Theoretical study of the hydrolysis of sulfur tetrafluoride

Theoretical study of the hydrolysis of sulfur tetrafluoride

Journal of Fluorine Chemistry 153 (2013) 114–120 Contents lists available at SciVerse ScienceDirect Journal of Fluorine Chemistry journal homepage: ...

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Journal of Fluorine Chemistry 153 (2013) 114–120

Contents lists available at SciVerse ScienceDirect

Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor

Theoretical study of the hydrolysis of sulfur tetrafluoride Chongsong Zhou a, Li Li b, Yongyan Zhou b, Hua Hou a, Baoshan Wang a,* a b

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei, PR China Electric Power Research Institute of Guangdong Power Grid Corporation, Guangzhou 510080, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 28 February 2013 Received in revised form 2 May 2013 Accepted 6 May 2013 Available online 13 May 2013

The reaction between sulfur tetrafluorine (SF4) and H2O was computationally investigated using the G4// MP2/6-311G(d, p) theoretical model. Among four reactive channels in the initial attack process, the primary path, which occurred via a SN2 displacement reaction to produce SF3OH, is the rate-determining step in the complete hydrolysis of SF4, resulting in the formation of SO2 (H2SO3). The energy barrier (i.e., 22.48 kcal mol1) in the gas phase will be substantially reduced when catalyzed by H2O and/or HF, and the Berry pseudorotation reaction (BPR) process of SF3OH becomes the rate-controlling step. The hydrolysis mechanism of SF4 in aqueous solution and the activation energy of the rate-determining step using the PCM model do not substantially change compared to the gas phase results. The hydrolysis of SF4 in an aqueous solution has a higher enthalpy. For secondary reactions in aqueous solution, the intermediates are prone to dehydrofluorination instead of hydrolysis. Although the activation energy of SOF2 hydrolysis is higher than that of SF4, the relative energy of the former is lower than that of the latter in aqueous solution. Therefore, SOF2 might be an important intermediate for the formation of SO2. ß 2013 Elsevier B.V. All rights reserved.

Keywords: SF4 Hydrolysis Sulfur tetrafluorine Theoretical Water

1. Introduction SF4 has C2v symmetry and a seesaw structure [1] arising from a trigonal bipyramidal geometry. The longer S–F bond in SF4 is oriented in the axial direction, while the shorter one is oriented in the equatorial direction. The two vacant sites on the sulfur atom in SF4 are prone to nucleophilic attack. Sulfur tetrafluorine (SF4) is the raw material used to produce SF6 [2], and SF4/HF solutions are widely applied for fluorination in organic synthesis [3]. Extensive research concerning SF4 [4–6] can be attributed to its interesting structure [7] and the presence of SF4 in the air around SF6 electrical equipment [8–10]. The decomposed gases (HF and SO2) most likely resulted from hydrolysis of the SF4 products in humid conditions [11]. The hydrolysis of SF4 has been known for a long time. Asmus et al. [12] first reported the hydrolysis rate constant (k) in aqueous solution in different pH environments using the pulse radiolysis conductivity technique. These researchers determined that the rate-determining step for the complete hydrolysis of SF4 was the nucleophilic attack of water on SF4. Sauers et al. [13] investigated the hydrolysis of SF4 in the gas phase with a mass spectrometer. Based on the measured rate constant of hydrolysis in

* Corresponding author at: College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei, PR China. Tel.: +86 027 68756347. E-mail address: [email protected] (B. Wang). 0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jfluchem.2013.05.002

the gas phase and the liquid water density, these authors’ calculated value in aqueous solution was consistent with the results of Asmus et al. [12]. Larin et al. [14] have investigated the hydrolysis of SF4 using HF/6-31G(d) and MP2/6-31G(d). These researchers proposed that SF3OH was produced by a displacement reaction via a 4-membered ring transition state in the initial process and that SF3OH could further produce SOF2 via a 6membered ring transition state catalyzed by two HF molecules. In this work, we attempted to answer two questions using theoretical calculations. First, we investigated the entire reactive potential energy surface (PES) of SF4 and H2O both in the gas phase and in aqueous solution, as well as self-catalysis. Second, we investigated the complete hydrolysis process from SF4 to SO2 (including secondary reactions). 2. Results and discussion 2.1. Theoretical model The hydrolysis reaction of SF4 is shown below. SF4 þ H2 O ! SOF2 þ 2HF

(1)

The reactants, products and certain sulfur-containing species were optimized using MP2/6-311G(d, p) and MP2/aug-ccpV(T + d)Z levels of theory. All of the calculated results and a portion of the experimental data [15–23] were shown in Figs. 1 and 2.

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Fig. 1. Structures of the sulfur containing species.

The calculated bond lengths of S–F and S–O using the aug-ccpV(T + d)Z basis sets are shorter than those determined using the 6-311G(d, p) basis sets and are closer to experimental structures. For example, the S–F bond length in the axial and equatorial orientation in SF4 calculated using MP2/aug-cc-pV(T + d)Z is 1.654 and 1.553 A˚, respectively, which are in good agreement with the experimental values of 1.646 and 1.545 A˚ [20], respectively, and slightly better than the theoretical results obtained from MP2/6311G(d, p) calculations in this work (within 0.030 A˚ deviation) and B3LYP/aug-cc-pV(Q + d)Z calculations [17] (within 0.020 A˚ deviation). Other sulfur-containing molecules are similar to SF4. This phenomenon is attributed to the aug-cc-pV(T + d)Z basis sets containing additional tight d augmented consistent functions for sulfur atoms, which will be beneficial to the convergence of the molecular orbits and the formation of slightly tighter bonds. Therefore, the expensive basis sets can be used to clarify the reliability of the 6-311G(d, p) basis sets. Compared to experimental results, the average absolute deviation of all of the bond lengths and all of the angles using the MP2/aug-cc-pV(T + d)Z level of theory are 0.009 A˚ and 0.3418, respectively, and the average relative deviation are 0.65 and 0.32%, respectively. These results indicate that the MP2/aug-ccpV(T + d)Z level of theory is a highly precise method for structure optimization of the sulfur-containing species. Interestingly, when using the MP2/6-311G(d, p) level of theory, the mean absolute deviation is 0.025 A˚ and 0.4638, and the mean relative deviation is 1.63 and 0.45%. These results indicate that the MP2/6-311G(d, p) level of theory is still reliable for exploring the hydrolysis of SF4, even though it may slightly overestimate the experimental bond lengths within 0.050 A˚. Therefore, the MP2/6-311G(d, p) level of theory is suitable for optimization of our system based on its accuracy and efficiency. The standard enthalpies of formation were calculated using G3MP2, CCSD(T)/CBS limits and G4 theories based on optimized structures using two levels of theory, and the computational results from six theoretical models and the experimental reference data [24–29] were shown in Table 1. For H2O, HF, SO, SO2 and SOF2

species, all of the standard enthalpies of formation from the six theoretical models were in agreement with each other. However, the calculated results for other species reflected the differences in these theoretical methods. The mean absolute deviation obtained with G3MP2, CCSD(T)/ CBS limits and G4 theory using the structures obtained from MP2/ 6-311G(d, p) calculations was 4.03 kcal mol1, 2.25 kca mol1 and 1.53 kcal mol1, respectively. The latter two theoretical models were nearly within chemical accuracy (2.0 kcal mol1) and performed better than the G3MP2 model for calculations of sulfur containing species. The G4 theory, which had the maximum deviation of 3.42 kcal mol1 for H2SO4, was relatively better than the CCSD(T)/CBS limits model. Hahn et al. proposed that G4 theory was the preferable method for calculation of accurate energies for this system [30]. The second-order Moller–Plesset perturbation (MP2) method was used to structure optimization. To calculate of the heats of formation using G3MP2, CCSD(T)/CBS limits and G4 theory, the difference in the mean absolute deviation between MP2/6311 G(d, p) and MP2/aug-cc-pV (T + d)Z was 0.67, 0.12 and 0.31 kcal mol1, respectively, and the maximum absolute relative deviation was 0.51%. These results indicated that the differences in structures obtained from using two different basis sets had little effect on the calculation of the standard enthalpies of formation. In particular, the discrepancy for G4 theory was smaller, and the heats of formation using MP2/aug-cc-pV(T + d)Z structure was always slightly lower than those obtained using MP2/6-311G(d, p) structure. The difference might decrease when the reactive heats of formation are calculated using relative energies. Therefore, the G4//MP2/6-311G(d, p) theoretical model was suitable for the calculation of the hydrolysis of SF4. 2.2. PES of hydrolysis of SF4 SF4 could react with H2O in humid air and in aqueous solution. The geometrical structures of the various species in the hydrolysis

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Fig. 2. PES showing the structures associated with the hydrolysis of SF4.

process in the gas phase and the PES of SF4 hydrolysis were shown in Figs. 2 and 3, respectively. SF4 combines a H2O molecule to form the SF4H2O complex with the release of 1.64 kcal mol1 of heat. Subsequently, reaction (1) required the elimination of two fluorine atoms. Therefore, SF4 hydrolysis can be divided into two stages. During the first stage, there are four primary reactive channels by which a water molecule can attack SF4 to eliminate a HF molecule. Three of these channels involve SN2 substitution reactions, while the remaining one proceeded via a combination reaction followed by an elimination reaction. The nucleophilic displacement (SN2), which has the lowest energy barrier, proceeds via a four-membered cyclic transition state (TS1, 22.48 kcal mol1), as shown in Fig. 2. The oxygen atom in the water molecule attacks the vacancy of the sulfur atom in SF4. During this attack, the O–H bond of water elongates and the activated H atom orients toward the axial fluorine atom of SF4. The transition state directly produces HF and SF3OH (Cs symmetry, marked as SF3OH-2). The other two substitution reaction paths via TS0 (34.21 kcal mol1) and TS3 (42.32 kcal mol1) are similar to channel 1. The difference in the reaction paths is the direction of attack by the water molecule on SF4. Both channels directly generate SF3OH. The SF3OH with C1 symmetry is the conformational isomer of SF3OH-2 and is referred to as SF3OH-1. The OH group is located in the equatorial site of SF4

in SF3OH-1 and is linked to the axial site of SF4 in SF3OH-2. SF3OH-1 and SF3OH-2 can be transformed into each other through a Berry pseudorotation reaction [31] (BPR, TS5 of 7.64 kcal mol1 barrier height) similar to the BPR of SF4 [32]. In the fourth channel, which has the highest energy barrier (TS2, 63.67 kcal mol1), both the hydrogen atom and OH group of the water molecule combine with the sulfur atom of SF4 to form the six-ligand SHF4OH. The SHF4OH intermediate will produce O = SF3OH, SF3OH-2 and SF3OH-1 via elimination of HF through TS6, TS7 and TS8, respectively. In the second stage, O = SF3OH, SF3OH-1 and SF3OH-2 eliminate HF and yield SOF2. SF3OH-2 must be transformed into SF3OH-1 via TS5 due to the long distance between the hydrogen atom and fluorine atoms in SF3OH-2. Throughout the PES, the channel with the lowest activation energies passes through TS1, which is also the rate-determining step in the hydrolysis of SF4, and these results are consistent with experimental results [12,13]. The barrier height of 22.48 kcal mol1 calculated using the G4//MP2/6-311G(d, p) model in the gas phase is slightly higher than that of 18.50 kcal mol1 obtained from the literature [14]. Six theoretical models were performed to calculate accurate energies for all of the stationary points. The maximum deviation and mean standard deviation for all of the relative energies on the PES were 2.37 kcal mol1 and 0.57 kcal mol1, respectively,

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Table 1 Experimental and calculated standard enthalpies of formation at 298.15 K.a Item

Experimental

57.80  0.01c H2O HF 65.32  0.17c 3 SO 1.14  0.06d 70.94  0.05d SO2 SO3 94.59  0.20d H2SO3 [127.00  3.00]e H2SO4 175.12  0.50d SF2 70.07  2.39f SF4 181.64  4.78f SF6 291.70  0.00d SOF2 138.60  0.00g SO2F2 181.30  2.01d DrH b 29.80  5.36 Mean deviation Mean absolute deviation Standard deviation Mean relative deviation (%) Mean absolute relative deviation (%) Standard deviation (%)

G3MP2

CCSD(T)/CBS limit

G4

MP2/6-311G(d, p)

aug-cc-pV(T + d)Z

MP2/6-311G(d, p)

aug-cc-pV(T + d)Z

MP2/6-311G(d, p)

aug-cc-pV(T + d)Z

57.28 65.35 1.71 68.48 89.97 120.29 165.94 67.12 177.90 284.24 136.22 174.95 31.74 4.02 4.03 5.17 3.13 3.14

57.60 65.54 1.36 69.05 90.53 121.32 166.92 67.43 178.69 285.45 137.10 176.21 31.89 3.32 3.36 4.39 2.57 2.63

58.20 65.93 2.40 67.31 90.42 122.90 169.77 70.40 184.82 294.45 139.40 179.46 28.25 1.14 2.25 2.90 1.11 1.94

58.55 66.10 2.03 68.13 91.28 124.39 171.14 70.97 186.01 295.98 140.69 181.04 28.34 0.17 2.13 2.63 0.32 1.84

57.49 65.40 1.70 68.71 91.82 122.38 170.28 69.94 182.93 290.75 139.35 180.15 29.72 1.40 1.53 2.34 1.23 1.34

57.86 65.60 1.26 69.69 92.80 123.59 171.70 70.43 183.99 292.20 140.58 181.83 29.92 0.44 1.22 1.74 0.44 1.06

3.73

3.18

2.64

2.21

2.00

1.43

a

Energies in kcal mol1. b Relative enthalpies of the reaction SF4 + H2O ! SOF2 + 2HF. c Ref. [24]. d Ref. [25]. e Ref. [26], the value in the gas phase was evaluated by Benson [26] based on the experimental result of 145.5 kcalmol1 in aqueous solution [27] and the difference of 18.5  3 kcal mol1 between gas phase and aqueous solution. f Ref. [28]. g Ref. [29].

indicating that the six theoretical models produce accurate energies for this system. Based on previous comparisons of structure optimization models, we adopted the G4//MP2/6311G(d, p) model to further investigate solvent effects, catalysis and secondary reactions in the hydrolysis of SF4. 2.3. Solvent effects All of the aqueous structures optimized using the MP2/6311G(d, p) level of theory with the PCM solvent model were shown in Fig. 2. The standard enthalpies of formation for all of the species were calculated using G4 theory and the PCM model, and the reactive PES of SF4 and H2O, both in gas phase and in aqueous solution, using the relative enthalpies were shown in Fig. 4.

The mean relative deviation for all of the structural parameters between in aqueous solution and in gas phase was 1.51%, indicating only a small structural difference for the species in the hydrolysis of SF4 between the two phases. Most of the discrepancies were primarily due to the transition state structures. For example, the mean relative deviation for the S–F, H–F, O–H and S–O bond lengths in the transition states was 2.25, 3.10, 1.92 and 1.82%, respectively. Therefore, the S–O bond lengths shortened, while the other bonds lengthened in the aqueous solution. The same trend was also observed for the molecular dipole moment where the molecular dipole moment of the transition states increased by an average of 1.44 Debye, while the others increased only 0.72 Debye on average. This trend in the dipole moment for the structures in aqueous solution was consistent with the literature [33].

Fig. 3. Reactive PES of SF4 and H2O.

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Fig. 4. PES showing the relative enthalpies obtained in the gas phase and in aqueous solution.

The relative enthalpies in aqueous solution were lower than those in the gas phase, as shown in Fig. 4. The average solvent stabilization energy of 11.90 kcal mol1 would be conducive to the hydrolysis of SF4. However, the solvent effects did not significantly change the reactive mechanism of SF4 hydrolysis, except for the switching the relative order of TS6 and TS7. Therefore, the ratedetermining step in the aqueous solution was the same as that determined in the gas phase. The reaction heats of the intermediates and products in aqueous solution were larger than those in the gas phase. For example, reaction (1) in aqueous solution released 9.60 kcal mol1 more heat than in gas phase, which could be attributed to the strong interaction between the water solvent and the generated HF molecule. This result indicated that SF4 hydrolysis is thermodynamically favored in aqueous solution. 2.4. Catalysis of H2O and/or HF Based on the PES of SF4 hydrolysis, the rate constant for the rate-determining step in the gas phase and in aqueous solution at 298.15 K was estimated to be 2.06  103 s1 using transition state

Table 2 Catalysis effects on activation energies (kcal mol1). Catalyzed species

Number of H2O molecules participating in catalysis

Number of HF molecules participating in catalysis

Number of atoms participating in cyclization

Activation energy

TS1 TS1 TS1 TS1 TS1 TS1 TS0 TS0 TS0 TS3 TS3 TS3 TS2 TS2 TS2

0 1 0 1 2 0 0 1 0 0 1 0 0 1 0

0 0 1 1 0 2 0 0 1 0 0 1 0 0 1

4 6 6 8 8 8 4 6 6 4 6 6 3 5 5

22.48 11.01 5.27 0.73 5.39 4.25 34.21 19.06 22.82 41.32 30.23 31.61 63.67 35.80 37.55

theory (TST) [34]. This value was less than the experimental result of 0.95  104 s1 in aqueous solution at 291.15 K [12] and 0.60  0.30  104 in the gas phase at 350 K [13]. Larin et al. [14] attributed this difference to the use of the 6-31G(d, p) basis sets. However, our application of the aug-cc-pV(T + d)Z basis sets did not significantly decrease the energy barriers. We decided to explore the self-catalysis of H2O and HF in this reaction due to the existence of the water dimer [35] and an increasing number of HF molecules. The catalysis of the key stationary points were investigated using the G4//MP2/6-311G(d, p) model, and the results were listed in Table 2. The barrier height of the SF4 hydrolysis decreases when catalyzed by HF and/or H2O. The activation energy of TS1 catalyzed by one water molecule and one HF molecule decreases by 11.47 and 17.21 kcal mol1, respectively. In fact, the hydrolysis of SF4 catalyzed by one water molecule is equivalent to the reaction between SF4 and the water dimer. When more molecules are involved in the catalysis, the energy barrier is lower and is less than zero when catalyzed by two HF molecules. The activation energies for the other three reaction paths can also be decreased by at least 10 kcal mol1 with catalysis by H2O or HF. The catalysis using H2O and HF in this system can be contributed to the formation of a larger ring in the transition states, as shown in the fourth column of Table 2. This larger ring reduces the steric effects, and the water and HF molecules may participate in proton transfer in the cyclic transition states. Therefore, self-catalysis will lead to a decrease in the total energies of the transition states and accelerate the hydrolysis of SF4. To generate SOF2, the primary reaction path proceeds through TS1, TS5 and TS4 without catalysis, and TS1 is the rate-determining step. For self-catalysis involved one molecule of H2O (or HF), the activation energy of TS1 decreases to 11.01 kcal mol1 (or 5.27 kcal mol1). Because the BPR of SF3OH-2 through TS5 cannot be catalyzed by H2O and HF, the barrier height of TS1 (11.01 kcal mol1) is equivalent to (even less than) the height of TS5 (11.27 kcal mol1). In addition, the other three channels still are required to overcome an energy barrier of at least 19.06 kcal mol1. Therefore, TS5 (or combined with TS1) will become the rate-determining step for the entire hydrolysis reaction. Based on the self-catalysis of H2O and HF, the rate constant of the rate-determining step in the gas phase at 298.15 K is 3.4  104 s1 using the 11.27 kcal mol1 barrier height of TS5, which is in good agreement with the experimental data [12,13].

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Fig. 5. Secondary reactions in aqueous solution.

2.5. Secondary reactions The complete hydrolysis of SF4 in aqueous solution was investigated using the PCM solvent model and G4//MP2/6311G(d, p) model, and the PES of the relative enthalpies was shown in Fig. 5. For the entire reactive PES, TS1 remains the ratedetermining step [13] for the formation of SO2 and H2SO3 in aqueous solution, as shown in Fig. 5. For the SF3OH-1, SF3OH-2, SF2(OH)2, SF(OH)3 and FSOOH intermediates, their hydrolysis barrier heights are higher than those of the dehydrofluorination reactions indicating that the fluorine containing sulfides prefer eliminate HF, rather than undergo hydrolysis. Therefore, SOF2 will be the primary intermediate from SF4 to SO2 (or H2SO3). However, Sauers et al. [13] proposed that the SOF2 intermediate might not appear during further hydrolysis in aqueous solution based on the reaction kinetics. The SOF2 hydrolysis activation energy (28.57 kcal mol1) in aqueous solution is higher than that of SF4 in this work, and this order is in agreement with experimental rate constants [13,36]. However, the relative energy for SOF2 hydrolysis in the complete PES (TS20, 10.82 kcal mol1) is 32.70 kcal mol1 lower than that of the rate-determining step (TS1, 21.88 kcal mol1). The reactive heat of reaction (1) can overcome the SOF2 hydrolysis energy barrier. Therefore, SOF2 may be a primary intermediate in the complete hydrolysis of SF4 from a thermodynamics standpoint. FSOOH is similar to SOF2.

reaction followed by an elimination reaction. The difference in the structures of the transition states obtained in aqueous solution and in the gas phase is slightly greater compared to the intermediates, reactants and products. The reactive mechanism for hydrolysis in aqueous solution does not substantially vary compared to that observed in the gas phase. However, the reaction is more thermodynamically favored in aqueous solution. The barrier height of the principal channel in the gas phase and in aqueous solution is 21.08 and 21.88 kcal mol1, respectively. This step is the rate-determining step in the complete hydrolysis of SF4 to generate SO2 and H2SO3. Self-catalysis of H2O and/or HF can remarkably decrease the energy barriers for the SF4hydrolysis such that the BPR of the SF3OH-2 intermediate will become the ratedetermining step with 11.27 kcal mol1 barrier height. Therefore, the rate constant for the hydrolysis of SF4 at 298.15 K is 3.4  104 s1 based on TST theory, which is in good agreement with the experimental result [13]. (3) In the secondary reactions, the intermediates prefer dehydrofluorination over further hydrolysis. In the complete PES for the hydrolysis reaction, the relative energy height of SOF2 hydrolysis is 32.70 kcal mol1 lower than that of SF4, notwithstanding that the higher absolute activation energy of the former. SOF2 and FSOOH are the important intermediates for the generation SO2 and H2SO3, both in the gas phase and in aqueous solution.

3. Conclusions Ab initio calculations using the G4//MP2/6-311G(d, p) model and the PCM model were performed to explore in detail the hydrolysis of SF4 in the gas phase and in aqueous solution. The important insights are summarized below. (1) MP2/6-311G(d, p) and MP2/aug-cc-pV(T + d)Z levels of theory provide precise structures of the sulfur-containing species, and the aug-cc-pV(T + d)Z basis sets are slightly better than the 6311G(d, p) basis sets for this system. CCSD(T)/CBS limits and G4 theories produce accurate energy calculations for fluorine containing sulfides. (2) Among the four primary channels for the initial H2O attack on SF4, three of these channels involve SN2 substitution reactions, while the other channel proceeds through a combination

4. Computational details All of the structures on the PES for the SF4 hydrolysis were optimized using the MP2/6-311G(d, p) and MP2/aug-cc-pV(T + d)Z [37] levels of theory. For the aug-cc-pV(T + d)Z basis sets, the augcc-pVTZ basis sets was used for all of the atoms, except for the sulfur atoms. The harmonic vibrational frequency and zero point energy were calculated using the same level of theory. All of the transition states were confirmed using the intrinsic reaction coordinates method. Single point calculations were performed with G3MP2 [38], CCSD(T)/CBS limits (n = D, T, Q) [39] and G4 [40] theory using the previously optimized structures. For CCSD(T)/CBS limits, the CCSD(T) method with the aug-cc-pVnZ (n = D,T,Q) basis sets were used for single point calculations, and the energetic

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results were further extrapolated to the CBS limits using a threeparameter formula [39], as shown below. EðnÞ ¼ Eð1Þ þ BexpðCnÞ

(2)

where n was the cardinal number of the basis sets (i.e., 2 for aug-ccpVDZ, etc.). The standard enthalpies of formation at 298.15 K were corrected as stated in the literature [30]. The solvent effects on SF4 hydrolysis had been taken into account using the polarizable continuum model (PCM) [41] with the united atom topological model (UAHF) [42]. All of the relative enthalpy calculations to investigate the solvent effects, catalysis and secondary reactions were performed using the G4 theory based on the optimized structures obtained at the MP2/6311G(d, p) level of theory. All of the calculations were performed with the Gaussian 09 program [43]. Acknowledgments We wish to thank the National Key Basic Research Program of China (2009CB23200) and the Science and Technology Project of Chenzhou City (2012cj034) for their supports. References [1] C. Szmytkowski, A. Domaracka, P. Mozejko, E. Ptasinska-Denga, S. Kwitnewski, J. Phys. B: At. Mol. Opt. Phys. 38 (2005) 745–755. [2] W.C. Smith, V.A. Engelhardt, J. Am. Chem. Soc. 82 (1960) 3838–3840. [3] W. Dmowski, K. Piasecka-Maciejewska, J. Fluorine Chem. 104 (2000) 273–276. [4] E. David, H. Woon, J. Thom, Dunning, J. Phys. Chem. A 113 (2009) 7915–7926. [5] K.D. Raffael, D.M. Smith, D.A. Newnham, Mol. Phys. 8 (2003) 1095–1104. [6] A.N. Taha, N.S. True, C.B. LeMaster, C.L. LeMaster, S.M. Neugebauer-Crawford, J. Phys. Chem. A 104 (2000) 3341–3348. [7] R.D. Harcourt, T.M. Klapotke, J. Fluorine Chem. 123 (2003) 5–20. [8] A.M. Casanovas, J. Casanovas, J. Phys. D: Appl. Phys. 38 (2005) 1556–1564. [9] K.R. Ryan, I.C. Plumb, Plasma Chem. Plasma Process. 8 (1988) 263–280. [10] D.H. Shi, J.F. Sun, Y.F. Liu, Z.L. Zhu, H. Ma, Eur. Phys. J. D 54 (2009) 43–50. [11] W.T. Tsai, J. Fluorine Chem. 128 (2007) 1345–1352. [12] K.D. Asmus, W. Grunbein, J.H. Fendler, J. Am. Chem. Soc. 92 (1970) 2625–2628. [13] I. Sauers, J.L. Adcock, L.G. Christophorou, H.W. Ellis, J. Chem. Phys. 83 (1985) 2618–2619. [14] A.V. Larin, N. Meurice, L. Leherte, M. Rajzman, D.P. Vercauteren, D.N. Trubnikv, Theoretical analysis of sulfur fluorines SFn (n = 3–6) in the gas phase, in: L.G. Christoporou, J.K. Olthoff (Eds.), Gaseous Dielectrics IX, Kluwer Academic/Plenum Publishers, New York, 2001, pp. 425–430. [15] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules, Van Nostrand Reinhold Co., USA, 1979, 468-481.

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