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

Abstract

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 ΔH0 = −22.3 ± 0.4 kcal mol−1 and ΔS0 = −35.2 ± 0.9 cal mol−1 K−1. Collision-assisted dissociation experiments suggest that the methyl groups retain their integrity in (CH3)7Si2+. Mixed ions such as (CH3)7SiGe+ and (CH3)7GeSn+ were not observed in mixtures of (CH3)4X and (CH3)4Y(X ≠ Y = Si, Ge, Sn). Instead CH3− transfer equilibrium reactions were observed viz. [Formula: see text] (ΔH0 = −10.2 ± 1.2 kcal mol−1, ΔS0 = −3.7 ± 2.4 cal K−1 mol−1) and [Formula: see text], ΔS0 = −0.9 ± 1.6 cal K−1 mol−1. These are in excellent agreement with some published differences in appearance potentials for (CH3)3X+ from (CH3)4X (X = Si, Ge, Sn).