Study on Pyrolytic Mechanisms of n-Perfluorosilanes Si_nF_2n+2 (2 ≤ n < 6) and Perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6)

Abstract

In this paper, the pyrolytic mechanisms of n-perfluorosilanes Si_nF_2n+2 (2 ≤ n < 6) and perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6) are studied in terms of kinetics and thermodynamics by theoretical calculation, and the pyrolytic reaction paths of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6) are obtained, which can be used to guide the experimental preparation research studies and separation operations of Si_nF_2n+2 (2 ≤ n < 6), Si_nF_2n (3 ≤ n ≤ 6), and their intermediate substances. The results of the kinetic analysis show that the pyrolytic mechanisms of Si_nF_2n+2 (2 ≤ n < 6) are as follows: first, the silicon–silicon bond breaking induces the generation of free radicals; then, in the chain transfer, the related free radicals participate in F-abstraction transfer with the molecules; and finally, the free radicals form the molecules, and the chain terminates. The F-abstraction transfer is the easiest process to initiate in the low-order silicon–fluorine substance during the chain transfer while releasing SiF₂ at the same time, whereas the generation of double free radicals is the most difficult process. The pyrolytic mechanisms of Si_nF_2n (3 ≤ n ≤ 6) are as follows: first, the α–Si–Si bond breaking induces the generation of double free radicals; then, the α–Si–Si or β–Si–Si bond breaks continually in the chain transfer; and finally, the double free radicals form the molecules, and the chain terminates. SiF₂ is most easily formed by breaking during the chain transfer. In the pyrolytic processes of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6), the chain initiation of silicon–silicon bond breaking requires the highest bond breaking energy, which is the control step of the pyrolytic reaction. The results of the thermodynamic analysis show that the pyrolytic reactions of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6) are endothermic. When Si_nF_2n+2 (2 ≤ n < 6) undergoes a pyrolytic reaction and the temperature is higher, the main pyrolytic products are SiF₄ and SiF₂. When 600 K < T < 1200 K, the main pyrolytic products of Si₄F₁₀ are Si₃F₈ and SiF₂, and when 900 K < T < 1400 K, Si₅F₁₂ can also convert to Si₃F₈ and SiF₂. The main pyrolytic products of Si_nF_2n (3 ≤ n ≤ 6) are SiF₂. When the temperature is higher, the pyrolytic order of Si_nF_2n (3 ≤ n ≤ 6) is as follows: Si₃F₆ (ring) < Si₄F₈ (ring) < Si₅F₁₀ (ring) < Si₆F₁₂ (ring). However, if the temperature is in the range of 1000 K < T < 1200 K, the pyrolytic order is the opposite.

1. Introduction

In 1958, Pease et al.¹ first obtained the (SiF₂)_x polymer by condensing SiF₂ gas at low temperature. In 1965, Bassler et al.² obtained the infrared spectra of the (SiF₂)_x polymer at ultra-low temperature (20–42 K) by using Ar as the carrier gas, and the spectra also showed the variation with the change in temperature, indicating that the structure of the substance changed. In the same year, Timms et al.³ heated the (SiF₂)_x polymer, collected the gas for mass spectrometry detection, and obtained a series of perfluorosilanes ranging from SiF₄ to Si₁₄F₃₀. Under the controlled temperature, two new substances, Si₃F₈ and Si₄F₁₀, were isolated from the distillate. In 1968, Hastie et al.⁴ observed an obvious characteristic peak of the (SiF₂)_x polymer in the infrared absorption spectra of SiF₂ isotopes. The above direct detection characterizations could not confirm the stable existence of the (SiF₂)_x polymer, so researchers needed to prove its existence by indirect methods. First, a SiF_n(n = 1–4)-mixed air flow was prepared under high temperature, and then, unsaturated hydrocarbons (propylene, butadiene, benzene, ethylene fluoride, etc.⁵⁻¹¹), BF₃,¹² H₂O,^3,13 H₂S,¹⁴ GeH₄,¹⁵ and PF₃¹⁶ were introduced. Finally, the mixed system was rapidly frozen at low temperature, and the system was heated for the detection and characterization to confirm the presence of the polymer. Besides, it was found that there were more two-chain and three-chain substances in the mixed system, which indicated that polymer (SiF₂)_x undergoes a pyrolytic reaction that causes chain breaking during the heating. In 1999, Lyman et al.¹⁷ pyrolyzed Si₂F₆ at 600–700 °C to obtain SiF₂ and SiF₄ gases. In the same year, Hrustak et al.¹⁸ theoretically predicted the thermodynamic parameters of SiF⁺ and SiF₂²⁺ generated by Si and F₂. In conclusion, the (SiF₂)_x polymer exactly emerged at low-temperature polymerization conditions, and with the increase in temperature, some unknown substances also emerged.

Due to the particularity of the Si–F bond, from the 1960s to the present, there have been various research reports on silicon–fluorine series, especially the various applications of SiF₂ and SiF₄ polymers.³⁻¹⁸ Recently, the teams of Sen et al.¹⁹ and Sinhababu et al.²⁰ also studied the separation of the stable SiF₂ monomer using a cyclic alkyl amino carbon (cAAC) ligand, but there exists no single, direct, and specific analysis of the intermediate change process. In our companion paper,²¹ we have first studied that there may be Si_xF_y (x ≤ 6, y ≤ 12) series at different temperatures, mainly including Si_nF_2n+2 (2 ≤ n < 6), Si₂F₆, Si₃F₈, Si₄F₁₀, and Si₅F₁₂, and Si_nF_2n (3 ≤ n ≤ 6), Si₃F₆, Si₄F₈, Si₅F₁₀, and Si₆F₁₂. We have analyzed and compared their stabilities, but their change paths have not been studied. Considering that it is too complicated to explore the process using experimental methods, computers have been used to solve this problem,²² and thus, this paper builds models of these substances, uses quantum computing methods to optimize the related structures, simulate the pyrolytic process, and determine and draw the relevant reaction path diagram, and discusses their pyrolytic mechanisms in terms of thermodynamics and kinetics to analyze their reaction paths. Analyzing the substances that may appear in the pyrolytic reaction process plays an important role in guiding the experimental preparations and separations of Si_nF_2n+2 (2 ≤ n < 6), Si_nF_2n (3 ≤ n ≤ 6), and their intermediate substances.

2. Results and Discussion

2.1. Pyrolytic Mechanisms of n-Perfluorosilanes Si_nF_2n+2 (2 ≤ n < 6)

2.1.1. All Possible Kinetic Paths in the Pyrolytic Reaction of n-Perfluorodisilane (Si₂F₆)

Figure 1 shows all possible kinetic paths in the pyrolytic reaction of Si₂F₆, and Table 1 shows the specific bond breaking energies and path numbers obtained by theoretical calculation. During the Si₂F₆ pyrolysis, the Si–Si bond first breaks to generate two SiF₃^• free radicals with the E of 363.33 KJ/mol, as shown in path 1. In the chain transfer, the SiF₃^• free radical in the chain initiation undergoes F-abstraction transfer with Si₂F₆ to generate a Si₂F₅^• free radical and SiF₄ with the E of 143.80 KJ/mol, as shown in path 2. Then, the Si₂F₅^• free radical undergoes β-decomposition and decomposes into a SiF₃^• free radical and SiF₂, and the E is 153.54 KJ/mol as shown in path 3. The chain termination may occur in the reaction, where two SiF₃^• free radicals generate Si₂F₆ as shown in path 4. By comparing the bond breaking energy of each path, it is found that path 1 has the highest bond breaking energy, so path 1 is the control step of the pyrolytic reaction of Si₂F₆. In other words, the pyrolysis of Si₂F₆ is initiated by the silicon–silicon bond breaking and the chain initiation of the silicon–silicon bond breaking requires the highest bond breaking energy, which is the control step of the pyrolytic reaction. From the kinetic paths of Si₂F₆, the pyrolysis reaction path of Si₂F₆ can be inferred, as shown in Figure 2, and the thermodynamic functions in the pyrolysis reaction path at different temperatures can be calculated, as shown in Figure 3.

Figure 1.

All possible kinetic paths in the pyrolytic reaction of Si₂F₆.

Table 1. Bond Breaking Energy (E) of Each Path in the Pyrolytic Reaction of Si₂F₆.

item	path	reaction	E (KJ/mol)
initiation	1	Si2F6 → SiF3• + SiF3•	363.33
F-abstraction transfer	2	SiF3• +Si2F6 → Si2F5• + SiF4	143.80
β-decomposition	3	Si2F5• → SiF3• + SiF2	153.54
termination	4	SiF3• + SiF3• → Si2F6	0

Figure 2.

Pyrolysis reaction path of Si₂F₆.

Figure 3.

Thermodynamic functions (a)Δ_rG_m^Θ and (b) Δ_rH_m in the pyrolysis reaction path of Si₂F₆ at different temperatures.

2.1.2. Pyrolysis Reaction Path of n-perfluorodisilane (Si₂F₆)

According to Figure 2, the main pyrolytic products of Si₂F₆ are SiF₂ and SiF₄. This is consistent with the experimental report of Lyman et al.¹⁷ in 1999. As can be seen from Figure 3a, when T < 1400 K and Δ_rG_m^θ > 0, the pyrolytic reaction does not occur, and when T > 1400 K and Δ_rG_m < 0, the pyrolytic reaction can occur; that is, the higher the temperature is, the smaller the Δ_rG_m^θ is, and the more easily the pyrolytic reaction goes on. As can be seen from Figure 3b, when the pyrolytic reaction occurs, Δ_rH_m > 0, so the pyrolytic reaction is endothermic, and the higher the temperature is, the smaller the Δ_rH_m is, and the less heat it absorbs.

2.1.3. All Possible Kinetic Paths in the Pyrolytic Reaction of n-Perfluorotrisilane (Si₃F₈)

Figure 4 shows all possible kinetic paths in the pyrolytic reaction of Si₃F₈, and Table 2 shows the specific bond breaking energies and path numbers obtained by theoretical calculation. During the pyrolysis of Si₃F₈, the Si–Si bond first breaks to generate a SiF₃^• free radical and a Si₂F₅^• free radical with an E of 333.92 KJ/mol, as shown in path 1. In the chain transfer, there are two paths, as shown in path 2 and path 3. In path 2, a SiF₃^• free radical in the chain initiation undergoes F-abstraction transfer with Si₃F₈ to generate a Si₃F₇^• free radical and SiF₄ with an E of 92.46 KJ/mol. However, in path 3, Si₂F₅^• initiates F-abstraction transfer with Si₃F₈ to generate Si₃F₇^• and Si₂F₆ with an E of 59.96 KJ/mol. By comparing the bond breaking energies of path 2 and path 3, the E of path 2 is higher than that of path 3, and thus, path 3 is more likely to occur; that is, the transfer is easier to initiate in a low-order silicon–fluorine substance (Si₂F₆). Then, Si₃F₇^• undergoes β-decomposition and decomposes into SiF₂ and a Si₂F₅^• free radical or a ^•Si₂F₄^• double free radical and SiF₃^•, as shown in path 4 and path 5, respectively. The bond breaking energies E are 134.06 and 353.03 KJ/mol, respectively. Additionally, the Si₂F₅^• free radical undergoes β-decomposition and decomposes into SiF₂ and a SiF₃^• free radical in path 6 with the E of 153.54 KJ/mol, while the ^•Si₂F₄^• double free radical decomposes into two SiF₂ units in path 7 with the E of 59.53 KJ/mol. This is in accordance with the fact that the lower the bond breaking energy is, the more likely it is to occur. In β-decomposition, the occurrence of path 5 is the most difficult and that of path 7 is the easiest; that is, the formation of the double free radical is the most difficult. Chain termination may occur in the reaction, and there are two possible paths: two SiF₃^• units in path 8 generate Si₂F₆ and Si₂F₅^• and SiF₃^• in path 9 generate Si₃F₈.

Figure 4.

All possible kinetic paths in the pyrolytic reaction of Si₃F₈.

Table 2. Bond Breaking Energy (E) of Each Path in the Pyrolytic Reaction of Si₃F₈.

In summary, the pyrolytic path of Si₃F₈ is as follows: first, SiF₃^• and Si₂F₅^• are generated in the chain initiation; then, in the chain transfer, the F-abstraction transfers occur according to path 2 and path 3 to generate SiF₄, Si₂F₆, and Si₃F₇^•; and then, β-decomposition occurs. Although path 7 is the easiest way, it is difficult to generate the ^•Si₂F₄^• double free radical required by path 7 in the chain transfer; that is, there is no basis for path 7. However, the bond breaking energies of path 4 and path 6 are second, and the difference between them is small, so the two paths exist at the same time, though in comparison, path 4 is more likely to occur. Pyrolytic product SiF₂ is mainly formed by the units breaking one by one from the main long chain. Thus, β-decomposition occurs according to path 4 and path 6 to generate Si₂F₅^•, SiF₃^•, and SiF₂. Finally, the chain terminates according to path 8 and path 9. By comparing the bond breaking energy of each path, path 1 has the highest bond breaking energy, so path 1 is the control step of the pyrolytic reaction of Si₃F₈. That is, the pyrolysis of Si₃F₈ is initiated by the silicon–silicon bond breaking, and the chain initiation of the silicon–silicon bond breaking requires the highest bond breaking energy, which is the control step of the pyrolytic reaction. From the kinetic paths of Si₃F₈, the pyrolysis reaction paths of Si₃F₈ can be inferred, as shown in Figure 5, and the thermodynamic functions of pyrolysis reaction paths at different temperatures can be calculated, as shown in Figure 6.

Figure 5.

Pyrolysis reaction paths of Si₃F₈.

Figure 6.

Thermodynamic functions (a) Δ_rG_m^Θ and (b) Δ_rH_m in the pyrolysis reaction paths of Si₃F₈ at different temperatures.

2.1.4. Pyrolysis Reaction Paths of n-Perfluorotrisilane (Si₃F₈)

Figure 5 shows that there are two pyrolytic reaction paths of Si₃F₈: path a, whose main pyrolytic products are SiF₂ and SiF₄, and path b, whose main pyrolytic products are Si₂F₆ and SiF₂. According to Figure 6a, when T < 1500 K and Δ_rG_m^θ > 0, path (1) and path (2) do not occur; when T > 1500 K and Δ_rG_m < 0, path (1) and path (2) can both occur; and path (1) is easier to occur than path (2). Thus, when Si₃F₈ undergoes a pyrolytic reaction and T > 1500 K, the main pyrolytic products are SiF₄ and SiF₂. According to Figure 6b, when the pyrolytic reactions proceed, Δ_rH_m > 0 and the Δ_rH_m of path (1) is larger than that of path (2), so path (1) and path (2) are endothermic, and the higher the temperature is, the smaller the Δ_rH_m is, and the less heat it absorbs.

2.1.5. All Possible Kinetic Paths in the Pyrolytic Reaction of n-Perfluorobutane (Si₄F₁₀)

Figure 7 shows all possible kinetic paths in the pyrolytic reaction of Si₄F₁₀, and Table 3 shows the specific bond breaking energies and path numbers obtained by theoretical calculation. For the pyrolysis of Si₄F₁₀, there are two paths in the chain initiation, as shown in path 1 and path 2. In path 1, the α–Si bond first breaks to generate SiF₃^• and Si₃F₇^• with the E of 327.36 KJ/mol. In path 2, the β–Si bond first breaks to generate two Si₂F₅^• units, and the bond breaking energy E is 307.88 KJ/mol. Since the energy difference between the two paths is similar, the two paths exist at the same time, and it is easier to generate Si₂F₅^•. In the chain transfer, there are three paths including path 3, path 4, and path 5. In path 3, SiF₃^• in the chain initiation undergoes F-abstraction transfer with Si₄F₁₀ to generate Si₄F₉^• and SiF₄ with the E of 89.78 KJ/mol. In path 4, Si₂F₅^• in the chain initiation undergoes F-abstraction transfer with Si₄F₁₀ to generate Si₄F₉^• and Si₂F₆ with the E of 81.33 KJ/mol. In path 5, Si₃F₇^• in the chain initiation undergoes F-abstraction transfer with Si₄F₁₀ to generate Si₄F₉^• and Si₃F₈, and the bond breaking energy E is 75.03 KJ/mol. Comparing the bond breaking energies of path 3, path 4, and path 5, path 5 is more likely to occur; that is, the transfer is easier to initiate in the low-order silicon–fluorine substance (Si₃F₈). Then, the ^•Si₂F₄^• double free radical, Si₄F₉^•, Si₃F₇^•, and Si₂F₅^• undergo β-decomposition, as shown from path 6 to path 13. The bond breaking energies E are 109.92, 203.22, 340.93, 353.03, 134.06, 153.54, 15.85, and 59.53 KJ/mol, respectively. The occurrence of path 9 is the most difficult and that of path 12 is the easiest; that is, the generation of the double free radical is the most difficult. The chain termination may occur in the reaction, and there are four possible paths: two SiF₃^• units in path 14 generate Si₂F₆; Si₂F₅^• and SiF₃^• in path 15 generate Si₃F₈; and Si₃F₇^• and SiF₃^• in path 16 and the two Si₂F₅^• units in path 17 both generate Si₄F₁₀.

Figure 7.

All possible kinetics paths in the pyrolytic reaction of Si₄F₁₀.

Table 3. Bond Breaking Energy (E) of Each Path in the Pyrolytic Reaction of Si₄F₁₀.

In summary, the pyrolytic path of Si₄F₁₀ is as follows: first, two Si₂F₅^• radicals, SiF₃^•, and Si₃F₇^• are generated in the chain initiation, according to path 1 and path 2. Then, in the chain transfer, SiF₄, Si₂F₆, Si₃F₈, and Si₄F₉^• are generated by F-abstraction transfer, according to path 3, path 4, and path 5, and then, β-decomposition occurs. Although path 12 and path 13 are the most likely to occur, it is difficult to generate the ^•Si₃F₆^• double free radical and ^•Si₂F₄^• double free radical in the F-abstraction transfer, so path 12 and path 13 cannot occur. However, the bond breaking energies of path 6, path 10, and path 11 are second, and the differences among them are small, so the three paths exist at the same time, though in comparison, path 6 is more likely to occur. Pyrolytic product SiF₂ is mainly formed by the units breaking one by one from the main long chain. Thus, β-decomposition occurs according to path 6, path 10, and path 11 to generate Si₂F₅^•, Si₃F₇^•, and SiF₂, respectively. Finally, the chain terminates according to path 14, path 15, path 16, and path 17. By comparing the bond breaking energies of each path, path 1 has the highest bond breaking energy, so path 1 is the control step of the pyrolytic reaction of Si₄F₁₀. That is, the pyrolysis of Si₄F₁₀ is initiated by the silicon–silicon bond breaking, and the chain initiation of the silicon–silicon bond breaking requires the highest bond breaking energy, which is the control step of the pyrolytic reaction. From the kinetic paths of Si₄F₁₀, the pyrolytic reaction paths of Si₄F₁₀ can be deduced, as shown in Figure 8, and the thermodynamic functions of pyrolytic reaction paths at different temperatures can be calculated, as shown in Figure 9.

Figure 8.

Pyrolytic reaction paths of Si₄F₁₀.

Figure 9.

Thermodynamic functions (a) Δ_rG_m^Θ and (b) Δ_rH_m in the pyrolytic reaction paths of Si₄F₁₀ at different temperatures.

2.1.6. Pyrolytic Reaction Paths of n-Perfluorobutane (Si₄F₁₀)

It can be seen from Figure 8 that there are three pyrolytic reaction paths of Si₄F₁₀, namely, path (1), whose main pyrolytic products are SiF₄ and SiF₂, path (2), whose main pyrolytic products are Si₂F₆ and SiF₂, and path (3), whose main pyrolytic products are Si₃F₈ and SiF₂. It can be seen from Figure 9a that when T < 1000 K and Δ_rG_m^θ > 0 for path (1) and path (2), the two pyrolytic reactions do not occur, and path (3) does not occur when T < 700 K. Then, when T > 700 K and Δ_rG_m < 0 for path (3), the pyrolytic reaction can occur, and path (1) and path (2) can occur when T > 1000 K. Additionally, when T = 1200 K, the Δ_rG_m^θ of path (1) and path (3) are the same. When 700 K < T < 1200 K, path (3) is the easiest to occur, and when T > 1200 K, path (1) is the easiest to occur. Thus, after Si₄F₁₀ undergoes a pyrolytic reaction, when 600 K < T < 1200 K, the main pyrolytic products are Si₃F₈ and SiF₂, and when T > 1200 K, the main pyrolytic products are SiF₄ and SiF₂. It can be seen from Figure 9b that when the pyrolytic reactions occur, Δ_rH_m > 0 for path (1), path (2), and path (3) and the Δ_rH_m of path (1) is the largest, so path (1), path (2), and path (3) are endothermic. Additionally, the higher the temperature is, the smaller the Δ_rH_m is, and the less heat it absorbs.

2.1.7. All Possible Kinetic Paths in the Pyrolytic Reaction of n-Perfluoropentasilane (Si₅F₁₂)

Figure 10 shows all possible kinetic paths in the pyrolytic reaction of Si₅F₁₂, and Table 4 shows the specific bond breaking energies and path numbers obtained by theoretical calculation. For the Si₅F₁₂ pyrolysis, there are two paths in the chain initiation, including path 1 and path 2. In path 1, the α–Si bond first breaks to generate SiF₃^• and Si₄F₉^• with the E of 325.35 KJ/mol. In path 2, the β–Si bond first breaks to generate Si₂F₅^• and Si₃F₇^• with the E of 300.52 KJ/mol. Since the energy difference between the two paths is similar, the two paths exist at the same time, and it is easier to generate the Si₂F₅^• and Si₃F₇^• pair. In the chain transfer, there are four paths from path 3 to path 6, as shown. In path 3, SiF₃^• in the chain initiation undergoes F-abstraction transfer with Si₅F₁₂ to generate Si₅F₁₁^• and SiF₄. The bond breaking energy E is 87.31 KJ/mol. In path 4, Si₂F₅^• in the chain initiation undergoes F-abstraction transfer with Si₅F₁₂ to generate Si₅F₁₁^• and Si₂F₆, with the E of 80.06 KJ/mol. In path 5, Si₃F₇^• in the chain initiation undergoes F-abstraction transfer with Si₅F₁₂ to generate Si₅F₁₁^• and Si₃F₈, and the bond breaking energy E is 75.27 KJ/mol. In path 6, Si₄F₉^• in the chain initiation undergoes F-abstraction transfer with Si₅F₁₂ to generate Si₅F₁₁^• and Si₄F₁₀, and the bond breaking energy E is 72.73 KJ/mol. Comparing the bond breaking energies among all paths from path 3 to path 6, path 6 is more likely to occur; that is, the transfer is easier to initiate in the low-order silicon–fluorine substance (Si₄F₁₀). Following this, β-decomposition occurs, and the bond breaking energies E of path 9, path 10, path 13, and path 14 are all larger than 300 KJ/mol, that of path 12 is larger than 200 KJ/mol, and those of path 7, path 8, path 11, path 15, and path 16 are all larger than 100 KJ/mol. The bond breaking energies E of path 17 to path 19 are all lower than 100 KJ/mol. Among them, the bond breaking energy of path 18 is the lowest and that of path 14 is the highest; that is, the occurrence of path 14 is the most difficult and that of path 18 is the easiest, and the formation of the double free radical is the most difficult. The chain termination may occur in the reaction, and there are four possible paths: two SiF₃^• radicals in path 20 generate Si₂F₆, Si₂F₅^• and SiF₃^• in path 21 generate Si₃F₈, and in path 22 and path 23, the Si₃F₇^• and SiF₃^• radicals and the two Si₂F₅^• radicals, respectively, both generate Si₄F₁₀.

Figure 10.

All possible kinetic paths in the pyrolytic reaction of Si₅F₁₂.

Table 4. Bond Breaking Energy (E) of Each Path in the Pyrolytic Reaction of Si₅F₁₂.

In summary, the pyrolytic path of Si₅F₁₂ is as follows: first, Si₂F₅^•, SiF₃^•, Si₄F₉^•, and Si₃F₇^• are generated according to the chain initiations of path 1 and path 2. In the chain transfer, SiF₄, Si₂F₆, Si₃F₈, Si₄F₁₀, and Si₅F₁₁^• are generated according to path 2, path 3, path 4, and path 5 by F-abstraction transfer, and then, β-decomposition occurs. Although path 17, path 18, and path 19 are more likely to occur, it is difficult to generate ^•Si₄F₈^•, ^•Si₃F₆^•, and ^•Si₂F₄^• double free radicals, so path 17, path 18, and path 19 cannot occur; however, the bond breaking energies of path 7, path 11, and path 15 are second and the differences are small, so the three paths exist at the same time, though in comparison, path 7 is more likely to occur. Pyrolytic product SiF₂ is mainly formed by the units breaking one by one from the main long chain. Thus, β-decomposition occurs according to path 7, path 11, and path 15 to generate Si₄F₉^•, Si₃F₇^•, Si₂F₅^•, and SiF₂. Finally, the chain terminates according to path 20, path 21, path 22, and path 23. By comparing the bond breaking energy of each path, path 1 has the highest bond breaking energy, so path 1 is the control step of the pyrolytic reaction of Si₄F₁₀. That is, the Si₅F₁₂ pyrolysis is initiated by the silicon–silicon bond breaking, and the chain initiation of the silicon–silicon bond breaking requires the highest bond breaking energy, which is the control step of the pyrolytic reaction. From the kinetic paths of Si₅F₁₂, the pyrolytic reaction paths of Si₅F₁₂ can be deduced, as shown in Figure 11, and the thermodynamic functions of pyrolytic reaction paths at different temperatures can be calculated, as shown in Figure 12.

Figure 11.

Pyrolytic reaction paths of Si₅F₁₂.

Figure 12.

Thermodynamic functions (a) Δ_rG_m^θ and (b) Δ_rH_m in the pyrolytic reaction paths of Si₅F₁₂ at different temperatures.

2.1.8. Pyrolytic Reaction Paths of n-Perfluoropentasilane (Si₅F₁₂)

It can be seen from Figure 11 that there are four pyrolytic reaction paths of Si₅F₁₂, namely, path (1), whose main pyrolytic products are SiF₂ and SiF₄, path (2), whose main pyrolytic products are Si₂F₆ and SiF₂, path (3), whose main pyrolytic products are Si₃F₈ and SiF₂, and path (4), whose main pyrolytic products are Si₄F₁₀ and SiF₂. It can be seen from Figure 12a that when T < 1200 K and Δ_rG_m^θ > 0 for path (1) and path (4), the two pyrolytic reactions do not occur; path (2) does not occur at T < 1100 K; and path (3) does not occur at T < 900 K. Additionally, when T > 900 K and Δ_rG_m < 0 for path (3), the pyrolytic reaction can occur; pyrolytic reaction path (2) occurs when T > 1100 K; path (1) and path (4) occur when T > 1200 K; and when T = 1400 K, the Δ_rG_m^θ of path (1), path (2), and path (3) are the same. When 900 K < T < 1400 K, pyrolytic reaction path (3) is the easiest to occur, and when T > 1400 K, path (1) is the easiest to occur. Thus, while Si₅F₁₂ undergoes pyrolytic reactions, when 900 K < T < 1400 K, the main pyrolytic products are Si₃F₈ and SiF₂, and when T > 1400 K, the main pyrolytic products are SiF₄ and SiF₂. It can be seen from Figure 12b that when the pyrolytic reactions occur, Δ_rH_m > 0 for path (1), path (2), path (3), and path (4) and the Δ_rH_m of path (1) is the largest, so path (1), path (2), path (3), and path (4) are endothermic, and the higher the temperature is, the smaller the Δ_rH_m is, and the less heat it absorbs.

2.2. Pyrolytic Mechanisms of Perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6)

Due to the ring structures of perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6), different from that of n-perfluorosilanes Si_nF_2n+2 (2 ≤ n < 6), the chain initiations will generate double free radicals, such as ^•Si₃F₆^•, ^•Si₄F₈^•, ^•Si₅F₁₀^•, and ^•Si₆F₁₂^• double free radicals, and then, chain transfer and chain termination will occur. For comparison with Si_nF_2n+2 (2 ≤ n < 6), this paper first speculates all possible kinetic paths of perfluorocyclopropane (Si₃F₆), perfluorocyclobutane (Si₄F₈), perfluorocyclopentane (Si₅F₁₀), and perfluorocyclohexane (Si₆F₁₂). Then, for compounds ranging from Si₆F₁₂ to Si₃F₆, the kinetic paths are calculated and analyzed, and the pyrolytic reaction paths are inferred. Finally, the thermodynamic functions of pyrolytic reaction paths at different temperatures are calculated.

2.2.1. All Possible Kinetic Paths in the Pyrolytic Reaction of Perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6)

Figure 13 shows all possible kinetic paths in the pyrolytic reaction of Si₃F₆, and the specific bond breaking energies and path numbers obtained by theoretical calculation of these paths are shown in Table 5. In the chain initiation, the α–Si–Si bond of the ring first breaks to generate a ^•Si₃F₆^• double free radical with the bond breaking energy of 129.84 KJ/mol, as shown in path 11. In the chain transfer, the ^•Si₃F₆^• double free radical continues to break the α–Si–Si bond to generate SiF₂ and a ^•Si₂F₄^• double free radical. The bond breaking energy E is 15.85 KJ/mol, as shown in path 12. After the chain transfer, the ^•Si₂F₄^• double free radical decomposes continually to generate two SiF₂ units, and the chain terminates. The bond breaking energy E is 59.53 KJ/mol, as shown in path 13. By comparing the bond breaking energy of each path, it is found that path 11 has the highest bond breaking energy, so path 11 is the control step of the pyrolytic reaction of Si₃F₆.

Figure 13.

All possible kinetic paths in the pyrolytic reaction of Si₃F₆.

Table 5. Bond Breaking Energy (E) of Each Path in the Pyrolytic Reaction of Si_nF_2n (3 ≤ n ≤ 6).

Figure 14 shows the possible kinetic paths in the pyrolysis reaction of Si₄F₈. According to the figure, there are six reaction paths in the pyrolytic reaction of Si₄F₈, and the specific bond breaking energies and path numbers obtained by the theoretical calculation of these paths are shown in Table 5. In the chain initiation, the α–Si–Si bond of the ring first breaks to generate a ^•Si₄F₈^• double free radical with the bond breaking energy of 195.95 KJ/mol, as shown in path 8. In the chain transfer, the ^•Si₄F₈^• double free radical continues to break the α–Si–Si or β–Si–Si bond to generate SiF₂ and a ^•Si₃F₆^• double free radical or two ^•Si₂F₄^• double free radicals with the bond breaking energy of 135.80 or 92.13 KJ/mol, respectively, as shown in path 9 and path 10. The bond breaking energy of path 9 is higher than that of path 10, which is different from that of other Si_nF_2n (3 ≤ n ≤ 6). The main reason is still unknown, and then, ^•Si₃F₆^• and ^•Si₂F₄^• double free radicals decompose continually to generate SiF₂, or the ^•Si₃F₆^• double free radical generates Si₃F₆, and the chain terminates. By comparing the bond breaking energy of each path, it is found that path 8 has the highest bond breaking energy, so path 8 is the control step of the pyrolytic reaction of Si₄F₈.

Figure 14.

All possible kinetic paths in the pyrolytic reaction of Si₄F₈.

Figure 15 shows the possible kinetic paths in the pyrolysis reaction of Si₅F₁₀. According to the figure, there are nine reaction paths in the pyrolytic reaction of Si₅F₁₀. The specific bond breaking energies and path numbers obtained by the theoretical calculation of these paths are shown in Table 5. In the chain initiation, the α–Si–Si bond of the ring first breaks to generate a ^•Si₅F₁₀^• double free radical with the bond breaking energy of 241.23 KJ/mol, as shown in path 5. In chain transfer, the ^•Si₅F₁₀^• double free radical continues to break the α–Si–Si or β–Si–Si bond to generate SiF₂ and a ^•Si₄F₈^• double free radical or a ^•Si₂F₄^• double free radical and a ^•Si₃F₆^• double free radical with the bond breaking energies of 131.87 and 208.15 KJ/mol, respectively, as shown in path 6 and path 7. The bond breaking energy of path 6 is lower than that of path 7, so path 6 is more likely to occur, that is, in the chain transfer, which is more likely to generate SiF₂, and then, ^•Si₄F₈^•, ^•Si₂F₄^•, and ^•Si₃F₆^• double free radicals decompose continually to generate SiF₂, or ^•Si₄F₈^• and ^•Si₃F₆^• double radicals generate Si₄F₈ and Si₃F₆. Finally, the chain termination may occur. By comparing the bond breaking energy of each path, it is found that path 5 has the highest bond breaking energy, so path 5 is the control step of the pyrolytic reaction of Si₅F₁₀.

Figure 15.

All possible kinetic paths in the pyrolytic reaction of Si₅F₁₀.

Figure 16 shows all possible kinetic paths in the pyrolytic reaction of Si₆F₁₂, and Table 5 shows the specific bond breaking energies and path numbers obtained by the theoretical calculation based on these paths. In the chain initiation, the α–Si–Si bond of the ring breaks first to generate a ^•Si₆F₁₂^• double free radical with the bond breaking energy of 258.10 KJ/mol, as shown in path 1. In the chain transfer, the ^•Si₅F₁₀^• double free radical continues to break the α–Si–Si or β–Si–Si bond to generate SiF₂ and ^•Si₅F₁₀^•, ^•Si₂F₄^• and ^•Si₄F₈^•, and ^•Si₃F₆^• double free radicals, respectively. The bond breaking energies E are 121.41, 193.75, and 321.58 KJ/mol, as shown in path 2, path 3, and path 4, respectively. Among them, the bond breaking energy of path 2 is the lowest and that of path 4 is the highest, that is, in the chain transfer, which makes it easier to generate SiF₂. Then, ^•Si₄F₈^•, ^•Si₂F₄^•, ^•Si₃F₆^•, and ^•Si₅F₁₀^• double free radicals continually decompose to generate SiF₂, or ^•Si₄F₈^•, ^•Si₃F₆^•, and ^•Si₅F₁₀^• double free radicals generate Si₃F₆, Si₄F₈, and Si₅F₁₀. Finally, the chain termination may occur. By comparing the bond breaking energy of each path, it is found that path 1 has the highest bond breaking energy, so path 1 is the control step of the pyrolytic reaction of Si₆F₁₂.

Figure 16.

All possible kinetic paths in the pyrolytic reaction of Si₆F₁₂.

In summary, the pyrolysis of Si_nF_2n (3 ≤ n ≤ 6) is initiated by the silicon–silicon bond breaking, and the chain initiation of the silicon–silicon bond breaking requires the highest bond breaking energy, which is the control step of the pyrolytic reaction. From the kinetic paths of Si₃F₆, Si₄F₈, Si₅F₁₀, and Si₆F₁₂, the possible pyrolytic reaction path of Si_nF_2n (3 ≤ n ≤ 6) can be inferred and is shown in Figure 17, and the thermodynamic functions of pyrolytic reaction paths at different temperatures are calculated, as shown in Figure 18.

Figure 17.

Pyrolytic reaction paths of Si_nF_2n (3 ≤ n ≤ 6).

Figure 18.

Thermodynamic functions (a) Δ_rG_m^Θ and (b) Δ_rH_m in the pyrolytic reaction paths of Si_nF_2n (3 ≤ n ≤ 6) at different temperatures.

2.2.2. Pyrolytic Reaction Paths of Perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6)

It can be seen from Figure 17 that the main pyrolytic products of Si₃F₆, Si₄F₈, Si₅F₁₀, and Si₆F₁₂ are SiF₂. It can be seen from Figure 18a that when T < 1000 K and Δ_rG_m^θ > 0 for the pyrolytic reaction of Si₃F₆, the pyrolytic reaction of Si₃F₆ does not occur, whereas the pyrolytic reactions of Si₄F₈ and Si₅F₁₀ do not occur when T < 1100 K, and the pyrolytic reaction of Si₆F₁₂ does not occur when T < 1200 K. Additionally, when T > 1200 K and Δ_rG_m < 0 for the pyrolytic reaction of Si₆F₁₂, the pyrolytic reaction of Si₆F₁₂ can occur; the pyrolytic reactions of Si₄F₈ and Si₅F₁₀ can both occur when T > 1100 K; and the pyrolytic reaction of Si₃F₆ occurs when T > 1000 K. Also, when T = 1200 K, the Δ_rG_m^θ values of Si₃F₆, Si₄F₈, and Si₅F₁₀ are the same. Thus, shile Si_nF_2n (3 ≤ n ≤ 6) undergoes a pyrolytic reaction, when 1000 K < T < 1200 K, the pyrolytic order of Si_nF_2n (3 ≤ n ≤ 6) is in the order from small to large as follows: Si₆F₁₂ (ring) < Si₅F₁₀ (ring) < Si₄F₈ (ring) < Si₃F₆ (ring), with the main pyrolytic product SiF₂ reduced, and when T > 1200 K, the pyrolytic order of Si_nF_2n (3 ≤ n ≤ 6) is in the order from small to large as follows: Si₃F₆ (ring) < Si₄F₈ (ring) < Si₅F₁₀ (ring) < Si₆F₁₂ (ring), with the main pyrolytic product SiF₂ increased. It can be seen from Figure 18b that when the pyrolytic reactions occur, Δ_rH_m > 0 for Si_nF_2n (3 ≤ n ≤ 6) and the Δ_rH_m of Si₆F₁₂ is the largest, so the pyrolytic reactions of Si_nF_2n (3 ≤ n ≤ 6) are endothermic, and the higher the temperature is, the smaller the Δ_rH_m is, and the less heat it absorbs.

3. Conclusions

In this paper, we studied all possible kinetic paths of n-perfluorosilanes Si_nF_2n+2 (2 ≤ n < 6) and perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6) by the theoretical calculation method, deduced the pyrolytic reaction paths of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6) from their kinetic paths, and obtained the thermodynamic functions of the pyrolytic reaction paths at different temperatures.

The pyrolytic mechanisms of Si_nF_2n+2 (2 ≤ n < 6) are as follows: first, the silicon–silicon bond breaking induces the generation of free radicals; then, in the chain transfer, the related free radicals initiate F-abstraction transfer with the molecules; and finally, the free radicals form the molecules, and the chain terminates. The F-abstraction transfer is the easiest to initiate in the low-order silicon–fluorine substance among the chain transfer paths and to release SiF₂ at the same time, while the generation of the double free radical is the most difficult.

The pyrolytic mechanisms of Si_nF_2n (3 ≤ n ≤ 6) are as follows: first, the α–Si–Si bond breaking induces the generation of double free radicals; then, the α–Si–Si or β–Si–Si bond breaks continually in the chain transfer; and finally, the double free radical forms the molecules, and the chain terminates. SiF₂ is the most easily formed compound by breaking among the chain transfer paths.

By comparing the bond breaking energy of each path of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6), it is found that the chain initiation of the silicon–silicon bond breaking of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6) requires the highest bond breaking energy, which is the control step of the pyrolytic reaction. Analysis of the thermodynamic functions of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6) shows that their pyrolytic reactions are endothermic. For the pyrolytic reaction of Si₃F₈, when T > 1500 K, the main pyrolytic products are SiF₄ and SiF₂. For the pyrolytic reaction of Si₄F₁₀, when 600 K < T < 1200 K, the main pyrolytic products are Si₃F₈ and SiF₂, and when T > 1200 K, the main pyrolytic products are SiF₄ and SiF₂. For the pyrolytic reaction of Si₅F₁₂, when 900 K < T < 1400 K, the main pyrolytic products are Si₃F₈ and SiF₂, and when T > 1400 K, the main pyrolytic products are SiF₄ and SiF₂. For the pyrolytic reaction of Si_nF_2n (3 ≤ n ≤ 6), the main pyrolytic product is SiF₂, where when 1000 K < T < 1200 K, the pyrolytic order of Si_nF_2n (3 ≤ n ≤ 6) is Si₆F₁₂ (ring) < Si₅F₁₀ (ring) < Si₄F₈ (ring) < Si₃F₆ (ring) and when T > 1200 K, the pyrolytic order is the opposite. At the same time, from the above conclusions, the pyrolytic reaction paths of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6) can be used to guide the experimental preparation research studies and separation operations of Si_nF_2n+2 (2 ≤ n < 6), Si_nF_2n (3 ≤ n ≤ 6), and their intermediate substances.

4. Computational Methods

In this paper, Gaussian 09²³ is used to optimize the structures of n-perfluorosilanes Si_nF_2n+2 (2 ≤ n < 6) and perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6) at the B3LYP/6-31(d, p) level, with the optimized structures and the parameters of the bond length and the bond angle shown in our companion paper,²¹ and the theoretical predictions of their pyrolytic mechanisms are presented. All possible kinetic reaction paths during the pyrolysis are predicted. The initial structures of the pivots are optimized at the B3LYP/6-31(d, p) level, and the frequency analysis is carried out at the same level. Combined with the existing related pyrolytic rules, molecular simulation calculation is carried out on the reaction to verify the feasibility of the molecular simulation method and obtain the bond breaking energy (E) of each path. In this paper, the bond breaking energy of the path is used to analyze the difficulty of the reaction of each path. Through kinetic analysis, the main products of various pyrolytic reactions of Si_nF_2n+2 (2 ≤ n < 6) and Si_nF_2n (3 ≤ n ≤ 6) are deduced. The optimized reactants and products are used to calculate the thermodynamic functions (the enthalpy change Δ_rH_m and the standard Gibbs free energy change Δ_rG_m^θ) in the pyrolytic reaction paths at different temperatures, which are used to study the influence of different temperatures on the reaction equilibrium. The detailed formulas taking Si₂F₆ as an example are as follows

1

2

3

4

where ΔH_m^e is the molar electron enthalpy change and D_e is the molecular ground-state dissociation energy.

The authors declare no competing financial interest.