Thermal Decomposition of Tin Tetrachloride Based on Cl- and Sn-Concentration Measurements

Abstract

The kinetics of the thermal decomposition of tin tetrachloride has been studied experimentally and theoretically. Ab initio MO calculations showed that SnCl4 finally decomposed into Sn(3P) and four chlorine atoms through four subsequent Sn−Cl bond dissociation channels. Two sets of kinetic experiments were performed using a shock tube equipped with atomic resonance absorption spectroscopy (ARAS). Chlorine atoms were first measured over the temperature range of 1250−1700 K and the total density range of 1.7 × 1018 to 8.9 × 1018 molecules cm-3. The rate coefficient for the initial reaction step, SnCl4 (+M) → SnCl3(2A1) + Cl (+M) (eq 1a), was found to be in the falloff region fairly close to the low-pressure limit under the present conditions. The second-order rate coefficient based on the Cl-atom measurements was determined to be k1a2nd = 10-5.37±0.62 exp[−(285 ± 18) kJ mol-1/RT] cm3 molecule-1 s-1 (error limits at the 2 standard deviation level). The second group of experiments was carried out by detecting tin atoms over the temperature range of 2250−2950 K and at a total density of 3.2 × 1018 molecules cm-3. The second-order rate coefficients for the subsequent reaction steps: SnCl2(1A1) (+M) → SnCl(2Π) + Cl (+M) (eq 3a) and SnCl(2Π) (+M) → Sn(3P) + Cl (+M) (eq 4a) were obtained to be k3a2nd = 10-8.36±0.86 exp[−(310 ± 42) kJ mol-1/RT] cm3 molecule-1 s-1 and k4a2nd = 10-9.50±0.78 exp[−(265 ± 40) kJ mol-1/RT] cm3 molecule-1 s-1, respectively. The Rice−Ramsperger−Kassel−Marcus (RRKM) calculations including variational transition state theory were also applied for reactions 1a and 3a. Structural parameters and vibrational frequencies of the reactants and transition states required for the RRKM calculations were obtained from the ab initio MO calculations. Energy barriers of the reactions, E0's, which are the most sensitive parameters in the calculations, were adjusted until the RRKM rate coefficients matched the observed ones. These fittings yielded E0,1a = 326 kJ mol-1 for reaction 1a and E0,3a = 368 kJ mol-1 for reaction 3a, in good agreement with the Sn−Cl bond dissociation energies of SnCl4 and SnCl2, demonstrating that the experimental data for k1a and k3a were theoretically reasonable and acceptable.