O) 2 -HF (see W2-PC3 in Figure 4).It is worth noting that W2-TS3 lies 0.53 kcal/mol below the reactants, suggesting this process in kinetically favorable as well.
It is well-known that the concentrations of larger complexes involving more than two molecules are very low in the troposphere [37]; as a result, only an additional one or two H 2 O molecules were taken into account.In summary, it is obvious that the participation of additional H 2 O molecules influences the reaction of NF 3 with OH dramatically.Taking the S N 2 mechanism, for example (see Figure 5), without the assistance of additional H 2 O molecules, the reaction is difficult to accomplish, as the energy barrier is high, and the products are endothermic.Fortunately, the energy barrier of the S N 2 mechanism decreases by 8.5 kcal/mol and 10.6 kcal/mol in the case of complex NF2O-(H2O)2-HF (see W2-PC3 in Figure 4).It is worth noting that W2-TS3 lies 0.53 kcal/mol below the reactants, suggesting this process in kinetically favorable as well.It is well-known that the concentrations of larger complexes involving more than two molecules are very low in the troposphere [37]; as a result, only an additional one or two H2O molecules were taken into account.In summary, it is obvious that the participation of additional H2O molecules influences the reaction of NF3 with OH dramatically.Taking the SN2 mechanism, for example (see Figure 5), without the assistance of additional H2O molecules, the reaction is difficult to accomplish, as the energy barrier is high, and the products are endothermic.Fortunately, the energy barrier of the SN2 mechanism decreases

Computational Methods
All reactants, products, pre-reactive complexes (RC, PC) and transition states (TS) were fully optimized using the density functional theory at the ωB97XD/aug-cc-pVTZ level of theory, as the long-range correction functional ωB97XD described the hydrogen bond well [51][52][53][54][55].The harmonic vibrational frequencies of all optimized structures were calculated at the same level of theory to confirm the stationary point (intermediate or transition states) and for the zero-point energy (ZPE) corrections.The intrinsic reaction coordinate (IRC) [50] calculations were carried out to verify that the predicted transition states connect the designated reactants and products.In order to obtain more accurate thermodynamics data, the single-point energies of all species were calculated using the CBS-QB3 method [56,57].The energy calculated at the CBS-QB3 level of theory was employed in the following discussion.All the DFT calculations were performed using the Gaussian 09 program [58].The bond length comparison of selected species and all the optimized cartesian coordinates of species involved in the reactions are available in the Supporting Information (SI).The zero-point energy (ZPE) and relative energies are listed in Table 1.The energy profiles and corresponding structures for the reaction of NF 3 with OH with the assistance of water molecules are illustrated in Figures 1-5.

Conclusions
The possibility of removal of NF 3 by the NF 3 + OH reaction was studied at the CBS-QB3 level of theory.It was found that the NF 3 + OH reaction in the absence of H 2 O molecules (naked reaction) is of no importance for the removal of NF 3 in atmospheric conditions, as both the S N 2 and F abstraction mechanisms must overcome a high energy barrier, while the products are endothermic.Although the participation of one H 2 O molecule has no influence on the NF 3 + OH reaction, as the H 2 O acts as a spectator, it significantly changes when (H 2 O) 2 takes part in the reaction as catalyst.The presence of (H 2 O) 2 not only reduces the energy barrier of the S N 2 mechanism, but also changes the products, i.e., with the formation of NF 2 O + (H 2 O) 2 -HF instead of NF 2 OH + (H 2 O) 2 -F.The reaction of NF 3 + OH is favorable in the presence of (H 2 O) 2 , both kinetically and thermodynamically.The results indicate that it is possible to remove NF 3 by reaction with OH radical in the presences of water molecules.

Figure 1 .
Figure 1.The energy profile of the NF3 + OH reaction.

Figure 2 .
Attributed to the formation of a hydrogen bond, W1-RC1 and W1-RC2 are 7.12 kcal/mol and 7.32 kcal/mol more stable than the reactants, respectively.Starting from W1-RC1, the reaction takes place via the SN2 mechanism.The corresponding transition state is W1-TS1.It should be noted that the structure of W1-TS1 is similar to that of TS1, except the breaking F atom bonded to the H2O due to the formation of an F•••H•••O hydrogen bond.The energy barrier of W1-TS1 is 14.64 kcal/mol, which is only 1.40 kcal/mol lower than that of TS1.Therefore, the influence of one H2O molecule on the SN2 mechanism is negligible in view of the kinetics.On the other hand, although the product complex W1-PC1 was located 18.84 kcal/mol below the reactants owing to the formation of a hydrogen bond, the final product, NF2OH + H2O-F,

Figure 1 .
Figure 1.The energy profile of the NF 3 + OH reaction.

Figure 2 .
Figure 2. Energy profiles of one additional H2O participating in the reaction of NF3 + OH.

Figure 2 .
Figure 2. Energy profiles of one additional H 2 O participating in the reaction of NF 3 + OH.
Moreover, the O-H bond in the OH radical is intended to break in the product direction of the IRC calculation, suggesting the products should change compared with the naked reaction and the reactions with one additional participating H 2 O.In fact, owing to the direct participation of (H 2 O) 2 , the broken H migrates along the (H 2 O) 2 skeleton, resulting in the formation of NF 2 O + (H 2 O) 2 -HF, as exhibited in Figure 4, which is different from the S N 2 mechanism of the NF 3 + OH reaction with one additional H 2 O.It is interesting that the formation of NF 2 O + (H 2 O) 2 -HF is exothermic by 70.49 kcal/mol.In a word, when two additional H 2 O molecules take part in the reaction of NF 3 + OH as catalyst, the formation of the products NF 2 O + (H 2 O) 2 -HF is favorable both thermodynamically and kinetically.

Figure 3 . 4 .Figure 3 .
Figure 3.The structures of intermediates and transition states involved in NF3 + OH reactions with

Figure 4 .
Figure 4.The energy profiles of NF3 + OH reactions with an additional two H2O molecules.

Figure 4 .
Figure 4.The energy profiles of NF 3 + OH reactions with an additional two H 2 O molecules.
H 2 O and (H 2 O) 2 catalyzed reactions.Especially, the thermodynamics of the reaction change as the products change from NF 2 OH + F to NF 2 O + HF with the formation of an O-H•••F hydrogen bond.

Figure 4 .
Figure 4.The energy profiles of NF3 + OH reactions with an additional two H2O molecules.

by 8 .
5 kcal/mol and 10.6 kcal/mol in the case of H2O and (H2O)2 catalyzed reactions.Especially, the thermodynamics of the reaction change as the products change from NF2OH + F to NF2O + HF with the formation of an O-H•••F hydrogen bond.

Figure 5 .
Figure 5.Comparison of naked NF 3 + OH reaction and the H 2 O and (H 2 O) 2 assisted reactions.

Table 1 .
The ZPE and calculated relative energies (in kcal/mol) for all reactants, transition states and products.