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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