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Theoretical Study of Silicon Monoxide Reactions with Ammonia and Methane Huyen Thi Nguyen, Tran Dieu Hang, and Minh Tho Nguyen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11665 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 16, 2017

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The Journal of Physical Chemistry

Theoretical Study of Silicon Monoxide Reactions with Ammonia and Methane Huyen Thi Nguyen,†,‡ Tran Dieu Hang§ and Minh Tho Nguyen†,‡,§,* †

Computational Chemistry Research Group, Ton Duc Thang University, District 7,

Ho Chi Minh City, 778000 Vietnam ‡

Faculty of Applied Sciences, Ton Duc Thang University, District 7, Ho Chi Minh City,

77800 Vietnam §

Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

*

Emails: [email protected]; [email protected]

(Abstract) High accuracy calculations were carried out to study the mechanisms of the reactions between the diatomic silicon monoxide (SiO) with NH3 and CH4. These reactions are relevant to the SiOrelated astrochemistry and atmospheric chemistry as well as the activation of the N–H and C–H bonds by the SiO triple bond. Energetic data used in the construction of potential energy surfaces describing the SiO + NH3/CH4 reactions were obtained at the coupled-cluster theory with extrapolation to the complete basis set limit (CCSD(T)/CBS) using DFT/B3LYP/aug-cc-pVTZ optimized geometries. Standard heats of formation of a series of small Si-molecules were predicted. Insertion of SiO into the N–H bond is exothermic with a small energy barrier of ~8 kcal/mol with respect to the SiO + NH3 reactants, whereas the C–H bond activation by SiO involves a higher energy barrier of 45 kcal/mol. Eight product channels are opened in the SiO + 1 ACS Paragon Plus Environment


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NH3 reaction including dehydrations giving HNSi/HSiN and dehydrogenations. These reactions are endothermic by 16 – 119 kcal/mol (calculated at 298.15 K) with the CCSD(T)/CBS energy barriers of 21 – 128 kcal/mol. The most stable set of products, HNSi + H2O, was also the product of the reaction pathway having lowest energy barrier of 21 kcal/mol. Ten product channels of the SiO + CH4 reaction including decarbonylation, dehydration, dehydrogenation, and formation of Si + CH3OH are endothermic by 19 – 118 kcal/mol with the energy barriers in the range of 71 – 126 kcal/mol. The formation of H2CSiO + H2O has the lowest energy barrier of 71 kcal/mol, whereas the most stable set of product, SiH4 + CO, is formed via a higher energy barrier of 90 kcal/mol. Accordingly while SiO can break the N-H bond of ammonia without the assistance of other molecules, it is not able to break the C-H bond of methane.

1. Introduction Gas phase chemistry of silicon containing molecules has been a subject of many studies in part due to their astrophysical and atmospheric interest.1-6 According to Suess and Urey,7 the first seven elements with high abundances included H, He, O, Ne, N, C and Si. In molecular astronomy, the dominant form of silicon and oxygen bearing species is the diatomic silicon monoxide (SiO). This homologue of CO is also the most widespread silicon containing molecules observed in the interstellar medium.4, 6 Although the reaction mechanism is still not clear, SiO has been considered to play the role of building block for the formation of silicate dust particles in the circumstellar and interstellar spaces.8-10 Upon entering the Earth atmosphere silicon is released from the dust particles mainly in the form of SiO and the Si atom, the latter of which is rapidly oxidized by O2 and O3 in the atmosphere to form the former,2, undergoes further reactions with small atmospheric molecules. 2 ACS Paragon Plus Environment

4

which then

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Reactions of SiO with hydrogen containing molecules such as H2, H2O, ... have been both experimentally and theoretically reported.2, 11-15 The potential energy surfaces (PESs) of the SiO + H2 reaction were reported in the studies of the Si + H2O and SiH4 + O reactions.2, 11-14 The breaking of H–H bond upon interacting with SiO leads to the formation of two possible adducts, namely HSiOH and H2SiO. Zachariah and Tsang reported the calculated energy barriers of the formation of both isomers from SiO + H2 to be 40 and 80 kcal/mol, respectively.14 In the case of SiO and H2O, a weakly bound adduct is formed and then dissociate to either the silylene Si(OH)2 or the sila-acid HSi(=O)OH, the former being more stable by ~5 kcal/mol than the latter.14-15 The cleavage of the O–H bond to form Si(OH)2 is associated with a very low energy barrier of only 1 – 2 kcal/mol, with respect to the reactants SiO + H2O,14-15 whereas formation of the isomer HSi(=O)OH is prohibited by a higher energy barrier of 38 kcal/mol.15 The [Si, O, C, 4H] and [Si, O, C, 4H]+● PESs were already reported in the studies of the Si + CH3OH, Si+● + CH3OH, and SiO+● + CH4 reactions.16-18 A rather strongly-bound complex is formed between the singlet atom Si(1D) and CH3OH, which undergoes rearrangement into either CH3OSiH or CH3SiOH.16 These compounds can then be transformed into two other open-chain species, CH3(H)Si=O and CH2=Si(H)OH, and a cyclic species, oxasilirane. The latter is a much more reactive silicon analogue of oxirane (ethylene oxide). Relative energies of several minima of the [Si, O, C, 4H]+● PES were calculated at the G2 level of theory.17 These local energy minima, being equilibrium structures, include CH3SiOH+●, CH2=Si(H)OH+●, CH3OSiH+●, CH3(H)OSi+●, HCOSiH3+●, CO(H)SiH3+●, SiH2CHOH+●, SiH3COH+●, and oxasilirane. In the study of the SiO+● + CH4 reaction, only the reaction pathways leading to SiOH + CH3+● and Si+● + CH3OH were explored.18 A complex is formed between SiO+● and CH4 with relatively large interaction energy of ~ -19 kcal/mol. A C–H bond cleavage from this complex yield CH3SiOH+●

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which subsequently either dissociates into SiOH + CH3+● or transforms into the complex of Si+● and CH3OH. The reactions of SiO with NH3 and CH4 have not been explored yet, even though they are most relevant astrochemical and atmospheric processes. In this context, we set out to focus in the present work on the study of the reactions of SiO in the neutral state with NH3 and CH4. The transformations between both [Si, O, N, 3H] and [Si, O, C, 4H] isomeric systems and their decompositions to smaller products are explored in detail using the high accuracy coupledcluster theory methods. For the SiO + NH3 reaction, we studied the dehydration pathways leading to [Si, N, H] species (HNSi and HSiN) and the dehydrogenation pathways leading to [Si, O, N, H] species. In case of the SiO + CH4 reaction, the formation of Si + CH3OH, the decarbonylation to SiH4 + CO, the dehydrations to [Si, C, 2H] species, and the dehydrogenations to [Si, O, C, 2H] species were studied. The isomerizations of the [Si, N,H],19 [Si, O, N, H],20 [Si, C, 2H],21 and [Si, O, C, 2H]22-24 species were already reported and thus are not further explored in the present paper. 2. Computational Details The structures of all species along the PESs of reactions between SiO and NH3 and CH4 are fully optimized using density functional theory (DFT) method with the hybrid B3LYP functional and the correlation-consistent polarized plus diffuse functions, aug-cc-pVTZ, basis set. Vibrational frequency analyses are carried out at the same level of theory to confirm the nature of the stationary points located and evaluate their zero-point vibrational (ZPE) corrections. The bond length of 1.520 Å and vibrational frequency of 1246 cm-1 of SiO obtained from B3LYP/aug-cc-pVTZ calculation are very close to the experimental values25 of 1.510 Å and 1242 cm-1, respectively. The CCSD(T)/aug-cc-pVTZ bond length of 1.529 Å and vibrational 4 ACS Paragon Plus Environment

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frequency of 1215 cm-1 of SiO are even less accurate than the values obtained from B3LYP calculation. The optimized geometries of relevant structures are then used for a series of single point electronic energy computations using coupled-cluster theory CCSD(T) in conjunction with the aug-cc-pVnZ (n = D, T, Q) basis sets. Total energies obtained from these single point calculations were then used to extrapolate the total coupled-cluster energies to the complete basis set limit (CBS) using the expression as follows:26 E(n) = E(CBS) + B exp[−(n − 1)] + C exp[−(n − 1)2 ] where E(n) and E(CBS) are the CCSD(T)/aug-cc-pVnZ and CCSD(T)/CBS energies, respectively. Relative energies of the relevant species are subsequently calculated using the extrapolated CCSD(T)/CBS energies and ZPE corrections obtained at the B3LYP/aug-cc-pVTZ level. The T1 diagnostic tests27 are carried out at CCSD(T)/aug-cc-pVQZ for all studied species to verify the amount of multi-reference character. All electronic structure calculations in this work are performed using the GAUSSIAN 09 suite of program.28 As for a preliminary calibration, energetic data of several main reaction pathways of the SiO + H2 and SiO + H2O reactions are obtained at the CCSD(T)/CBS//B3LYP/aug-cc-pVTZ level and compared with previously reported data (see Table 1).12, 14-15 The energy barrier of a reaction is calculated as the difference in relative energy between the transition structure (TS) and the reactants. The energy of reaction is evaluated as the difference in relative energies between products and reactants. The corresponding enthalpy barriers and enthalpies of reaction are also calculated using the enthalpy values obtained at 298.15 K using CCSD(T)/CBS



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electronic energies. The reaction SiO + H2 was previously explored using perturbation theory (MPn) computations.12 There is a large discrepancy between our CBS results and those from previous MP2/6-31G(d,p).12 Our calculated energy barriers and energies of reactions of both SiO + H2 and SiO + H2O reactions are however closer to those of the MP4/6-311G(2df,p) calculations14 with the maximum difference of ~3 kcal/mol. The present enthalpy barriers and enthalpies of reaction for the SiO + H2O reaction also agree better with the data reported by Becerra et al. obtained at the G3 level.15 As for a convention, the labels of all the species involved in the reaction between SiO and NH3 start with capital letter A, followed by a number and a letter. The number indicates a set of compound with a specific arrangement of non-hydrogen atoms and the different lowercase letters indicate different compounds of the same set. For example, the set of seven molecules A1a – A1g are those with the N–Si–O arrangement, whereas the set of four molecules A2a – A2d implies those with N, Si, and O atoms forming a three-membered ring. The TS connecting two equilibrium structures, A1a and A1b for example, is denoted as A1a/A1b. The names of the relevant species in the reaction between SiO and CH4 follow the same convention but start with capital letter M. The letters A and M simply stand for ammonia and methane, respectively. 3. Results and Discussion. 3.1 Thermochemical Parameters of Si Species. In view of the fact that basic thermochemical parameters of many simple Si-containing molecules are not experimentally determined yet, let us first evaluate their heats of formation. The total atomization energy (TAE) of each species is determined using both the ZPE-corrected CCSD(T)/CBS and Gaussian-4 (G4) energies. The G4 method can provide us with good results


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for heats of formation with the absolute deviation of ± 1 kcal/mol.29 By combining our computed TAE values with the known heats of formation for the elements C, N, O, Si, and H at 0 K (ΔHf,0 = 170.0, 112.5, 59.0, 106.5, and 51.6 kcal/mol, respectively)30 and 298 K (ΔHf,298 = 171.4, 113.0, 59.5, 108.0, and 52.1 kcal/mol, respectively),31 we derive the heat of formation ΔHf,0 and ΔHf,298 for the molecules under study in the gas phase. The differences between the calculated ΔHf,0 and ΔHf,298 values are quite small, being in the range of 0.0 - 1.7 kcal/mol. The ΔHf,0 values are listed in Table 2 along with several previously calculated data.20-21 The ΔHf,298 values are listed in Table S1 of the Supporting Information (ESI) file. The ΔHf,298 values obtained at the G4 and CCSD(T)/CBS methods are -24.1 and -23.7 kcal/mol for SiO and 8.6 and 6.6 kcal/mol for SiH4, respectively. In comparison to the expt.

expt.

experimental heats of formation of SiO (∆Hf,298 (SiO) = -24.3 kcal/mol) and SiH4 (∆Hf,298 (SiH4) = 8.2 kcal/mol),32 the G4 method produces better results than the CCSD(T)/CBS method with the errors of only 0.2 and 0.4 kcal/mol, respectively. Except for HSiCH and c-HSiNO, the deviation between the CCSD(T)/CBS and G4 values are in the range of 0 – 3 kcal/mol. Large differences of 8 – 9 kcal/mol are observed for HSiCH and c-HSiNO. The large deviation of the heat of formation obtained at the CCSD(T)/CBS level as compared to the G4 and experimental data for some species was already reported.33 It should also be pointed out that in the present work, apart from the ZPE corrections, some smaller corrections are not included in the evaluation of the TAEs using CCSD(T)/CBS energies. These include the corrections for corevalence correlation, spin-orbit interaction and scalar relativistic effect. For small species such as those considered here, their negligence is expected to give rise to an additional error of, at most, ± 1.0 kcal/mol on the computed standard heat of formation.33



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In general, calculated G4 values agree well with previously reported G1, G2 and G3B3 data.20, 34 The CCSD(T)/CBS values for CH2Si and HSiCH are also close to those obtained at the CCSD(T)/TZ2P(df) level.21 3.2. Reaction SiO + NH3 3.2.1. Formation and Transformation of SiO + NH3 Adduct. The interconversions between different [Si, O, N, 3H] species are showed in Figure 1 with the starting materials being SiO + NH3. The TS’s located on these PE profiles are plotted in Figure 2. A weakly bound adduct (Arc) is initially formed between the Si atom of SiO and N atom of NH3 with the interaction energy of ~ -7 kcal/mol. Hydrogen transfer reactions from Arc lead to seven different species, namely A1α, α = a – g, of the first set of molecules, which have a N–Si–O arrangement. Subsequently, the second set containing four molecules with three-membered ring arrangement, A2β, β = a – d, can be formed from A1b and A1c. A third set containing two molecules A3γ, γ = a, b with an Si–N–O arrangement is formed from the molecule A2a of the second set. As shown in Figure 1, the compounds of the first set A1α are much more energetically stable than those of the A2β and A3γ sets. All seven molecules of the A1α set have relative energies lower than those of the reference SiO + NH3 and the reactant complex Arc. Based on calculated energy profiles, the most favorable pathways leading to the formation of these molecules are SiO + NH3 → Arc → A1a → A1b → A1c → A1d → A1e → A1f, and A1d → A1g. The highest-energy TS along this pathway is A1b/A1c with relative energy (∆E) of 30 kcal/mol. A shorter pathway leading to A1g involves a H transfer from A1a with TS A1a/A1g which has a slightly higher relative energy (∆E = 35 kcal/mol with respect to the reactant reference). The one-step formation of A1c from Arc is not competitive with the indirect pathway via A1a and A1b, and involves a higher energy TS Arc/A1c (∆E = 43 kcal/mol). 8 ACS Paragon Plus Environment

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Note that formation of pre-association complex Arc and its subsequent transformation to A1a actually constitute the bond activation of ammonia using silicon monoxide SiO. The cleavage of the N–H bond has been the subject of many studies in search for mild catalytic transformations of NH3.35-38 The addition of NH3 to the SiO triple bond of a silanone precursor was experimentally reported to be a facile reaction taking place under mild conditions. 38 Calculated data obtained in the current work suggests that the activation of NH3 by SiO is also kinetically feasible with the energy barrier of the process SiO + NH3 → Arc → A1a being only 8 kcal/mol. In comparison to its carbon analogue (CO), SiO is obviously more reactive toward NH3 as compared to the insertion of CO into NH3 to form formamide (NH2COH) having a much higher energy barrier of 71 kcal/mol.39 The reaction of SiO + NH3 is also more thermodynamically favorable. The adduct A1a is more stable than the original set of reactants (SiO + NH3), whereas the NH2COH is substantially much less stable than CO + NH3 (by 30.6 kcal/mol at the CCSD(T)CSB//MP2/aug-cc-pVTZ level). Formations of both A2β and A3γ sets of molecules from the A1α set involve TSs characterized by high relative energies, implying large energy barriers with respect to the reactants SiO + NH3. The most stable species A2a of the A2β set, being a silicon analogue of the three-membered oxaziridine, is formed from A1c via TS A1c/A2a (∆E = 59 kcal/mol). In this TS, the H transfer from N to Si is accompanied by ring closure yielding a N–O bond in A2a. The optimized N–Si–O bond angle is reduced from 127º in A1c to 105º in A1c/A2a. Of the three possible pathways leading to formation of A2b from either A2a, A1b, or A1c, the latter is the most energetically favorable with the ring closure TS A1c/A2b (∆E = 75 kcal/mol) being ~ 20 kcal/mol lower in energy than the other two TSs A2a/A2b and A1b/A2b. The remaining two



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species of the A2β set, A2c and A2d, are formed from A2b via the H-transfer TSs A2b/A2c and A2b/A2d (∆E = 96 and 94 kcal/mol, respectively). The set of open-chain species A3a (SiH3–N=O) are formed from the cyclic compound A2a via the significantly high-energy TS A2a/A3a (∆E = 110 kcal/mol) in which the transfer of H from N to Si is accompanied by a ring opening. Even though A2a/A3a is still cyclic (see Figure 2), intrinsic reaction coordinate (IRC) calculations carried out at B3LYP/aug-cc-pVTZ level of theory indeed show the breaking of Si–O bond generating A3a. The H transfer from Si to O of A3a giving rise to A3b involves an even less stable TS A3a/A3b with very high relative energy of 125 kcal/mol. 3.2.2. Different Product Channels of the SiO + NH3 reaction. Eight product channels of the SiO + NH3 reaction including two dehydration and six dehydrogenation channels are studied. The schematic potential energy profiles illustrating the decompositions of different [Si, O, N, 3H] species are presented in Figures S1-S3 in the electronic Supporting Information (ESI) file. The optimized structures and the relative energies of the involved TSs are given in Figure 3. All of these product channels are endothermic as listed below in the increasing order of the 0 enthalpies of reactions calculated at 298.15 K (∆r𝐻298𝐾 ) using the CCSD(T)/CBS electronic

energies and B3LYP/aug-cc-pVTZ thermal corrections to enthalpy:

SiO + NH3

→

HNSi + H2O

(AP1)

0 ∆r𝐻298𝐾 = 15.9 kcal/mol

→

HNSiO + H2

(AP2)


(AP3)


→


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→

NSiOH + H2

(AP4)


→

HSiN + H2O

(AP5)


(AP6)


(AP7)


(AP8)

0 ∆r𝐻298𝐾 = 119.0 kcal/mol

→ → →

HSiNO + H2

Eliminations of a water molecule from various molecules of the A1α set yields two isomeric products HNSi (AP1) and HSiN (AP5). Different from the carbon homologues HCN/HNC in which HCN being more stable than HNC, the HNSi isomer turns out to be much more energetically stable than the its isomer HSiN with an energy difference of 65 kcal/mol in favor of the former (see Figure S1). The product HNSi + H2O is also the most energetically stable one, as compared to seven other sets of products. HNSi can be formed from dehydrations of either A1b and A1d, or A1g. Of the three possible pathways, H2O-elimination from A1b is the most favorable channel with the relative energy of TS A1b/Apc1 being 21 kcal/mol. The product complex Apc1 is a weak complex in which the two molecules HNSi and H2O are linked to each other by a hydrogen bond. The product complex Apc5 of HSiN and H2O is formed from A1f via TS A1f/Apc5 having rather high relative energy of 77 kcal/mol. Six dehydrogenation pathways of the SiO + NH3 reaction lead to different [Si, O, N, H] species including three open-chain HNSiO, NSiOH and HSiNO and three cyclic c-SiONH, cSiNOH and c-HSiNO isomers (see Figures S2 and S3, respectively). These dehydrogenation products are considerably less stable than the product AP1. The most stable set of 11 ACS Paragon Plus Environment


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dehydrogenation products AP2 (HNSiO + H2, ∆E = 31 kcal/mol) can be formed from either A1a, A1c and A1e, or A1f. Production of the tetra-atomic species HNSiO via TS A1c/AP2 is associated with an energy barrier of 45 kcal/mol, which is slightly lower than that of pathways via TSs A1e/AP2 and A1f/AP2 (cf. Figure 3). While the dehydrogenation of A1a has the highest energy barrier of 71 kcal/mol that leading to HNSiO via A1c is also the most favorable among the studied dehydrogenation pathways of SiO + NH3 reaction. The H2-loss from A1d and A3d yielding NSiOH and HSiNO are demanding, with very high energy barriers of 85 and 118 kcal/mol, respectively. The cyclic products generated from the A2β molecules have relative energies in the range of 65 – 119 kcal/mol. Of these pathways, the lowest energy barrier of 90 kcal/mol involves the dehydrogenation of A2b giving AP3. However, this energy barrier is substantially much higher than those of the reaction pathways leading to AP1 and AP2. Overall, the product channel of the SiO + NH3 reaction associated with the lowest energy barrier is actually the dehydration from A1b producing the HNSi species. We also explored several transformation pathways from A2a leading to noncyclic Si-O-N species including SiH-OH-NH, SiH2-O-NH, and SiH2-OH-N. These compounds are rather unstable with high relative energies of 92.0 - 161.2 kcal/mol. It is reasonable to expect that the decomposition TSs of these compounds would also have high relative energies. Indeed, the TS involved in the dehydrogenation of SiH-OH-NH leading to SiONH + H2 has a high relative energy of 149.0 kcal/mol. These pathways are thus not discussed further here. 3.3. Reaction SiO + CH4 3.3.1. Formation and Transformation of SiO + CH4 Adduct. Similar to the above reaction, three sets of compounds are formed from addition of SiO to CH4, namely M1α, M2β, and M3γ.


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Their transformations are schematically shown in Figure 4, along with related optimized structures. The involved TSs are separately displayed in Figure 5. The first set M1α contains five molecules M1a – M1e with the C–Si–O arrangement and relative energies lying in the range of -2 – 11 kcal/mol with respect to the reactants. The second set M2β contains four cyclic compounds, including oxasilirane M2a and its isomers M2b – M2d. The molecules M3a and M3b of the third set M3γ have a Si–O–C connectivity and higher relative energies than those of the first set. Addition of the SiO triple bond into CH4 with the breaking of a C–H single bond is more difficult to achieve than the insertion of SiO into NH3. This reaction SiO + CH4 → M1a is characterized by a rather high energy barrier of 45 kcal/mol. A natural bond orbital (NBO) analysis was carried out at the CCSD(T)/aug-cc-pVTZ level for SiO, NH3, CH4, and the two N-H and C-H bond breaking TSs Arc/A1a and MR/M1a. The NBO analysis points out a clear and large difference in the bonding characteristics of both TSs. While Arc/A1a is basically a molecule NH3SiO, MR/M1a looks like more a complex of two unstable fragments, CH3 and HOSi. As shown in Table S2 in the ESI file, the new N–Si bond is already formed in TS Arc/A1a but the three N–H bond orbitals still exist. There is a strong donation from the lone pair of O to the antibonding orbital of the N–H bond in Arc/A1a which eventually leads to the breaking of this bond, and creates a new O–H bond in the product A1a. Meanwhile, the existence of only three C–H bonding orbitals indicates that the breaking of C–H bond already occurs within MR/M1a (cf. Table S3 in the ESI file). A new O–H bond is already formed, but there is still no bonding between Si and C. Two main electron donations in this TS include the donations from the lone pair of C to the antibonding orbital of O–H bond and to an empty p orbital of Si. Obviously, the number of NBOs remains almost unchanged in going 13 ACS Paragon Plus Environment


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from the SiO + CH4 to MR/M1a, whereas the TS Arc/A1a has one more NBO than the reactants SiO + NH3. The increase in the number of NBOs in the latter case might explain the lower reaction barrier for the N–H bond cleavage as compared to that of the C–H bond cleavage. The C–H cleavage by the neutral SiO diatomic molecule is also much more difficult to achieve than that by the radical cation SiO+●. The relative energy of the TS for C–H bond breaking in the process of the radical cation SiO+● + CH4 reaction is even lower than starting point SiO+● + CH4 by 14 kcal/mol.18 As shown in Figure 4, the reaction pathways forming M1a, M1b, M1d and M1e have energy barriers of ~45 kcal/mol, whereas formation of M1c involves TS M1b/M1c with a higher relative energy of 54 kcal/mol. The most stable molecule M3a of the M3γ set can be formed from M1c via a three-membered ring TS M1c/M3a (∆E = 76 kcal/mol), in which CH3, HSi, and O arrange in a three-membered ring pattern (see Figure 5). The H transfer from C to Si of either M1c or M3a yields oxasilirane M2a. Another H transfer from C to Si of M2a accompanied by the breaking of the Si–C bond yields M3b, which is much less stable than M3a. The other cyclic compounds of the M2β set also come from rearrangement of M2a with substantial energy barriers of 80 – 92 kcal/mol. Our calculated energy barriers for the transformations M1a → M1d, M1b → M1c, M1c → M3a, and M3a → M2a are close to the previously reported data16 with differences being less than 3 kcal/mol. 3.3.2. Different Product Channels of the SiO + CH4 Reaction. Ten product channels including decarbonylation, dehydration, dehydrogenation and formation of methanol were studied. The schematic potential energy profiles illustrating the decompositions of different [Si, O, C, 4H] species are presented in Figures S4-S7 in the ESI file. The optimized structures and the relative energies of the involved TSs are given in Figure 6. The products are listed below in the 14 ACS Paragon Plus Environment

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increasing order of enthalpy of reaction. Similar to the product channels of the SiO + NH3 reaction, all of the product channels of the SiO + CH4 reaction are also endothermic.

SiO + CH4

→

SiH4 + CO

(MP1)


(MP2)


→

→

H2CSiO + H2

(MP3)


→

H2CSi + H2O

(MP4)


→

H2SiOC + H2

(MP5)


→

HSiCHO + H2

(MP6)


→

HCSiOH + H2

(MP7)


→

SiCHOH + H2

(MP8)


→

HSiCH + H2O

(MP9)


→

Si + CH3OH

(MP10)

0 ∆r𝐻298𝐾 = 118.3 kcal/mol

Most of the product channels of the SiO + CH4 reaction involve TSs with high relative energies of 63 – 128 kcal/mol. The most stable set of products including SiH4 + CO, and the least stable one, Si + CH3OH, are shown in Figure S4. The H transfer from C to Si of M3b giving rise to SiH4 + CO involves the high-lying TS M3b/MP1 (∆E = 90 kcal/mol). The reaction pathways describing a Si-loss M3a → Mpc10 → Si + CH3OH and M1b → Mpc10 → Si + CH3OH are simply the reverse processes of the insertions of Si into the O – H and C – O bonds, respectively, reported in a previous theoretical study of the Si + CH3OH reaction.16 Our CCSD(T)/CBS + ZPE energies of reactions of 95 and 120 kcal/mol for the 15 ACS Paragon Plus Environment


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Page 16 of 33

reactions M3a → Si + CH3OH and M1b → Si + CH3OH confirm the previously reported DFT values of 96 and 119 kcal/mol, respectively, using the B3LYP functional and 6-311G(d,p) basis set.16 However, the differences between energy barriers of these two reactions turn out to be larger, being up to ~4 kcal/mol. Such a larger deviation is due to the difference in the theoretical methods used. As for a comparison, let us note that the energy barriers obtained using B3LYP/aug-cc-pVTZ computations, the differences with the earlier DFT results amount to only ~1 kcal/mol. Water loss reactions from the SiO + CH4 system yielding the isomers CH2Si and HSiCH are shown in Figure S5. The more stable product CH2Si can be formed from M1b, M1d, or M2c, and HSiCH can be formed from M2d. Of the four pathways, the dehydration from M1b has the lowest energy barrier of 63 kcal/mol. The dehydrations of the cyclic compounds M2c and M2d involve much higher energy barriers of 117 and 111 kcal/mol, respectively. Dehydrogenation pathways illustrated in Figures S6 and S7 point out that that this process is not favored at all. Formation of MP3 via TS M1c/MP3 is defined by the lowest energy barrier, but such an energy barrier amounts up to 71 kcal/mol. Remaining reaction pathways including the dehydrogenations of M1a, M1c, M1d, M1e, M2c (via TS M2c/MP8), M3a, and M3b involve even higher energy barriers of 76 – 128 kcal/mol. Although both dehydrogenation products MP2 and MP3 are located at lower position on the relative energy scale than the dehydration product MP4 (CH2Si + H2O), the energy barriers for H2-loss to MP2 and MP3 are in fact higher than that for dehydration from M1b. Of the ten reaction pathways, dehydration of M1b is also a product channel with the lowest energy barrier of the SiO + CH4 reaction.


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3.4. T1 Diagnostic Test The T1 values of all species involved in the reactions SiO + NH3 and SiO + CH4 are obtained at the CCSD(T)/aug-cc-pVQZ level. All studied [Si, O, N, 3H] and [Si, O, C, 4H] species have T1 values lower than the suggested threshold of 0.020. However, the T1 values of SiO, several TSs and products are higher than 0.020 (see Table S4 in the ESI file). The largest T1 values of 0.060 - 0.144 obtained for TS A1c/A2a, A1c/A2b, M1b/Mpc10, and M1c/M2a indicate that these systems have high multi-reference character. Such systems need to be treated with multireference methods to derive more reliable results. However, although the T1 diagnostic test provides a criterion to judge the multi-reference character of a chemical system, it does not mean that the results for systems with T1 value larger than 0.020 cannot be correctly described by single reference methods. As mentioned in a previous section, the bond length of SiO obtained from single reference B3LYP method agrees well with experiment even though SiO has a T1 value of 0.025. In addition, although the T1 value

of

0.054

of

the

triplet

SiO

is

also

larger

than

the

threshold,

the

CCSD(T)/CBS//B3LYP/aug-cc-pVTZ method provides accurate S0 → T1 adiabatic excitation energy for SiO with a very small difference of 0.02 kcal/mol between theory and experiment.40 4. Concluding Remarks High accuracy coupled-cluster theory calculations were carried out to study the mechanisms of the reactions between silicon monoxide SiO with NH3 and CH4. The geometries of different species were optimized using density functional theorey at the B3LYP/aug-cc-pVTZ level. Energetic data used to construct the potential energy surfaces (PESs) of the SiO + NH3/CH4 reactions were obtained at the CCSD(T)/CBS + ZPE computations. Enthalpy barriers and enthalpies of reaction were also calculated using the CCSD(T) electronic energies. Heats of



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Page 18 of 33

formation of a set of small silicon-containing molecules were predicted using both CCSD(T)/CBS and G4 approaches. Formation of SiO + NH3/CH4 adducts and their subsequent transformations leading to different isomers on the [Si, O, N, 3H] and [Si, O, C, 4H] PESs were examined in detail. Insertion of SiO into the N–H bond is exothermic with a small energy barrier of 8 kcal/mol with respect to the starting reactant SiO and NH3. Meanwhile, insertion of SiO into the C–H bond of methane involves a much higher energy barrier of 45 kcal/mol. The formation of other [Si, O, N, 3H] and [Si, O, C, 4H] species from these adducts mostly involve high energy barriers. Eight product channels were studied for the SiO + NH3 reaction including dehydrations to HNSi/HSiN and dehydrogenations. All of these channels are endothermic by 16 – 119 kcal/mol and have energy barriers in the range of 21 – 128 kcal/mol. The most stable set of product, HNSi + H2O, was also the product of the reaction pathway having lowest energy barrier of 21 kcal/mol. Ten product channels of the SiO + CH4 reaction including decarbonylation, dehydration, dehydrogenation, and formation of Si + CH3OH are endothermic by 19 – 118 kcal/mol. The energy barriers for these reaction pathways are in the range of 71 – 126 kcal/mol. The reaction pathway with lowest energy barrier of 71 kcal/mol is the dehydration leading to the formation of the third most stable set of product, H2CSiO + H2O. The formation of the most stable set of product, SiH4 + CO, involves a higher energy barrier of 90 kcal/mol. Overall, the detailed mechanisms of the SiO + NH3/CH4 reactions that are relevant to the studies of SiO-related astrochemistry and atmospheric chemistry were established. In addition, the results provide new insights into the activation of N–H and C–H bonds by the SiO triple bond.


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Acknowledgements. The authors thank Ton Duc Thang University (TDTU-Demasted) for support. We are indebted to the KU Leuven Research Council (GOA, PDM and IRO programs). MTN is grateful to the Gridchem for allocating computer times. This work used the Extreme Science and Engineering Discovery Environment (XSEDE) facilitites, which are supported by National Science Foundation grant number ACI-1053575.

Supporting Information. Tables list calculated heats of formation at 298 K, natural bond orbitals of the TSs Arc/A1a and MR/M1a from natural bond orbital analysis (CCSD(T)/aug-ccpVTZ), and Cartesian coordinates of the relevant structures optimized at the B3LYP/aug-ccpVTZ level. Figures display schematic potential energy profiles illustrating the decompositions of different [Si, O, N, 3H] and [Si, O, C, 4H] species. This material is available free of charge via the Internet at http://pubs.acs.org.

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Page 24 of 33

Tables Table 1. Energy barriers and energies of reactions of several reaction pathways of the SiO + H 2 and SiO + H2O reactions. Enthalpy barriers and enthalpies of reaction at 298.15 K are given in the parentheses (kcal/mol). Reactions

Energy barrier (kcal/mol)

The SiO + H2 reaction

This work

Ref 14

Ref 12

This work

Ref 14

Ref 12

SiO + H2 → HSiOH (cis)

41 (39)

40

51

0 (-2)

-3

11

SiO + H2 → H2SiO

79 (78)

80

95

-1 (-2)

-1

6

10

0 (0)

59

1 (1)

HSiOH (cis) → HSiOH (trans)

8 (8)

H2SiO → HSiOH (trans)

56 (57)

HSiOH (trans) → Si–OH2

89 (87)

55

0 -2

4

Ref 14

Ref 15

79 (80)

SiO + H2 → Si (triplet) + H2O

73 (72) Ref 14

This work

The SiO + H2O reaction

Ref 15

SiO + H2O → SiO-H2O complex SiO + H2O → Si(OH)2

Energy of reaction (kcal/mol)

This work -5 (-5)

(-6)

3 (2)

1

(2)

-38 (-39)

-36

(-38)

SiO + H2O → HSiO(OH)

38 (36)

36

(38)

-31 (-32)

-31

(-31)

Si(OH)2 → HSiO(OH)

59 (59)

56

(59)

7 (7)

5

(7)

HSiO(OH) → SiO2 + H2

59 (59)

(61)

42 (43)


(44)

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Table 2. Calculated heats of formation at 0 K(ΔHf,0) of SiO and several decomposition products of the SiO + NH3/CH4 reactions. Values are given in kcal/mol. ΔHf,0

Compounds

Theory(a)

CCSD(T)

G4

SiO

-23.7

-24.1

SiH4

7.5

9.5

HNSi

40.9

39.0

HSiN

106.4

105.1

HNSiO

-0.9

-0.3

0.2 (Ref. 20)

NSiOH

34.3

34.7

34.9 (Ref. 20)

HSiNO

61.5

58.2

58.7 (Ref. 20)

c-SiONH

32.9

31.5

33.0 (Ref. 20)

c-SiNOH

59.3

57.7

58.8 (Ref. 20)

c-HSiNO

86.2

77.8

77.1 (Ref. 20)

CH2Si

77.8

75.5

80.6 (Ref. 21)

HSiCH

118.2

108.9

114.7 (Ref. 21)

H2CSiO

18.6

18.3

H2SiOC

35.7

34.7

HSiCHO

37.1

35.8

HCSiOH

44.5

42.4

SiCHOH

49.3

47.0

c-SiOCH2

11.0

(a)

20

8.9 21

Ref. : G3B3 method; ref. : CCSD(T)/TZ2P(fd) method.



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Figure captions. Figure 1. Transformations of different [Si, O, N, 3H] species. Energetic data in kcal/mol are obtained at CCSD(T)/CBS//B3LYP/aug-cc-pVTZ + ZPE. Figure 2. B3LYP/aug-cc-pVTZ optimized structures of the transition states on the [Si, O, N, 3H] potential energy surfaces. Figure 3. B3LYP/aug-cc-pVTZ optimized structures of the transition states involved in the decompositions of different [Si, O, N, 3H] species. Relative energies in kcal/mol are obtained at CCSD(T)/CBS//B3LYP/aug-cc-pVTZ + ZPE. Figure 4. Transformations of different [Si, O, C, 4H] species. Energetic data in kcal/mol are obtained at CCSD(T)/CBS//B3LYP/aug-cc-pVTZ + ZPE. Figure 5. B3LYP/aug-cc-pVTZ optimized structures of the transition states on the [Si, O, C, 4H] potential energy surfaces. Figure 6. B3LYP/aug-cc-pVTZ optimized structures of the transition states involved in the decompositions of different [Si, O, C, 4H] species. Relative energies in kcal/mol are obtained at CCSD(T)/CBS//B3LYP/aug-cc-pVTZ + ZPE.


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Figures Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Figure 6.


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TOC Graphic


Theoretical Study of Silicon Monoxide Reactions ... - ACS Publications

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