ESTIMATION OF THE ARRHENIUS PARAMETERS FOR SIH4 REVERSIBLE SIH2+H-2 AND DELTA-HFO (SIH2) BY A NONLINEAR-REGRESSION ANALYSIS OF THE FORWARD AND REVERSE REACTION-RATE DATA

We have extended a calculational procedure for the estimation of Arrhenius parameters for unimolecular reactions by direct regression of RRKM-derived predictions of experimental data to include nonlinear regression of experimental data for the reverse reaction. Both the forward and reverse reaction experimental data may then be used to access the validity of kinetic parameters assumed in the RRKM model. Using recent direct measurements of the rate of SiH2 insertion into H-2, a recent SiH4 pyrolysis study in a static reactor, and earlier shock tube and static reactor SiH4 pyrolysis studies, estimates for the high-pressure Arrhenius parameters for the SiH4 decomposition reaction are obtained. The RRKM model is a good representation of the data over a 900 K temperature range, at pressures from 1 to 4000 Torr. The 1100 K Arrhenius expression is: log k(T) (s-1) = (15.79 +/- 0.5) - (59.99 +/- 2.0) kcal mol-1/2.3RT. The use of a correction factor to the density of states due to the effects of a anharmonicity is seen to lead to improvement in the agreement with the data. The large uncertainty in the preexponential was due to the existence of a range of permissible A factors which provided adequate fits. Since the use of the detailed balance principle necessitates estimation of the thermodynamic equilibrium constant, we have also determined estimates for DELTA-H(f)-degrees(SiH2) consistent with the above high-pressure Arrhenius parameters. DELTA-H(f)-degrees (SiH2) is predicted to be 65.5 +/- 1.0 kcal mol-1. The current thermal kinetic experimental data precludes a more accurate determination of DELTA-H(f)-degrees(SiH2) due to the uncertainties in the temperature extrapolation of the high-pressure rate constant and in the bath gas collision efficiencies.