Dávalos Juan Z, Baer Tomas
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA.
J Phys Chem A. 2006 Jul 13;110(27):8572-9. doi: 10.1021/jp0566358.
Threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) has been used to study the dissociation kinetics and thermochemistry of Me(4)Si, Me(6)Si(2), and Me(3)SiX, (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, Delta fH(o)298K(Me(3)Si(+)) = 617.3 +/- 2.3 kJ/mol, has been determined from the measured onsets for methyl loss (10.243 +/- 0.010 eV) from Me(4)Si, and Br and I loss from Me(3)SiBr (10.624 +/- 0.010 eV) and Me(3)SiI (9.773 +/- 0.015 eV), respectively. The methyl loss onsets for the trimethyl halo silanes lead to Delta fH(o)298K(Me(2)SiBr(+)) = 590.3 +/- 4.4 kJ/mol and Delta fH(o)298K(Me(5)Si(2)(+)) = 487.6 +/- 6.2 kJ/mol. The dissociative photoionization of Me(3)SiSiMe(3) 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, Delta fH(o)298K(Me(3)Si()) = 14.0 +/- 6.6 kJ/mol. In combination with other experimental values, we propose a more accurate average value for Delta fH(o)298K(Me(3)Si()) of 14.8 +/- 2.0 kJ/mol. Finally, the bond dissociation enthalpies (DeltaH(298K)) Si-H, Si-C, Si-X (X=Cl, Br, I) and Si-Si are derived and discussed in this study.
阈值光电子-光离子符合光谱法（TPEPICO）已被用于研究四甲基硅、六甲基二硅和三甲基硅卤化物（X = Br，I）分子的解离动力学和热化学。通过分解图和离子飞行时间分布测量了这些物种精确的0 K解离光电离起始点，并根据统计RRKM理论和DFT计算对它们进行了分析和模拟。根据从四甲基硅中甲基损失（10.243±0.010 eV）以及三甲基硅溴（10.624±0.010 eV）和三甲基硅碘（9.773±0.015 eV）中溴和碘损失的测量起始点，确定了三甲基硅离子的平均生成焓，ΔfH°298K（Me3Si+） = 617.3±2.3 kJ/mol。三甲基卤代硅烷的甲基损失起始点得出ΔfH°298K（Me2SiBr+） = 590.3±4.4 kJ/mol和ΔfH°298K（Me5Si2+） = 487.6±6.2 kJ/mol。三甲基硅硅烷的解离光电离通过一个非常缓慢的Si-Si键断裂步骤进行，其速率常数作为离子内能的函数进行了测量。将该速率常数外推至解离极限得到0 K解离起始点（9.670±0.030 eV）。这个起始点，连同先前确定的三甲基硅离子能量，得出三甲基硅自由基的生成焓，ΔfH°298K（Me3Si*） = 14.0±6.6 kJ/mol。结合其他实验值，我们提出ΔfH°298K（Me3Si*）的更准确平均值为14.8±2.0 kJ/mol。最后，本研究推导并讨论了Si-H、Si-C、Si-X（X = Cl、Br、I）和Si-Si的键解离焓（ΔH298K）。
