A kinetic study of the gas-phase reactions of 1-methylsilacyclobutane in hot wire chemical vapor deposition

Abstract

The reaction kinetics of the decomposition of 1-methylsilacyclobutane (MSCB) in a hot wire chemical vapor deposition (HWCVD) reactor was investigated. The stable reaction products were monitored using vacuum ultraviolet laser single photon ionization in tandem with time-of-flight mass spectrometry. Steady-state approximation was used to determine the rate constants of three individual decomposition pathways of MSCB, i.e., cycloreversion to form ethene and methylsilene (R1), ring opening to form propene and methylsilylene (R2), and exocyclic Si–CH₃ bond cleavage to form ˙CH₃ radicals (R3). The activation energies (E_a) for R2 and R3 in a HWCVD reactor were determined to be 86.6 kJ mol⁻¹ and 106 kJ mol⁻¹, respectively. The fact that these E_a values are close to those obtained for the MSCB decomposition on metal surfaces under collision-free conditions indicates that the heterogeneous reactions on the hot wire surface govern the gas-phase reaction kinetics in the HWCVD reactor. In addition, the E_a values obtained from a theoretical study of the decomposition kinetics using ab initio calculations at the CCSD(T)/6-311++G(3d,2p)//MP2/6-311++G(d,p) level were 62.9 kcal mol⁻¹ (i.e., 263 kJ mol⁻¹), 62.0 kcal mol⁻¹ (i.e., 259 kJ mol⁻¹), and 86.2 kcal mol⁻¹ (i.e., 361 kJ mol⁻¹) for R1, R2, and R3, respectively. The much lower experimental E_a values compared with those from the theoretical calculations clearly suggest that the tungsten filament in the HWCVD reactor catalyzed the decomposition.