Browsing by Author "Lin, M. C."

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A Computational Study on the Adsorption Configurations and Reactions of Phosphorous Acid on TiO(2) Anatase (101) and Rutile (110) Surfaces
(2009-05-14)
Raghunath, P.
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Lin, M. C.
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應用化學系分子科學碩博班
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Institute of Molecular science
We report the result of a density functional theory study on the adsorption and decomposition pathways of phosphorous acid (H(3)PO(3)) on TiO(2) anatase (101) and rutile (110) surfaces. The most stable adsorption structure for H(3)PO(3) and its isomer, HP(O)(OH)(2), is a monodentate adsorption mode for the anatase surface with calculated adsorption energies 23.5 and 38.5 kcal/mol and a bidentate adsorption mode for rutile surface with 26.7 and 36.6 kcal/mol. The mechanisms for the surface reactions of these species have been explicitly elucidated with the computed potential energy surfaces. The barriers for the stepwise H(3)PO(3) H-migration to two nearby bridged O atoms reactions on anatase leads to Ti-OP(OH)O-Ti(a) + 2H-O(b)(a) with 7.9 and 6.8 kcal/mol barriers. Even lower activation barriers (1.3 and 2.9 kcal/mol) have been obtained on the rutile (110) surface for the same bond breaking modes. The intermediate Ti-OP(OH)O-Ti(a) thus formed on both surfaces can further decompose via two distinct pathways through H-migration to the P atom and H(2)O elimination to produce Ti-OP(H)(O)O-Ti(a) and Ti-OPO-Ti(a), respectively. In addition, we have calculated the adsorption and reactions of the dimer of H(3)PO(3) on both surfaces. The most noticeable difference occurs in the energy levels of the H(3)PO(3) reactions on the anatase and rutile surfaces, with the rutile being more reactive than the anatase surface. The predicted adsorption energies show that Ti-OP(OH)O-Ti(a) with two hydrogen atoms on bridged surface oxygen atoms is 47.1 kcal/mol for anatase and 42.4 kcal/mol for rutile; both are low when compared with the Ti-OB(OH)O-Ti(a) on the same surfaces, 140.1 and 134.6 kcal/mol, respectively. Our density of states analysis shows that OB(OH)O has a larger overlap with the TiO(2) surface than OP(OH)O has, favoring the former's charge transfer efficiency.
A Computational Study on the Adsorption Configurations and Reactions of Phosphorous Acid on TiO2 Anatase (101) and Rutile (110) Surfaces
(2009-05-14)
Raghunath, P.
;
Lin, M. C.
;
應用化學系分子科學碩博班
;
Institute of Molecular science
We report the result of a density functional theory study on the adsorption and decomposition pathways of phosphorous acid (H3PO3) on TiO2 anatase (101) and rutile (110) surfaces. The most stable adsorption structure for H3PO3 and its isomer, HP(O)(OH)(2), is a monodentate adsorption mode for the anatase surface with calculated adsorption energies 23.5 and 38.5 kcal/mol and a bidentate adsorption mode for rutile surface with 26.7 and 36.6 kcal/mol. The mechanisms for the surface reactions of these species have been explicitly elucidated with the computed potential energy surfaces. The barriers for the stepwise H3PO3 H-migration to two nearby bridged O atoms reactions on anatase leads to Ti-OP(OH)O-Ti(a) + 2H-O-b(a) with 7.9 and 6.8 kcal/mol barriers. Even lower activation barriers (1.3 and 2.9 kcal/mol) have been obtained on the rutile (110) surface for the same bond breaking modes. The intermediate Ti-OP(OH)O-Ti(a) thus formed on both surfaces can further decompose via two distinct pathways through H-migration to the P atom and H2O elimination to produce Ti-OP(H)(O)O-Ti(a) and Ti-OPO-Ti(a), respectively. In addition, we have calculated the adsorption and reactions of the dimer of H3PO3 on both surfaces. The most noticeable difference occurs in the energy levels of the H3PO3 reactions on the anatase and rutile surfaces, with the rutile being more reactive than the anatase surface. The predicted adsorption energies show that Ti-OP(OH)O-Ti(a) with two hydrogen atoms on bridged surface oxygen atoms is 47.1 kcal/mol for anatase and 42.4 kcal/mol for rutile; both are low when compared with the Ti-OB(OH)O-Ti(a) on the same surfaces, 140.1 and 134.6 kcal/mol, respectively. Our density of states analysis shows that OB(OH)O has a larger overlap with the TiO2 surface than OP(OH)O has, favoring the former's charge transfer efficiency.
A computational study on the adsorption configurations and reactions of SiHx(x=1-4) on clean and H-covered Si(100) surfaces
(2016-11-30)
Le, Thong N-M
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Raghunath, P.
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Huynh, Lam K.
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Lin, M. C.
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應用化學系
Possible adsorption configurations of H and SiHx (x= 1 4) on clean and H-covered Si(100) surfaces are determined by using spin-polarized DFT calculations. The results show that, on the clean surface, the gas-phase hydrogen atom and SiH3 radicals effectively adsorb on the top sites, while SiH and SiH2 prefer the bridge sites of the first layer. Another possibility for SiH is to reside on the hollow sites with a triple-bond configuration. For a partially H-coverd Si(100) surface, the mechanism is similar but with higher adsorption energies in most cases. This suggests that the surface species become more stable in the presence of surface hydrogens. The minimum energy paths for the adsorption/migration and reactions of H/SiHx species on the surfaces are explored using the climbing image-nudged elastic band method. The competitive surface processes for Si thin-film formation from SiHx precursors are also predicted. The study reveals that the migration of hydrogen adatom is unimportant with respect to leaving open surface sites because of its high barriers (>29.0 kcal/mol). Alternatively, the abstraction of hydrogen adatoms by H/SiHx radicals is more favorable. Moreover, the removal of hydrogen atoms from adsorbed SiHx, an essential step for forming Si layers, is dominated by abstraction rather than the decomposition processes. (C) 2016 Elsevier B.V. All rights reserved.
A computational study on the decomposition of NH4ClO4: Comparison of the gas-phase and condensed-phase results
(2006-11-24)
Zhu, R. S.
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Lin, M. C.
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交大名義發表
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National Chiao Tung University
The decomposition of NH4ClO4 (AP) in both gaseous and condensed phases has been studied at the CCSD(T)/6-311 + G(3df,2p)// B3LYP/6-311 + G(3df,2p) level. Formation of NH3 + HClO3 is the main channel in the gas phase with 14.2 kcal/mol enthalpy change. In solution, the enthalpy change for this channel is 29.5 kcal/mol, which is close to the sublimation activation energy, similar to 30 kcal/mol in crystalline AP. Formation of H3N-HO +ClO3 has higher enthalpy changes, 56.3 kcal/mol and 77.1 kcal/mol in the gas phase and in solution, respectively. Strong solvent effects may exist in the AP decomposition reaction. (c) 2006 Elsevier B.V. All rights reserved.
A computational study on the energetics and mechanisms for the dissociative adsorption of SiHx(x=1-4) on W(111) surface
(2016-01-30)
Lin, Y. H.
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Raghunath, P.
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Lin, M. C.
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應用化學系
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Department of Applied Chemistry
The adsorption and dissociation mechanisms of SiHx(x = 1-4) species on W(111) surface have been investigated by using the periodic density functional theory with the projector-augmented wave approach. The adsorption of all the species on four surface sites: top (T), bridge (B), shallow (S), and deep (D) sites have been analyzed. For SiH4 on a top site, T-SiH4(a), it is more stable with an adsorption energy of 2.6 kcal/mol. For SiH3, the 3-fold shallow site is most favorable with adsorption energy of 46.0 kcal/mol. For SiH2, its adsorption on a bridge site is most stable with 73.0 kcal/mol binding energy, whereas for SiH and Si the most stable adsorption configurations are on 3-fold deep sites with very high adsorption energies, 111.8 and 134.7 kcal/mol, respectively. The potential energy surfaces for the dissociative adsorption of all SiH species on the W(111) surface have been constructed using the CINEB method. The barriers for H-atom migration from SiHx(a) to its neighboring W atoms, preferentially on B-sites, were predicted to be 0.4, 1.0, 4.5 and, 8.0 kcal/mol, respectively, for x= 4, 3, 2, and 1, respectively. The adsorption energy of the H atom on a bridge site on the clean W(1 1 1) surface was predicted to be 65.9 kcal/mol, which was found to be slightly affected by the co-adsorption of SiHx-1 within +/- 1 kcal/mol. (C) 2015 Elsevier B.V. All rights reserved.
A computational study on the kinetics and mechanism for the unimolecular decomposition of o-nitrotoluene
(2006-08-24)
Chen, S. C.
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Xu, S. C.
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Diau, E.
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Lin, M. C.
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應用化學系分子科學碩博班
The kinetics and mechanism for the unimolecular decomposition of o-nitrotoluene (o-CH3C6H4NO2) have been studied computationally at the G2M(RCC, MP2)//B3LYP/6-311G(d, p) level of theory in conjunction with rate constant predictions with RRKM and TST calculations. The results of the calculations reveal 10 decomposition channels for o-nitrotoluene and its six isomeric intermediates, among them four channels give major products: CH3C6H4 + NO2, C6H4C(H) ON (anthranil) + H2O, CH3C6H4O (o-methyl phenoxy) + NO, and C6H4C(H-2) NO + OH. The predicted rate constants in the 500-2000 K temperature range indicate that anthranil production, taking place initially by intramolecular H-abstraction from the CH3 group by NO2 followed by five-membered ring formation and dehydration, dominates at temperatures below 1000 K, whereas NO2 elimination becomes predominant above 1100 K and CH3C6H4O formation by the nitro-nitrite isomerization/decomposition process accounts for only 5-11% of the total product yield in the middle temperature range 800-1300 K. The branching ratio for CH2C6H4NO formation by the decomposition process of CH2C6H4N(O)OH is negligible. The predicted high-pressure-limit rate constants with the rate expression of 4.10 x 10(17) exp[-37000/T] s(-1) for the NO2 elimination channel and 9.09 x 10(12) exp[-25800/T] s(-1) for the H2O elimination channel generally agree reasonably with available experimental data. The predicted high-pressure-limit rate constants for the NO and OH elimination channels are represented as 1.49 x 10(14) exp[-30000/T] and 1.31 x 10(15) exp[-38000/T] s(-1), respectively.
A first-principles analysis for sulfur tolerance of CeO2 in solid oxide fuel cells
(2007-07-26)
Chen, Hsin-Tsung
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Choi, YongMan
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Liu, Meilin
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Lin, M. C.
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應用化學系分子科學碩博班
The mechanism for H2S-CeO2(111) interactions in solid oxide fuel cells (SOFCs) has been investigated by using periodic density functional theory (DFT) calculations. In order to properly characterize the effect of the localization of Ce-4f states on the interactions, DFT + U calculations were applied. Adsorption of H2S, SH, and atomic S was initially examined to locate energetically favorable intermediates. The species adsorb favorably at the Ce-top, O-top, and Ce-O bridging sites, respectively. Potential energy profiles for the H2S-CeO2 (111) interactions along the three product channels producing H-2, H2O, and SO2 were constructed using the nudged elastic band (NEB) method. Calculations show that H2S weakly bounds on CeO2(111) with a small binding energy, followed by dehydrogenation processes, forming surface sulfur species with an exothermicity of 29.9 kcal/mol. Molecular-level calculations demonstrated that the SO2-forming pathway is energetically most favorable.
A model study on the mechanism and kinetics for reactions of the hydrated electron with H3O+ and NH4+ ions
(2019-09-16)
Le Huyen, Trinh
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Pham, Tien, V
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Minh Tho Nguyen
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Lin, M. C.
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交大名義發表
Mechanism and kinetics of the reactions of the hydrated electron (e(aq)(-)) with H3O+ and NH4+ cations were determined using quantum chemical computations with both density functional theory (B3LYP) and wavefunction (MP2 and CCSD(T)) methods and the 6-311 + + G(3df,2p) basis set, in conjunction with a PCM method for treating structures in solution. Although both reactions occur with a hydrogen atom transfer, their mechanisms differ from each other by kinetic results. While the reaction of e(aq)(-) and H3O+ in solution is diffusion-controlled, NH4+ reacts with e(aq)(-) via a transition structure with H-tunneling. The predicted rate constants agree well with available experimental results.
A novel and facile decay path of Criegee intermediates by intramolecular insertion reactions via roaming transition states
(2015-03-28)
Trong-Nghia Nguyen
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Putikam, Raghunath
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Lin, M. C.
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應用化學系
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應用化學系分子科學碩博班
We have discovered a new and highly competitive product channel in the unimolecular decay process for small Criegee intermediates, CH2OO and anti/syn-CH3C(H)OO, occurring by intramolecular insertion reactions via a roaming-like transition state (TS) based on quantum-chemical calculations. Our results show that in the decomposition of CH2OO and anti-CH3C(H)OO, the predominant paths directly produce cis-HC(O)OH and syn-CH3C(O)OH acids with >110 kcal/mol exothermicities via loose roaming-like insertion TSs involving the terminal O atom and the neighboring C-H bonds. For syn-CH3C(H)OO, the major decomposition channel occurs by abstraction of a H atom from the CH3 group by the terminal O atom producing CH2C(H)O-OH. At 298 K, the intramolecular insertion process in CH2OO was found to be 600 times faster than the commonly assumed ring-closing reaction. (c) 2015 AIP Publishing LLC.
A selective acetate anion binding receptor: participation via cationic CH3 donors
(2015-10-28)
Pandian, Thiravidamani Senthil
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Srinivasadesikan, Venkatesan
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Lin, M. C.
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Kang, Jongmin
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應用化學系
A novel receptor 1, which has only C-H hydrogen bonds as recognition element has been designed and synthesized. The receptor 1 utilizes cationic CH3 in 2-methyl benzimidazole and naphthalene benzylic C-H as hydrogen bonding donor. The binding cavity formed in receptor 1 prefers to accommodate small negatively charged atom such as oxygen. Therefore, acetate and nitrite show strong affinity for receptor 1 in acetonitrile due to negatively charged oxygen in them. In addition, the order of binding affinity for halides are Br->Cl->I-. These results reflect the size and basicities of halides. Further, H-1 NMR titration of halide clearly indicates the anion-pi interaction. The experimental data from UV-vis, fluorescence and 1H NMR titration are consistent with DFT results. (C) 2015 Elsevier Ltd. All rights reserved.
A theoretical study of surface reduction mechanisms of CeO2 (111) and (110) by H-2
(2007-04-23)
Chen, Hsin-Tsung
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Choi, Yong Man
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Liu, Meilin
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Lin, M. C.
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應用化學系分子科學碩博班
Reaction mechanisms for the interactions between CeO2(111) and (110) surfaces are investigated using periodic density functional theory (DFT) calculations. Both standard DFT and DFT+U calculations to examine the effect of the localization of Ce4f states on the redox chemistry of H-2-CeO2 interactions are described. For mechanistic studies, molecular and dissociative local minima are initially located by placing an H-2 molecule at various active sites of the CeO2 surfaces. The binding energies of physisorbed species optimized using the DFT and DFT+U methods are very weak. The dissociative adsorption reactions producing hydroxylated surfaces are all exothermic; exothermicities at the DFT level range from 4.1 kcalmol(-1) for the (111) to 26.5 kcalmol(-1) for the (110) surface, while those at the DFT+U level are between 65.0 kcalmol(-1) for the (111) and 81.8kcalmol(-1) for the (110) surface. Predicted vibrational frequencies of adsorbed OH and H2O species on the surfaces are in line with available experimental and theoretical results. Potential energy profiles are constructed by connecting molecularly adsorbed and dissociatively adsorbed intermediates on each CeO2 surface with tight transition states using the nudged elastic band (NEB) method. It is found that the U correction method plays a significant role in energetics, especially for the intermediates of the exit channels and products that are partially reduced. The surface reduction reaction on CeO2(110) is energetically much more favorable. Accordingly, oxygen vacancies are more easily formed on the (110) surface than on the (111) surface.
Ab Initio Chemical Kinetic Study for Reactions of H Atoms with SiH(4) and Si(2)H(6): Comparison of Theory and Experiment
(2010-01-14)
Wu, S. Y.
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Raghunath, P.
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Wu, J. S.
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Lin, M. C.
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機械工程學系
The reactions of hydrogen atom with silane and disilane are relevant to the understanding of catalytic chemical vapor deposition (Cat-CVD) and plasma enhanced chemical vapor deposition (PECVD) processes. In the present study, these reactions have been investigated by means of A initio molecular-orbital and transition-state theory calculations. In both reactions, the most favorable pathway was found to be the H abstraction leading to the formation of SiH(3) and Si(2)H(5) products, with 5.1 and 4.0 kca/mol barriers, respectively. For H + Si(2)H(6), another possible reaction pathway giving SiH(3) + SiH(4) may take place with two different mechanisms with 4.3 and 6.7 kcal/mol barriers for H-atom attacking side-way and end-on, respectively. To validate the calculated energies of the reactions, two isodesmic reactions, SiH(3)+CH(4)-> SiH(4)+CH(3) and Si(2)H(5)+C(2)H(6)-> Si(2)H(6)+C(2)H(5) were employed; the predicted heats of the formation for SiH(3) (49.0 kcal/mol) and Si(2)H(5) (58.6 kcal/mol) were found to agree well with the experimental data. Finally, rate constants for both H-abstraction reactions predicted in the range of 290-2500 K agree well with experimental data. The result also shows that H+Si(2)H(6) producing H(2)+Si(2)H(5) is more favorable than SiH(3)+SiH(4).
Ab Initio Chemical Kinetic Study for Reactions of H Atoms with SiH4 and Si2H6: Comparison of Theory and Experiment
(2010-01-14)
Wu, S. Y.
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Raghunath, P.
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Wu, J. S.
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Lin, M. C.
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機械工程學系
The reactions of hydrogen atom with silane and disilane are relevant to the understanding of catalytic chemical vapor deposition (Cat-CVD) and plasma enhanced chemical vapor deposition (PECVD) processes. In the present study, these reactions have been investigated by means of A initio molecular-orbital and transition-state theory calculations. In both reactions, the most favorable pathway was found to be the H abstraction leading to the formation of SiH3 and Si2H5 products, with 5.1 and 4.0 kca/mol barriers, respectively. For H + Si2H6, another possible reaction pathway giving SiH3 + SiH4 may take place with two different mechanisms with 4.3 and 6.7 kcal/mol barriers for H-atom attacking side-way and end-on, respectively. To validate the calculated energies of the reactions, two isodesmic reactions, SiH3+CH4 -> SiH4+CH3 and Si2H5+C2H6 -> Si2H6+C2H5 were employed; the predicted heats of the formation for SiH3 (49.0 kcal/mol) and Si2H5 (58.6 kcal/mol) were found to agree well with the experimental data. Finally, rate constants for both H-abstraction reactions predicted in the range of 290-2500 K agree well with experimental data. The result also shows that H+Si2H6 producing H-2+Si2H5 is more favorable than SiH3+SiH4.
Ab Initio Chemical Kinetics for H plus NCN: Prediction of NCN Heat of Formation and Reaction Product Branching via Doublet and Quartet Surfaces
(2013-07-18)
Teng, Wen-Shuang
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Moskaleva, Lyudmila V.
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Chen, Hui-Lung
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Lin, M. C.
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應用化學系
The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (CCO)-C-3 + N-2 = (NCN)-N-3 + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN -> CH + N-2 (k(P1)), HCN + N-4 (k(QP1)), and HNC + N-4 (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N-2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by k(r) = 4.01 x 10(-15) T-0.90 exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.
Ab initio chemical kinetics for hypergolic reactions of nitrogen tetroxide with hydrazine and methyl hydrazine
(2019-09-01)
Trinh Le Huyen
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Raghunath, Putikam
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Lin, M. C.
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交大名義發表
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應用化學系
The kinetics and mechanisms for the hypergolic reactions of N2O4 (NTO) with N2H4 and CH3NHNH2 have been investigated by ab initio molecular orbital theory based on the UCCSD(T) method with the 6-311 + G(3df, 2p) basis set. These reactions are important to the propulsion chemistry of the N2O4-N2H3X (X = H, CH3) propellant systems. The results of our calculations show that the hypergolic ignition reactions of NTO with N2H4 and CH3NHNH2, producing N2H3NO + HNO3 and CH3NHN(H)NO/CH3N(NO)NH2 + HNO3, respectively, can be initiated by the rapid attack of hydrazine molecules by ONONO2, following the isomerization of NTO via loose, roaming-like transition states with 5.9 and 6.3 kcal/mol above the reactants in the presence of the hydrazine reactants as spectators. The rate constants for the reactions have been predicted in the temperature range 298-2000 K with the transition state theory; the result indicates, for example, that the half-life of N2O4 at 300 K in a mixture containing an excess amount of N2H4 at atmospheric pressure was predicted to be as short as 1.4 x 10(-3) s; similarly, a 50-50 mixture of N2O4 and N2H4 at atmospheric pressure decays by half within 4.1 x 10(-5) s upon mixing at 300 K, both reflecting the very effective hypergolic initiation of these propellant systems.
Ab Initio Chemical Kinetics for SiH(3) Reactions with Si(x)H(2x+2) (x=1-4)
(2010-12-30)
Raghunath, P.
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Lin, M. C.
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應用化學系分子科學碩博班
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Institute of Molecular science
Gas-phase kinetics and mechanisms of SiH(3) reactions with SiH(4), Si(2)H(6), Si(3)H(8), and Si(4)H(10), processes of relevance to a-Si thin-film deposition, have been investigated by ab initio molecular orbital and transition-state theory (TST) calculations. Geometric parameters of all the species involved in the title reactions were optimized by density functional theory at the B3LYP and BH&HLYP levels with the 6-311++G(3df,2p) basis set. The potential energy surface of each reaction was refined at the CCSD(T)/6-311++G(3df,2p) level of theory. The results show that the most favorable low energy pathways in the SiH3 reactions with these silanes occur by H abstraction, leading to the formation of SiH(4) + Si(x)H(2x+1)(silanyl) radicals. For both Si(3)H(8) and n-Si(4)H(10) reactions, the lowest energy barrier channels take place by secondary Si-H abstraction, yielding SiH(4) + s-Si(3)H(7) and SiH(4) + s-Si(4)H(9), respectively. In the i-Si(4)H(10) reaction, tertiary Si-H abstraction, has the lowest barrier producing SiH(4) + t-Si(4)H(9). In addition, direct SiH(3)-for-X substitution reactions forming Si(2)H(6) + X (X = H or silanyls) can also occur, but with significantly higher reaction barriers. A comparison of the SiH(3) reactions with the analogous CH(3) reactions with alkanes has been made. The rate constants for low-energy product channels have been calculated for the temperature range 300-2500 K by TST with Eckart tunneling corrections. These results, together with predicted heats of formation of various silanyl radicals and Si(4)H(10) isomers, have been tabulated for modeling of a-Si:H film growth by chemical vapor deposition.
Ab Initio Chemical Kinetics for SiH2 + Si2H6 and SiH3 + Si2H5 Reactions and the Related Uninnolecular Decomposition of Si3H8 under a-Si/H CVD Conditions
(2013-10-24)
Raghunath, P.
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Lin, M. C.
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應用化學系
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Department of Applied Chemistry
The kinetics and mechanisms for SiH2 + Si2H6 and SiH3 + Si2H5 reactions and the related unimolecular decomposition of Si3H8 have been investigated by ab initio molecular orbital theory based on the QCISD(T)/CBS//QCISD/6-311++G(d,p) method in conjunction with quantum statistical variational Rice-Ramsperger-Kassel-Marcus (RR.KM) calculations. For the barrierless radical association processes, their variational transition states have been characterized by the CASPT2//CASSCF method. The species involved in the study are known to coexist under CVD conditions. The results show that the association reaction of SiH2 and Si2H6 producing Si3H8 occurs by insertion via its lowest-energy path forming a loose hydrogen-bonding molecular complex with 8.3 kcal/mol binding energy; the reaction is exothermic by 55.0 kcal/mol. The chemically activated Si3H8 adduct can fragment by several paths, producing SiH4 + SiH3SiH (-0.7 kcal/mol), Si(SiH3)(2) + H-2 (-1.4 kcal/mol), and SiH3SiH2SiH + Hy (-1.4 kcal/mol). The predicted enthalpy changes as given agree well with available thermochemical data. Three other decomposition channels of Si3H8 occurring by Si-H or Si-Si breaking were found to be highly endothermic, and the reactions take place without a well-defined barrier. The heats of formation of Si3H8, SiH2SiH, Si2H4, n-Si3H7, Si(SiH3)(2), and SiH3SiH2SiH have been predicted and found to be in close agreement with those available data in the literature. The product branching rate constants for SiH2 + Si2H6 and SiH3 + Si2H3 reactions and the thermal unimolecular decomposition of Si3H8 for all low-energy paths have been calculated with multichannel variational RRKM theory covering varying P,T conditions typically employed in PECVD and Cat-CVD processes for hydrogenated amorphous silicon (a-Si/H) film growth. The results were also found to be in good agreement with available kinetic data. Our kinetic results may be employed to model and control very large-area a-Si/H film growth for a new generation of solar cell applications.
Ab Initio Chemical Kinetics for SiH3 Reactions with SixH2x+2 (x=1-4)
(2010-12-30)
Raghunath, P.
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Lin, M. C.
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應用化學系分子科學碩博班
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Institute of Molecular science
Gas-phase kinetics and mechanisms of SiH3 reactions with SiH4, Si2H6, Si3H8, and Si4H10, processes of relevance to a-Si thin-film deposition, have been investigated by ab initio molecular orbital and transition-state theory (TST) calculations. Geometric parameters of all the species involved in the title reactions were optimized by density functional theory at the B3LYP and BH&HLYP levels with the 6-311++G(3df,2p) basis set. The potential energy surface of each reaction was refined at the CCSD(T)/6-311++G(3df,2p) level of theory. The results show that the most favorable low energy pathways in the SiH3 reactions with these silanes occur by H abstraction, leading to the formation of SiH4 + SixH2x+1(silanyl) radicals. For both Si3H8 and n-Si4H10 reactions, the lowest energy barrier channels take place by secondary Si-H abstraction, yielding SiH4 + s-Si3H7 and SiH4 + s-Si4H9, respectively. In the i-Si4H10 reaction, tertiary Si-H abstraction, has the lowest barrier producing SiH4 + t-Si4H9. In addition, direct SiH3-for-X substitution reactions forming Si2H6 + X (X = H or silanyls) can also occur, but with significantly higher reaction barriers. A comparison of the SiH3 reactions with the analogous CH3 reactions with alkanes has been made. The rate constants for low-energy product channels have been calculated for the temperature range 300-2500 K by TST with Eckart tunneling corrections. These results, together with predicted heats of formation of various silanyl radicals and Si4H10 isomers, have been tabulated for modeling of a-Si:H film growth by chemical vapor deposition.
Ab Initio Chemical Kinetics for SiHx Reactions with Si2Hy (x=1,2,3,4; y=6,5,4,3; x plus y=7) under a-Si:H CVD Condition
(2017-03-01)
Trong-Nghia Nguyen
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Lin, M. C.
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應用化學系
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Department of Applied Chemistry
Gas-phase reactions of SiHx with Si2Hy (x = 1,2,3,4; y = 6,5,4,3) species, respectively, which may coexist under chemical vapor deposition (CVD) conditions, have been investigated by means of ab initio molecular orbital and statistical theory calculations. Potential energy surface (PES) predicted at the CCSD(T)/CBS//B3LYP/6-311++G(3df,2p) level shows that these reactions take place primarily via trisilany radicals, n-Si3H7 and i-Si3H7. For example, SiH2 can associate with Si2H5 producing n-H2SiSiH2SiH3 exothermically by 55.8 kcal/mol; SiH3 can undergo addition to H2SiSiH2 to produce n-Si3H7 or associate with H3SiSiH barrierlessly forming i-Si3H7; whereas SiH can insert into one of the SiH bonds of Si2H6 to give excited n-Si3H7. Similarly, H2SiSiH and SiSiH3 can undergo insertion reactions with SiH4 producing n/i-Si3H7 intermediates, respectively, to be followed by fragmentation to various smaller species. These processes are fully depicted in the complete PES. The predicted heats of formation of various species agree well with available thermochemical data. The rate constants and product branching ratios for the low-energy channel products have been calculated for the temperature range 300-1000 K by variational RRKM (Rice-Ramsperger-Kassel-Macus) theory with Eckart tunneling corrections. The results may be employed for realistic kinetic modeling of the plasma-enhanced chemical vapor deposition growth of a-Si:H thin films under practical conditions. (C) 2017 Wiley Periodicals, Inc.
Ab Initio Chemical Kinetics for Singlet CH(2) Reaction with N(2) and the Related Decomposition of Diazomethane
(2010-04-22)
Xu, Shucheng
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Lin, M. C.
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應用化學系分子科學碩博班
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Institute of Molecular science
The kinetics and mechanism for the reaction of singlet state CH(2) with N(2) have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of the reactions has been calculated by single-point calculations at the CCSD(T)/6-311+G(3df,2p) level based on geometries optimized at the B3LYP/6-311+G(3df,2p) level. By comparing the differences in the predicted heats of reaction with the available experimental values, we estimate the uncertainties in the calculated heats of reactions are +/- 1.4 kcal/mol. Rate constants for various product channels in the temperature range of 300-3000 K are predicted by the variational transition state and RRKM theories. The predicted total rate constants for (1)CH(2) + N(2) at 760 Torr Ar pressure can be represented by the expressions s-k(total) = 9.67 x 10(+7) x T(-6.88) exp (-1345/T) cm(3) molecule(-1) s(-1) at T = 300-2400 K and 3.15 x 10(-229) x T(+56.18) exp (128 000/T) cm(3) molecule(-1) s(-1) at T = 2400-3000 K. The branching ratios of the primary channels for (1)CH(2) + N(2) are predicted: k(1) for forming singlet s-CH(2)N(2)-a (diazomethane) accounts for 0.97-0.01, k(2) + k(4) for producing HCNN-a + H accounts for 0.00-0.69, k(3) for forming singlet s-CH(2)N(2)-b (3H-diazirine) accounts for 0.03-0.00, k(5) for producing HCN + NH accounts for 0.00-0.18, and k(6) for producing CNNH + H accounts for 0.00-0.11 in the temperature range of 300-3000 K. The rate constant predicted for the unimoclecular decomposition of diazomethane producing (1)CH(2) + N(2) agrees closely with experimental results. Because of the low stability of the two isomeric CH(2)N(2) adducts and the high barriers for production of CN-containing products, the contribution of the CH(2) + N(2) reaction to NO formation becomes very small.