American Chemical Society
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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

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journal contribution
posted on 2006-07-13, 00:00 authored by Juan Z. Dávalos, Tomas Baer
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 (ΔH298K) Si−H, Si−C, Si−X (X=Cl, Br, I) and Si−Si are derived and discussed in this study.