Ab Initio Molecular Orbital Study of the Thermochemistry and Reactions of the Chlorinated Disilenes and Their Isomers (Si₂H_nCl_4-n)

Abstract

Structures, vibrational frequencies, and energies for the chlorinated disilenes, their dibridged isomers, and the transition states connecting the chlorinated disilenes to the corresponding silylsilylenes are presented. Geometries and frequencies were obtained at the MP2/6-31G(d,p) level, and energetics were calculated at the G2, G2(MP2), MP4/6-31+G(2df,p), and/or MP2/6-31+G(2df,p) levels of theory, depending on the number of chlorine atoms in the molecule. The silylsilylene isomer with the structure H n Cl 3-n SiSiCl was found to be lowest in energy for all of the chlorinated compounds. The dibridged structures are all significantly higher in energy than the silylsilylene and disilene structures. Barriers for isomerization by H transfer and Cl transfer ranged from 5 to 18 kcal/mol and 9 to 23 kcal/mol above the disilene, respectively. Energies along paths for decomposition of the chlorinated disilenes to pairs of silylenes are presented, confirming that the reverse reactions are barrierless. Finally, energetics of various decomposition products of the Si 2 H n Cl 4-n compounds are considered. It is shown that for the species with two or more chlorines, decomposition to a pair of silylenes should be the dominant reaction path based on the energetics of competing paths. The isomerization barriers are much smaller than decomposition barriers, so the isomerization reactions will be fast compared to decomposition.

Pressure-dependent rate coefficients of chemical reactions involving Si2H4 isomerization from QRRK calculations

“…The value of E À1 was set to 16.8 kcal mol À1 in this study, supported by experimental results [40]. To our knowledge, no other experimental measurement have been carried out for evaluating the activation energy of isomerization reaction (R1), but several theoretical estimates of E 1 or E À1 have been published [5,38,39,[41][42][43], which agree generally well with those of Becerra and Walsh.…”

Numerical Modeling of Gas-Phase Nucleation and Particle Growth during Chemical Vapor Deposition of Silicon

“…For all silylene-to-silene isomerizations we have assumed a forward rate coefficient with a pre-exponential factor of 10 13 s Ϫ1 and an activation energy of 7.5 kcal/mol (31.4 kJ/mol), based on an ab initio electronic structure calculation for Si 2 H 4 B isomerization to Si 2 H 4 A. 18 Little is known experimentally about these reactions, however, at temperatures high enough for silane pyrolysis to occur these isomerization reactions are much faster than any other reactions in our mechanism. Because of this, they are near equilibrium in all of our simulations, and our results are insensitive to the selection of rate parameters for them.…”

Thermochemistry and Kinetics of Silicon Hydride Cluster Formation during Thermal Decomposition of Silane

“…[1][2][3][4][5][6][7] There is also a substantial body of work on the kinetics of gas-phase reactions of small silicon hydrides. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] On the basis of this body of research, models of thermal CVD of silicon from silane can now predict film growth rates and precursor utilization with reasonable accuracy and reliability, at least under conditions where particle formation is negligible. However, understanding of the processes that lead to gas-phase particle nucleation is still quite limited, and models for nucleation and growth of particles in this system do not have the level of predictive capability that has been achieved in modeling film growth rates and gas-phase chemical composition.…”