Ab initio study on the rate constants of SiCl4 + H→SiCl3 + HCl

Abstract

The reaction is studied using an ab initio dynamic method. The direct chlorine SiCl 4 ] H ] SiCl 3 ] HCl abstraction of reaction processes via a symmetry transition state is calculated by the intrinsic reaction C 3v coordinate (IRC) method at the UMP2/6-311G(d,p) level. The forward and reverse barriers are reÐned by UMP4(SDTQ) and UQCISD(T) single point energy calculations using the same basis set of 6-311G(d,p). The forward barrier is calculated to be 24.34 kcal mol~1 at the UMP4(SDTQ) level and 22.62 kcal mol~1 at the UQCISD(T) level, while the reverse barrier is 22.46 and 21.90 kcal mol~1, respectively. The enthalpy of reaction is calculated to be 1.94 kcal mol~1 at the UMP4(SDTQ) level and 0.72 kcal mol~1 at the UQCISD(T) level. The forward rate constants in the temperature range 1500È1800 K are calculated by the conventional and variational transition state theory (CVT) with small-curvature tunneling (SCT) correction. The variational e †ect is important for the calculation of forward rate constants but the SCT correction is small. The CVT and CVT/SCT rate constants of the forward reaction at the UMP4(SDTQ) and UQCISD(T) levels are consistent with the lower limit of the experimental result. However, the theoretical activation energies in the temperature range 1500È1800 K, 28.43 kcal mol~1 at the UMP4(SDTQ) level and 27.10 kcal mol~1 at the UQCISD(T) level, are much higher than the experimental value, 9.54 ^5.17 kcal mol~1. The theoretical temperature dependence of the rate constants di †ers from the experimental result.

Theoretical Reassessment and Model Validation of Some Kinetic Parameters Relevant to Si/Cl/H Systems

“…The abstraction reaction proceeds through a C3v symmetry transition state, consistent with the findings of Zhang et al 3 3 The T1 diagnostic values are rather modest, ranging from 0.006 to 0.021, for the abstraction path R1 highlighting the weak multi-reference character of TS1. The forward energy barrier is computed at 20.8 kcal mol -1 , which is on the upper range of the previously published energy barriers.…”

“…). The dashed and dotted lines depict the effect of substituting the herein calculated rate constants of reaction R1 and R2 with the published values of Zhang et al3 (dotted line) and Kunz and Roth 4 (dashed line), respectively.3.2.2.3. Sensitivity and rate of production analysisA brute-force sensitivity analysis has been performed to identify the most sensitive reactions.…”

“…SiCl4 + H ⇄ SiCl3 + HCl Rate constants were obtained by monitoring the H atom ARAS profile decay and assuming a pseudo first-order kinetics, without any contributions from secondary chemistry. However, since this pioneering work, two theoretical studies by Ge et al 2 and Zhang et al 3 thermal decomposition 4 suggested that the first-order assumption did not hold and thus that secondary chemistry initiated by SiCl4 decomposition may account for a large extent to the H atom consumption observed experimentally (SiCl3 + H for example) as indicated by a rate of consumption analysis. Indeed, using the experimental rate constant of Kunz and Roth for k2 and the value of Catoire et al for k1 reveals that SiCl4 is consumed for one third by thermal decomposition at 1530 K and that the abstraction reaction R1 is totally overwhelmed by the reaction R2 at higher temperatures (1725 K).…”

“…After the publication of the first (and last) kinetic experimental work, the features of reaction R1 have been investigated theoretically, spanning a large set of theoretical methods. Zhang et al 3 were the first to report a predicted rate constant, however limited to the same temperature range as the experimental data of Catoire et al 1 . The potential energy surface of the reaction was investigated based on MP2/6-311G(d,p) optimized structures and final single point energies evaluated at the UMP4(SDTQ) and UQCISD(T) level of theory with the same basis set.…”

Measurements and modeling of SiCl4 combustion in a low-pressure H2/O2 flame

“…The rate calculated by Zhang is within the error of the experimental measurement over the temperature range in which the measurements were made. Some reasons for the large difference in activation energy between the experiment and the calculation are discussed in Zhang's paper 50 Another consideration is that the basis set used in the calculations, QCISD, has been found to give higher reaction energies and barrier heights than other basis sets for reaction of chlorine with methane. 51 The value for this rate that we used in our model in order to provide the best fit to our data is closer to the experi-mental rate than to the calculated one.…”