Pressure Dependence Of Rate Coefficients Research Articles

Pressure Dependence of Rate Coefficients of Unimolecular and Chemical Activation Reactions Connected to the Potential Energy Wells of Si2H2Cl4, Si2Cl6, and Si2Cl4 via Rice-Ramsperger-Kassel-Marcus Calculations.

Rate coefficients for elementary reactions connected to the potential energy wells of Si2H2Cl4, Si2Cl6, and Si2Cl4, which are important Si2 species in chemical vapor deposition (CVD) processes that use chlorosilanes as silicon source gases, were determined through the Rice-Ramsperger-Kassel-Marcus theory under various conditions of temperature and pressure. The optimized structures and vibrational frequencies of the reactants, products, and transition state were obtained using (U)B3LYP/6-31+G(d,p), and the single-point energies of the optimized structures were recalculated using the coupled cluster method with single and double excitations plus triple perturbation (U)CCSD(T) with complete basis set extrapolation. Many of the unimolecular decomposition channels and chemical activation reactions investigated in this work were found to be in the fall-off regime under subatmospheric to moderately high-pressure conditions so that it is expected that accurate modeling of the gas phase in the chlorosilane CVD reactor requires careful determination of the rate coefficients as functions of temperature and pressure for the conditions of interest, instead of using high-pressure limit rate coefficients. The rate coefficients determined here were expressed through Chebyshev coefficients and also modified Arrhenius parameters to be used in simulations of systems under a wide range of temperature and pressure conditions.

Pressure dependence of rate coefficients of unimolecular and chemical activation reactions connected to the potential energy wells of chlorinated monosilanes by RRKM calculations

AbstractRate coefficients for elementary reactions connected to the potential energy wells of SiHCl3, SiH2Cl2, SiHCl2, and SiH3Cl, which are important Si1 species in chemical vapor deposition (CVD) processes that use chlorosilanes as silicon source gases, were determined through Rice–Ramsperger–Kassel–Marcus (RRKM) theory for various conditions of temperature and pressure. Many of the unimolecular decomposition channels and chemical activation reactions investigated in this work were found to be in the fall‐off regime under subatmospheric to moderately high‐pressure conditions, so that it is expected that accurate modeling of the gas phase in chlorosilane CVD reactors requires careful determination of the rate coefficients as functions of temperature and pressure for the conditions of interest, instead of using high‐pressure limit rate coefficients. The rate coefficients determined here were tabulated using Chebyshev coefficients and can be used in simulations of systems under a wide range of temperature and pressure conditions.

Pressure Dependence of Rate Coefficients for Formation and Dissociation of Pentachlorodisilane and Related Chemical Activation Reactions

Four entrance channels to the pentachlorodisilane (Si2HCl5) potential energy well, namely, SiCl2 + SiHCl3, SiCl4 + SiHCl, Cl3SiSiCl + HCl, and SiCl3 + SiHCl2, were analyzed in detail through transition state theory, Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and solution of the multichannel master equation. The stationary points in the potential energy surface were optimized, and their vibrational frequencies and rotational constants calculated at the (U)B3LYP/6–31+G(d,p) level of theory; the (U)CCSD(T)/aug-cc-pVTZ level was then used for accurate estimation of activation energies. The pressure and temperature dependence of the rate coefficients of the channels related to Si2HCl5 stabilization/dissociation was determined along a wide range of conditions, for the first time. All channels showed strong pressure dependence in the four cases, at least at low-to-moderate pressure conditions. Each entrance channel leads to the formation of different products under different conditions, and the mechanism was analyzed in detail. The results indicated that at atmospheric pressure the reactions are in the falloff region, and therefore do not support the adoption of high-pressure limit rate coefficients in reaction models designed for simulation of systems at atmospheric or subatmospheric pressure conditions.

Temperature- and pressure-dependent rate coefficients for the HACA pathways from benzene to naphthalene

RRKM-Master Equation calculations have been performed to evaluate temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6H4C2H, C6H5C2H2, and C6H4C2H3 radicals. These calculations indicate a strong pressure dependence for the role of various Hydrogen-Abstraction-C2H2-Addition (HACA) sequences for the formation of naphthalene from benzene. At atmospheric and lower pressures the C8H7 radicals, C6H4C2H3 and C6H5C2H2, cannot be stabilized above 1650K. As a result, both the Bittner–Howard HACA route, in which a second acetylene molecule adds to C6H5C2H2, and the modified Frenklach route, where a second C2H2 adds to the aromatic ring of C6H4C2H3 obtained by internal hydrogen abstraction, are unrealistic under low pressure flame conditions. At the higher pressures of some practical combustion devices (e.g., 100atm) these routes may be operative. Naphthalene is predicted to be the main product of the C6H5C2H2+C2H2 and C6H4C2H3+C2H2 reactions in the entire 500–2500K temperature range independent of pressure (ignoring the issues related to the instability of C8H7 species). Frenklach's original HACA route, where the second C2H2 molecule adds to the aromatic ring activated by intermolecular H abstraction from C8H6, involves the C6H4C2H+C2H2 reaction, which is shown to predominantly form dehydrogenated species with a naphthalene core (naphthyl radicals or naphthynes) at T <2000K and diethynylbenzene at higher temperatures. The temperature and pressure dependence of rate coefficients for the various reaction channels has been analyzed and the results clearly demonstrate the importance of pressure for the reaction outcome. Thus, one must use caution when using low-pressure flame studies to validate PAH mechanisms for use in broader ranges of pressure.

97/03076 Soot formation modeling for turbulent flames

RRKM-Master Equation calculations have been performed to evaluate temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6H4C2H, C6H5C2H2, and C6H4C2H3 radicals. These calculations indicate a strong pressure dependence for the role of various Hydrogen-Abstraction-C2H2-Addition (HACA) sequences for the formation of naphthalene from benzene. At atmospheric and lower pressures the C8H7 radicals, C6H4C2H3 and C6H5C2H2, cannot be stabilized above 1650 K. As a result, both the Bittner–Howard HACA route, in which a second acetylene molecule adds to C6H5C2H2, and the modified Frenklach route, where a second C2H2 adds to the aromatic ring of C6H4C2H3 obtained by internal hydrogen abstraction, are unrealistic under low pressure flame conditions. At the higher pressures of some practical combustion devices (e.g., 100 atm) these routes may be operative. Naphthalene is predicted to be the main product of the C6H5C2H2 + C2H2 and C6H4C2H3 + C2H2 reactions in the entire 500–2500 K temperature range independent of pressure (ignoring the issues related to the instability of C8H7 species). Frenklach's original HACA route, where the second C2H2 molecule adds to the aromatic ring activated by intermolecular H abstraction from C8H6, involves the C6H4C2H + C2H2 reaction, which is shown to predominantly form dehydrogenated species with a naphthalene core (naphthyl radicals or naphthynes) at T < 2000 K and diethynylbenzene at higher temperatures. The temperature and pressure dependence of rate coefficients for the various reaction channels has been analyzed and the results clearly demonstrate the importance of pressure for the reaction outcome. Thus, one must use caution when using low-pressure flame studies to validate PAH mechanisms for use in broader ranges of pressure.

The pressure dependence of ion–molecule association rate coefficients

Two new techniques are presented for calculating the pressure dependence of ion/molecule association rates: (i) a semiclassical method of incorporating the effect of the hindered dipole rotation into an RRKM calculation of the microscopic rate coefficients k(E,J), and (ii) a solution for the master equation for unimolecular dissociation and recombination reactions, which incorporates angular momentum (J) conservation and is applicable when the moments of inertia of reactant and activated complex differ by a large amount. These techniques provide the optimal currently available means for calculating the pressure dependence of rate coefficients for ion–molecule reactions, which are highly sensitive to J-conservation effects. The method may be used for reliable estimates and fitting of experimental fall-off data. The new technique shows that Troe’s solution of the low-pressure J-conserving master equation is accurate for the nonequilibrium population distribution but overestimates (by up to a factor of 2) the rate coefficient; extensions to Troe’s method to provide more accurate analytical approximations are derived. Application is presented to the association of CH3+ with HCN. The experimental pressure dependence, which cannot be fitted without proper accounting for J conservation, is fitted using the new treatment with values of ∼125 cm−1 for the average downward energy transfer for both rotational and internal energies. The new J-conservation method is also applicable to reactions of neutrals where the moment of inertia of the activated complex is more than six times that of the stable reactant.