Mechanism of the silane decomposition. I. Silane loss kinetics and rate inhibition by hydrogen. II. Modeling of the silane decomposition (all stages of reaction)

Part I: Kinetic data for the static system silane pyrolysis (from 640–703 K, 60–400 torr) are presented. For conversion from 3–30%, first-order kinetics are obtained, with silane loss rates equal to half the hydrogen formation rates. At conversions greater than 40%, rate inhibition attributable to the back reaction of hydrogen with silylene occurs. Overall reaction rates are not surface sensitive, but disilane and trisilane yield maxima under some conditions are. A nonchain mechanism capable of describing quantitatively all stages of the silane pyrolysis is proposed. Post 1.0% initiation is both homogeneous (gas phase) and heterogeneous (on the walls), and reaction intermediates are silylenes and disilenes. Free radicals are not involved at any stage of the reaction. Rate data at high conversions and with added hydrogen provide kinetics for the addition of silylene to hydrogen [reaction (?1)1] relative to its addition to silane [reaction (2)]: k_?1,/k₂ = 10^?0.65 × e^{?3200 cal/RT}. With E₂ = 1300 cal, this gives a high pressure activation energy for silylene insertion into hydrogen of E_?1 = 8200 cal. Part II: An analysis is made of each rate constant of the silane mechanism and the modeling results are compared with experimental results. Agreement is excellent. It is concluded that the dominant sink reaction for silylene intermediates is 1,2—H₂ elimination from disilane (followed by Si₂H₄ polymerization and wall deposition). The model is in accord with slow isomerization between disilene and silylsilylene and near exclusive 1,2—H₂ elimination from Si₂H₆. It is also concluded that disilene is about 10 kcal/mol more stable than silylsilylene and that the activation energy for isomerization of silylsilylene to disilene is greater than 26 kcal/mol.     

Bridged structures in multiply bonded silicon compounds: Disilyne,protonated disilyne and disilene

Ab initio SCF and electron correlation calculations are reported for the singlet ground state of the title compounds. These calculations confirm earlier findings that non-planar bridged Si₂H₂ is the most stable structure. For protonated disilyne (Si₂H³⁺) a bridged D_3h structure is the global mimimum. Two bridged structures of C_2v and C_2h symmetry are found in the case of disilene (Si₂H₄) which are only 14–17 kcal/mol above the D_2h structure.     

The N4 molecule has an open-chain triplet C2h structure

The lowest-energy N₄ is computed ab initio to be the planar C_2h(³B_u) open-chain structure 13 . The open-chain N₄ singlet-state structures dissociate on geometry optimization. The tetraazatetrahedrane T_d structure 1 and the tetrazete D_2h structure 2 are minima at MP 2/6-31G *. However, both are higher in energy than 13 (24.1 and 21.2 Kcal/mol [UQCISD ) (T )(full)/6-311+G *//MP 2/6-31G * + ZPE (MP 2/6-31G )*, respectively]. The energy of 13 is 157.5 kcal/mol higher than that of two N₂(¹∑ molecules [UQCISD (T )(Full)/6-311+G *//MP 2/6-31G *] © 1993 John Wiley & Sons, Inc.     

The structure of protonated disilene

The open and cyclic forms of Si₂ H₅⁺ have been calculated at the SCF level and with inclusion of electron correlation energy. Our final results indicate that both structures are about equally stable. The proton affinity of disilene is calculated as 207 kcal/mol.     

Thermal decomposition kinetics of polysilanes: Disilane,trisilane, and tetrasilane

The decomposition kinetics of disilane with added butadiene, trisilane both neat and with added butadiene, trimethylsilane or H₂, and normal and iso-tetrasilane both neat and in the presence of added butadiene are reported. Arrhenius parameters of the primary dissociation reactions are determined: A-factors suggest that polysilane decompositions (1) have similar intrinsic activation entropies (ΔS? ≈? 6.2 ± 5 e.u.) and (2) have activation energies which increase with increasing reaction endothermicities. Relative trapping efficiencies of SiH₄, Si₂H₆, Si₃H₈, C₄H₆, Me₃SiH, and H₂ toward SiH₂ and SiH₃SiH are also determined. Other results include the heat of formation of silylsilylene, ΔH ${}_{\text{°}}^{\text{f}}$ (SiH₃SiH) = 75.3 Kcal/mol, and the activation energy for 1,1-H₂ elimination from disilane (EH₂ = 57.8 kcal/mol).     

Is the stereomutation of methane possible?

Large basis set ab initio calculations at correlated levels, including MP2, single reference, as well as multireference configuration interaction, carried out on the methane potential energy surface, have located and characterized a transition structure for stereomutation (one imaginary frequency). This structure is best described as a pyramidal complex between singlet methylene and a side-on hydrogen molecule with C_s symmetry. At the single reference CI level, it lies 105 kcal/mol above the methane T_d-ground state but is stable relative to dissociation into CH₂(¹A₁) and H₂ by 13 kcal/mol at 0 K (with harmonic zero point energy (ZPE) corrections for all structures). Dissociation of the transition state into triplet methylene and hydrogen also is endothermic (by 4 kcal/mol), but single bond rupture to give CH and H^. is 3 kcal/mol exothermic. Thus, it does not appear likely that methane can undergo stereomutation classically beneath the dissociation limit. Confirming earlier conclusions, side-on insertion of ¹A₁ CH₂ into H₂ in a perpendicular geometry occurs without activation energy. Planar (D_4h) methane (130.5 kcal/mol) has four imaginary frequencies. Two of these are degenerate and lead to equivalent planar C_2v structures with one three-center, two-electron bond and two two-electron bonds and two imaginary frequencies. The remaining imaginary frequencies of the D_4h form lead to tetrahedral (T_d) and pyramidal (C_4v) methane. The latter has three negative eigenvalues in the force-constant matrix; one of these leads to the T_d global minimum and the other to the C_s (parallel) stereomutation transition structure. Multireference CI calculations with a large atomic natural orbitals basis set produce similar results, with the electronic energy of the C_s stereomutation transition state 0.7 ± 0.5 kcal/mol higher than that of CH + H^. dissociation products, and a ZPE-corrected energy which is 5 ± 1 kcal/mol higher. Also considered are photochemical pathways for stereomutation and the possible effects of nuclear spin, inversion tunneling, and the parity-violating weak nuclear interaction on the possibility of an experimental detection of stereomutation in methane. © 1995 by John Wiley & Sons, Inc.     

On the dimerization process of nitroso compounds

Summary Many organic C-nitroso compounds R-NO form stable dimers with a covalent NN bond. To gain insight into the dimerization reaction 2 R-NO (R-NO)₂ a theoretical study of the dimerization to atrans-form was performed using HNO as a model compound. Complete geometry optimizations were carried out at the HF, MP2 and QCISD levels using a 6–31G* basis. In the stationary points energies were calculated at the MP4(SDTQ) and QCISD(T) levels. For the equilibrium structure of the monomer and dimers stable RHF solutions were found, whereas for the TS UHF and UMPn calculations were applied. Extensive spin contamination was found in the UHF wavefunction, and projections up tos+4 were invoked. Relative energies were corrected for differences in ZPE. Calculations were made (a) for the least-motion path (C _2h symmetry) and (b) for a path with complete relaxation of all internal coordinates. Along the latter path a TS having virtuallyC _i symmetry was found. Along path (a) an activation energy of around 150 kcal/mol was predicted, in conformity with a symmetry forbidden reaction. On the relaxed path (b) the barrier to dimerization was estimated to be 10.7 kcal/mol at the MP4(SDTQ)//MP2 level, and 10.9 kcal/mol at the QCISD(T)//QCISD level. Unscaled ZPE corrections, calculated at the SCF level, changed these values to 12.7 and 12.9 kcal/mol, respectively. The reaction energy for the dimerization process is predicted to be – 17.2 kcal/mol at the MP4(SDTQ)//MP2 level corrected for ZPE. Calculations at the G1 level gave a corresponding value of – 16.4 kcal/mol. The equilibrium constant for the association to thetrans dimer is estimated to beK _p =259 atm, indicating that the dimer should be an observable species in the gas phase.     

Aromatic stability energy studies on five-membered heterocyclic C4H4M (M = O, S, Se, Te, NH, PH, AsH and SbH): DFT calculations

Energetic, geometric and magnetic criteria were applied to examine the stability and/or aromatic character for the cyclic molecules C ₄ H ₄ M (M = O, S, Se, Te, NH, PH, AsH and SbH) at B3LYP/6-311++G^** and MP2/6-311++G^** levels of theory. The isodesmic reactions and nuclear independent chemical shifts (NICS) calculations were utilized to examine the molecules for energetic and magnetic criteria, respectively. The isodesmic reaction energies reveal that thiophene (C ₄ H ₄ S, ?23.269 kcal/mol) and pyrrole (C ₄ H ₄ NH, ?20.804 kcal/mol) have the greatest aromatic stabilization energies and tellurophene (C ₄ H ₄ Te, ?15.114 kcal/mol) and stibole (C ₄ H ₄ SbH, ?1.169 kcal/mol) have the lowest aromatic stabilization energies in their corresponding groups at MP2/6-311++G^**. The NICS calculations confirmed the results obtained through isodesmic reaction energies.     

Theoretical predictions of the structure,gas‐phase acidity,and aromaticity of tetrathiosquaric acid

Results of ab initio self‐consistent‐field and density functional theory calculations of the gas‐phase structure, acidity (free energy of deprotonation, ΔG⁰), and aromaticity of tetrathiosquaric acid (3,4‐dithiohydroxy‐3‐cyclobutene‐1,2‐dithione, H₂C₄S₄) are reported. The global minimum found on the potential energy surface of tetrathiosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very close in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanedithione, and cyclobutenedithiol. The computed aromatic stabilization energy by homodesmotic reaction is −18.4 (MP2(fu)/6‐311+G**//RHF/6‐311+G**) and −15.1 kcal/mol (B3LYP//6‐311+G**// B3LYP/6‐311+G**). The aromaticity of tetrathiosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (Λ) −11.77 (CSGT(IGAIM)‐RHF/6‐311+G**// RHF/6‐311+G**) and −18.08 (CSGT(IGAIM)‐B3LYP/6‐311+G**// B3LYP/6‐311+G**). Thus, tetrathiosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The most reliable theoretical gas‐phase acidities are $\Delta G^{0}_{1(298\mathrm{K})}=303.7$ and $\Delta G^{0}_{2(298\mathrm{K})}=394.1$ kcal/mol. Hence, tetrathiosquaric acid is a stronger acid than squaric acid (3,4‐dihydroxy‐3‐ cyclobutene‐1,2‐dione, H₂C₄O₄). Comparisons of the computed results of tetrathiosquaric acid with squaric acid have also been made. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 443–449, 2000     

Ab initio chemical kinetics for the unimolecular decomposition of Si2H5 radical and related reverse bimolecular reactions

For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat‐CVD) processes using small silanes as precursors, disilanyl radical (Si₂H₅) is a potential reactive intermediate involved in various chemical reactions. For modeling and optimization of homogeneous a‐Si:H film growth on large‐area substrates, we have investigated the kinetics and mechanisms for the thermal decomposition of Si₂H₅ producing smaller silicon hydrides including SiH, SiH₂, SiH_3, and Si₂H₄, and the related reverse reactions involving these species by using ab initio molecular‐orbital calculations. The results show that the lowest energy path is the production of SiH + SiH₄ that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si₂H₅. Additionally, the dissociation energies for breaking the Si? Si and H? SiH₂ bonds were predicted to be 53.4 and 61.4 kcal/mol, respectively. To validate the predicted enthalpies of reaction, we have evaluated the enthalpies of formation for SiH, SiH₂, HSiSiH₂, and Si₂H₄(C_2h) at 0 K by using the isodesmic reactions, such as ²HSiSiH₂ + ¹C₂H₆→¹Si₂H₆ + ²HCCH₂ and ¹Si₂H₄(C_2h) + ¹C₂H₆ → ¹Si₂H₆ + ¹C₂H₄. The results of SiH (87.2 kcal/mol), SiH₂ (64.9 kcal/mol), HSiSiH₂ (98.0 kcal/mol), and Si₂H₄ (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si₂H₅ and the related bimolecular reverse reactions have been predicted and tabulated for different T, P‐conditions with variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory by solving the master equation. The result indicates that the formation of SiH + SiH₄ product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH₂ + SiH₃, HSiSiH₂ + H₂, and Si₂H₄(C_2h) + H under T, P‐conditions typically used in PECVD and Cat‐CVD. © 2013 Wiley Periodicals, Inc.     

The isomers of Si2H4: disilene and silylsilylene

Geometries and relative energies of the lowest singlet and triplet states of disilene and silylsilylene have been investigated using ab initio SCF and CI methods. At the most reliable level of theory employed, singlet disilene is predicted to be 8–10 kcal/mol more stable than singlet silylsilylene.     

A theoretical characterization of the structure formation enthalpy,and fluzional behaviour of B2H6 and AlBH6

Summary A comprehensive study of the binary association complexes B₂H₆ and AlBH₆ has been performed by ab initio molecular orbital theory. Reliable formation enthalpies can be computed only be extended basis sets and a reasonably complete account of correlation. The greater stability towards neutral dissociation of AlBH₆ with respect to B₂H₆ obtained at the Hartree-Fock level employing the 6-21G^* basis set ( 10 kcal/mol) is reduced to only 2 kcal/mol when the basis set is sufficiently saturated and correlation energy properly included. The value of the activation energy for hydrogen scrambling in AlBH₆ is much less sensitive to the method used, although correlation still plays a significant rôle reducing the potential energy barrier from 11.4 to 7.7 kcal/mol.     

Mechanism of pyrolysis of C2Cl6

Using published data on the kinetics of pyrolysis of C₂Cl₆ and estimated rate parameters for all the involved radical reactions, a mechanism is proposed which accounts quantitatively for all the observations: The steady-state rate law valid for after about 0.1% reaction is and the reaction is verified to proceed through the two parallel stages suggested earlier whose net reaction is A reported induction period obtained from pressure measurements used to follow the rate is shown to be compatible with the endothermicity of reaction A, giving rise to a self-cooling of the gaseous mixture and thus an overall pressure decrease. From the analysis, the bond dissociation energy DH⁰(C₂Cl₅? Cl) is found to be 70.3 ± 1 kcal/mol and ΔH_f300⁰(·C₂Cl₅) = 7.7 ± 1 kcal/mol. The resulting π? bond energy in C₂Cl₄ is 52.5 ± 1 kcal/mol.     

Structures of gas-phase C2F 7 + ions

In an ion cyclotron resonance spectrometer, less than 96% of the C₇F ₇ ⁺ cation formed on electron ionization of perfluorotoluene reacts with hexamethyldisilazane. In contrast, the C₇F ₇ ⁺ from perfluoronorbornadiene or perfluorobicyclo[3.2.O]hepta-2,6-diene is nonreactive with hexamethyldisilazane. Collision-induced dissociation results support this dichotomy, although the evidence is not as clear-cut. The reactive ion is assigned the benzyl structure and the nonreactive ion the tropyl structure, on the basis of analogy with the protio cases. By AM1 calculations, the perfluorobenzyl ion is 25 kcal/mol more stable than the perfluorotropyl ion, the opposite of the situation for the protio analogs (? 12 kcal/mol). Ab initio calculations at the 3–21G level agree with the semiempirical energy difference to within 0.4 kcal/mol; at the more appropriate 6–31G*/MP2 level, the perfluorobenzyl cation is 9.7 kcal/mol more stable than the perfluorotropyl cation.     

Does CH prefer a C2v rather than a Cs structure?

The closely related C_s ( 1 ) and C_2v ( 3 ) structures of CH have been reinvestigated at many ab initio levels using MP2/6-31G** and MP2/6-311 + + G(2df, 2pd) geometries. The largest basis sets employed were 6-311G(3df, 2p), 6-311 + + G(3df, 3pd), and the Dunning “correlation consistent” polarized triple-split valence basis set (cc-pVTZ). Electron correlation was probed at the MP4 level, but the QCISD method was also used with the largest basis sets. While electron correlation favors 3 over 1 by about 2 kcal/mol, the correlated relative energies with all basis sets employed range from 0.36–1.03 kcal/mol in favor of 1 . The best estimate of this difference, 0.86 kcal/mol, is essentially identical with the (scaled) zero-point energy difference, 0.84 kcal/mol, favoring 3 over 1 . These results indicate that 1 and 3 have almost exactly the same energy at 0 K. Our best value for the dissociation energy of CH is 42.0 kcal/mol [QCISD(T)/6-311 + + G(3df, 3pd)//MP2(fu)/6-311 + + G(2df, 2pd), corrected to 298 K], which agrees very well with the experimental value. © 1992 by John Wiley & Sons, Inc.     

Theoretical predictions of the structure, gas-phase acidity and aromaticity of 1,2-diseleno-3,4-dithiosquaric acid

Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure, acidity (free energy of deprotonation, ΔG^o), and aromaticity of 1,2-diseleno-3,4-dithiosquaric acid (3,4-dithiohydroxy-3-cyclobutene-1,2-diselenone, H₂C₄Se₂S₂) are reported. The global minimum found on the potential energy surface of 1,2-diseleno-3,4-dithiosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very close in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutenedithiol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction (Eq 1) is −20.1 kcal/mol (MP2(fu)/6-311+G** //RHF/6-311+G**) and −14.9 kcal/mol (B3LYP//6-311+G**//B3LYP/6-311+G**). The aromaticity of 1,2-diseleno-3,4-dithiosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (Λ) −17.91 (CSGT(IGAIM)-RHF/6-311+G**//RHF/6-311+G**) and −31.01 (CSGT(IGAIM)-B3LYP/6-311+G**//B3LYP/6-311+G**). Thus, 1,2-diseleno-3,4-dithiosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated theoretical gas-phase acidity is ΔG^o _1(298K)=302.7 kcal/mol and ΔG^o _2(298K)=388.4 kcal/mol. Hence, 1,2-diseleno-3,4-dithiosquaric acid is a stronger acid than squaric acid(3,4-dihydroxy-3-cyclobutene-1,2-dione, H₂C₄O₄). Received: 11 April 2000 / Accepted: 7 July 2000 / Published online: 27 September 2000     

Ab initio predictions of the molecular structures and harmonic vibrational frequencies of Ga2O2 isomers

The potential energy surface of Ga₂O₂ is examined at the SCF and MP2 levels employing basis set of triple- plus double polarization quality. Four stationary points located at the SCF level are characterized via their Hessian index. Electron correlation is important for the energy ordering and splitting of the isomers. For example, two minimum energy structures, a cyclicD _2h form and a linear Ga-O-Ga-O, separated by 25.69 kcal/mol at the SCF level have an energy difference of only 1.70 kcal/mol at the MP2 levels. Our computed harmonic vibrational frequency at 962 cm^–1 for the Ga-O-Ga-O minimum structure in in good agreement with the experimental predicted value of 967 cm^–1.     

A theoretical study of protonation of triatomic silicon–carbon compounds

Ab initio molecular orbital methods are employed to study the low-lying states of C₃H⁺, SiC₂H⁺, Si₂CH⁺, and Si₃H⁺. Special attention is paid to a comparative study between C₃H⁺ and Si₃H⁺. In both cases a ³B₂ state is found to lie the lowest at the HF level, although inclusion of correlation effects favor a linear structure (¹Σ⁺ state) for C₃H⁺, which lies 25 kcal/mol below the ³B₂ state at the MP 4 level, and a bent structure (¹A′ state) for Si₃H⁺, which lies just 2 kcal/mol below the ³B₂ state. The proton affinities of C₃, SiC₂, Si₂C, and Si₃ are estimated at different levels of theory. Both protonation at carbon and silicon atoms are considered for SiC₂ and Si₂C. It is found that C₃ comparatively has a low proton affinity. On the other hand, Si₃ has a relatively high proton affinity compared with the protonation at silicon atom for both SiC₂ and Si₂C. These results are discussed on the basis of electronic structure arguments.     

Theoretical Investigation of the C2H2B2 Potential Energy Surface

Fifteen unique energy minima and thirteen transition states on the C ₂H₂B₂ potential surface have been located and optimized at the MP2 level of theory with the 6-311G(d,p) basis set. The planar four-membered ring isomer , 1, an analog of cyclobutadiene, is a transition state lying 37 kcal/mol above the nonplanar four-membered ring , 3. The planar , 10, is the second most stable species found, lying 72.2 kcal/mol below 3. The nonplanar, butterfly-shaped ring, 4, is a local minimum 33.7 kcal/mol more stable than 3. A four-membered ring isomer with alternating boron–carbon locations, , 5, lies 67.0 kcal/mol below 3 and 33.3 kcal/mol below 4. The ring of 5 is planar with one hydrogen above and one below the plane (C _2h symmetry). The borylene-substituted boracyclopropene, , 8, is a planar local minimum lying 36.0 kcal/mol above 5. The most stable C₂H₂B₂ isomer found was the planar, four-membered ring system 22 (D _2h symmetry) composed of two BCC three-membered rings fused across the C-C bond. Structure 22 lies 22.2 kcal/mole below 10, 105.4 kcal/mol below 3, 71.7 kcal/mol below 4, and 38.2 kcal/mol below 5. Isomer 22 is the structural analog of the trialene form of C₄H₂. The most stable linear isomer, HB BH, 26, was surprisingly 50.5 kcal/mol less stable than 22. The stabilities of the two most stable cyclic isomers 10 and 22 may be explained by aromaticity.     

The aromatic diboracyclopropenyl (diboriranyl) anion; CB2H3 −: An ab initio study

The most stable structure of CB₂H₃ ^–, as established computationally, is the aromatic diboracyclopropenyl (diboriranyl) anion (5), while open-chainC _2v, isomer H₂BCBH (7) is only 3 kcal/mol higher in energy at the QCISD(T)/6-311 +G^**//MP2/6-31+G^*+ZPE (HF/6-31 +G^*). The 47-kcal/mol barrier between cyclic,5, and open-chain,7, structures suggests that both of them may be observed. The aromatic stabilization energy of the diboriranyl anion (18 kcal/mol) is half the value in the isoelectronic cyclopropenium ion, C₃H₃ ⁺. The computed, by IGLO method (5a), and experimental (6a) chemical shifts,(¹³C) and(¹¹B), agree within 4 ppm range. The theoretical vibrational frequencies of the most stable isomers,5 and7, are presented for experimental verification of these species.