The formation of (CH3)7Si2+ in (CH3)4Si/CH4 mixtures and CH3− exchange reactions between (CH3)4Si, (CH3)4Ge, and (CH3)4Sn studied by high pressure mass spectrometry

The association equilibrium [Formula: see text] has been studied in a high pressure mass spectrometer ion source using tetramethylsilane/methane mixtures. Measurement of the equilibrium constant over a range of temperatures yields ΔH 0  = −22.3 ± 0.4 kcal mol −1 and ΔS 0  = −35.2 ± 0.9 cal mol −1 K −1 . Collision-assisted dissociation experiments suggest that the methyl groups retain their integrity in (CH 3 ) 7 Si 2 + . Mixed ions such as (CH 3 ) 7 SiGe + and (CH 3 ) 7 GeSn + were not observed in mixtures of (CH 3 ) 4 X and (CH 3 ) 4 Y(X ≠ Y = Si, Ge, Sn). Instead CH 3 − transfer equilibrium reactions were observed viz. [Formula: see text] (ΔH 0  = −10.2 ± 1.2 kcal mol −1 , ΔS 0  = −3.7 ± 2.4 cal K −1  mol −1 ) and [Formula: see text], ΔS 0  = −0.9 ± 1.6 cal K −1 mol −1 . These are in excellent agreement with some published differences in appearance potentials for (CH 3 ) 3 X + from (CH 3 ) 4 X (X = Si, Ge, Sn).