Thermochemistry of Silaethylene and Methylsilylene from Experiment and Theory

Fourier transform ion cyclotron resonance spectroscopy has been used to examine the deprotonation energetics of the methylsilyl cation, CH₃SiD₂⁺, to yield silaethylene and methylsilylene proton affinities of 205 ± 3 and 215 ± 4 kcal/mol, respectively. These values, combined with the known heat of formation of methylsilyl cation, yield ΔH_f°₂₉₈(CH₂SiH₂) = 43 ± 3 kcal/mol and ΔH_f°₂₉₈(CH₃SiH) = 53 ± 4 kcal/mol. These results are corroborated by ab initio generalized valence bond-configuration interaction calculations which indicate that silaethylene is more stable than methylsilylene by 11.6 kcal/mol, in excellent agreement with the experimental difference (10 ± 3 kcal/mol). The adiabatic ionization potential of methylsilylene is calculated to be 8.22 eV, which is lower than the value of 8.85 eV determined for silaethylene using photoelectron spectroscopy.