Potential energy profiles have been determined for the two series
of reactions: (i) X- + HCOY, where X =
Y = H, F, or Cl, and (ii) X- + RCOX, where X = F or
Cl and R = SiH3, CH3, H, CN, or
NO2. Energies
of all stationary points, including reactants, ion−dipole complexes,
stable adducts, transition states, and products,
were evaluated at the Hartree−Fock (HF) and the second-order
Møller−Plesset (MP2) correlation levels
with the 6-311++G** for reaction series i and with the 6-31+G*
basis set for reaction series ii. The results
predict that acyl-transfer reactions can proceed through single-well,
double-well, and triple-well energy profiles
in the gas phase depending on the nucleophile, X-,
nucleofuge Y-, and acyl group R. Factors that favor
the
single-well or triple-well profile with a stable tetrahedral adduct are
(a) stronger bond formation of the C−X
or C−Y bond, (b) stronger nucleophilicity of X- and
poorer leaving ability of the nucleofuge, Y-, (c)
wide
energy gap between the two antibonding MOs,
π*C
O and
σ*C
-
X, and (d) strong electron
acceptor acyl group,
R. Whenever gas-phase experimental results are available, good
agreements were obtained with our MP2
predictions.
