The translational energy dependence of several gas-phase SN2 and carbonyl addition−elimination reaction rate constants has been measured in an FT-ICR. The relative energy dependences of the reactions are qualitatively consistent with variations in the reaction potential surfaces that are inferred from their thermal rate constants, which comprise a wide range of reaction efficiencies. Furthermore, the observed energy dependence of each reaction agrees quantitatively with RRKM calculations of the reaction rate constants. We conclude that, although nonstatistical dynamics are observed in some SN2 reactions, statistical reaction rate theory is, in general, an accurate means of interpreting the translational energy dependence of bimolecular ion−molecule reaction rate constants.

The electron affinity of the bis(dimethylphosphino)methyl radical was measured to be 35.3 ± 0.2 kcal/mol, using electron photodetachment spectroscopy in an ion cyclotron resonance spectrometer. Using equilibrium measurements, of bis(dimethylphosphino)methane and bis(trimethylsilyl)methane was determined to be 370 ± 3 and 373 ± 3 kcal/mol, respectively. From measured and known electron affinities and gas-phase acidities, we derive C−H bond dissociation energies:  bis(dimethylphosphino)methane, 92 ± 3 kcal/mol, and bis(trimethylsilyl)methane, 95 ± 3 kcal/mol. of trimethylphosphine was bracketed at 383−387 kcal/mol. The α-stabilization effect of silyl and phosphino substitution is large and comparable in size to stabilization by thio and chloro substitution. Possible mechanisms of stabilization are discussed.

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