Thermochemistry and Dissociative Photoionization of Si(CH₃)₄, BrSi(CH₃)₃, ISi(CH₃)₃, and Si₂(CH₃)₆ Studied by Threshold Photoelectron−Photoion Coincidence Spectroscopy^†

Threshold photoelectron−photoion coincidence spectroscopy (TPEPICO) has been used to study the dissociation kinetics and thermochemistry of Me₄Si, Me₆Si₂, and Me₃SiX, (X = Br, I) molecules. Accurate 0 K dissociative photoionization onsets for these species have been measured from the breakdown diagram and the ion time-of-flight distribution, both of them analyzed and simulated in terms of the statistical RRKM theory and DFT calculations. The average enthalpy of formation of trimethylsilyl ion, (Me₃Si⁺) = 617.3 ± 2.3 kJ/mol, has been determined from the measured onsets for methyl loss (10.243 ± 0.010 eV) from Me₄Si, and Br and I loss from Me₃SiBr (10.624 ± 0.010 eV) and Me₃SiI (9.773 ± 0.015 eV), respectively. The methyl loss onsets for the trimethyl halo silanes lead to (Me₂SiBr⁺) = 590.3 ± 4.4 kJ/mol and (Me₅Si₂⁺) = 487.6 ± 6.2 kJ/mol. The dissociative photoionization of Me₃SiSiMe₃ proceeds by a very slow Si−Si bond breaking step, whose rate constants were measured as a function of the ion internal energy. Extrapolation of this rate constant to the dissociation limit leads to the 0 K dissociation onset (9.670 ± 0.030 eV). This onset, along with the previously determined trimethylsilyl ion energy, leads to an enthalpy of formation of the trimethylsilyl radical, (Me₃Si^•) = 14.0 ± 6.6 kJ/mol. In combination with other experimental values, we propose a more accurate average value for (Me₃Si^•) of 14.8 ± 2.0 kJ/mol. Finally, the bond dissociation enthalpies (ΔH_298K) Si−H, Si−C, Si−X (X=Cl, Br, I) and Si−Si are derived and discussed in this study.