Essentials of Computational Chemistry

392 11 IMPLICIT MODELS FOR CONDENSED PHASES
the energy surface that generates the gas-phase reaction coordinate may be quite different
from the one-dimensional slice that generates the reaction coordinate on the solvated surface.
Put differently, solvation can move a stationary point not only along the gas-phase reaction
coordinate but in other directions as well. Thus, if one constructs a solution coordinate by
computing solvation free energies point-by-point for the gas-phase coordinate, one may miss
important effects associated with movement off the gas-phase reaction coordinate. This is
illustrated by the example of the Claisen rearrangement of allyl vinyl ether in Figure 11.5.
Here the reaction coordinate may be thought of as the difference in distance between the
initially bonded O3 and C4 atoms and the ultimately bonded C1 and C6 atoms. In water,
aldehydes are better solvated than ethers, and this differential solvation is felt by the TS
structure, so that it shifts to the right along the reaction coordinate (new TS location not
shown). However, it also moves along an orthogonal coordinate best described by the distance
between the C1C2O3 and C4C5C6 fragments. Polar solvents interact more strongly with a
TS structure having greater interfragment separation because of the zwitterionic character
associated with this structure. This move off the reaction coordinate can significantly lower
the activation free energy.
Another situation that can complicate the interpretation of a reaction in solution by comparison
to the gas phase involves a reaction that fails to have corresponding stationary points in
the gas phase. If there is no stationary point in the gas phase, there is no real sense in talking
about the free energy of solvation of the structure that exists in solution. A good example
E
O
O
O
O
Reaction coordinate
Figure 11.5 Gas-phase reaction coordinate for the Claisen rearrangement of allyl vinyl ether
−
+
O