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. J. Chem. 65, 2849Chem. 65, (1987. IThe association equilibrium (CH3)4Si + (CH3)3Si+ (CH3)7SiZ+ 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 AH0 = -22.3 2 0.4 kcal mol-I and AS0 = -35.2 ? 0.9 cal mol-I K-'. 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. (CH3)3Si+ + (CH3)4Ge e (CH3)4Si + (CH3)3Ge+ (AH0 = -10.2 5 1.2 kcal mol-I, AS0 = -3.7 ? 2.4cal K -I mol-I) and (CH3)3Ge+ + (CH3)4Sn = (CH3)4Ge + (CH3)3Sn+ (AH0 = -8.1 2 0.9 kcal mol-l, AS' = -0.9 2 1.6 cal K-' mol-I).These are in excellent agreement with some published differences in appearance potentials for (CH3)3X+ from (CH3)4X (X = Si, Ge, Sn). (X + Y = Si, Ge, Sn). Toutefois, on a observC des riactions d'kquilibre de transfert de CH3-; par exemple, (CH3)3Si+ + (CH3)4Ge (CH3)4Si + (CH3)3Ge+ (AH0 = -10,2 5 1,2 kcal mol-', AS0 = -3,7 ? 2,4 cal K-' mol-') et (CH3)3Ge+ + (CH3)4Sn = (CH3)4Ge + (CH3)3Sn+ (AH0 = -8 , l t 0,9 kcal mol-l, AS0 = -0,9 2 1,6 cal K-' mol-I). Ces valeurs sont en bon accord avec des valeurs publikes pour les diffkrences dans les potentiels d'apparition des ions (CH3)3X+ ii partir des

The proton affinity (PA) of trimethylsilanol has been determined by high pressure mass spectrometry from the enthalpy change for the reaction [Formula: see text]. ΔH0 = −30.1 ± 1.9 kcal mol−1 giving PA (Me3SiOH) = 183.7 kcal mol−1. This latter value is 10 kcal mol−1 less than that of the carbon analogue, tert-butanol.

The reactions of trichloromethylium (CCl3+) with benzene and the lower alkyl aromatics (ArH) have been studied by high pressure mass spectrometry at pressures in the range 2–4 Torr and temperatures from 300 to 560 K. The only two primary products are the adduct ArHCCl3+ and ArCCl2+, which is formed by loss of HCl from the adduct. The relative yields of adduct increase with increasing number of methyl substituents on the aromatic ring (benzene → mesitylene). The disappearance of CCl3+ is kinetically second order with specific rate constants increasing from benzene to mesitylene, the latter reacting essentially at every ion–molecule collision. All rate constants are fairly large (>1010 cc molecule−1 s−1) and show negative temperature coefficients. ArCCl2+ is unreactive but ArHCCl3+ reacts further by proton transfer to ArH.

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