constraints, true transition states were not obtained); solution reactions of these
kinds are known to be up to 10^20 times slower than in the gas phase. Also, in con-
trast to gas-phase conditions, substitution was favored over elimination. The con-
clusion was that “the inclusion of a few solvent molecules in the quantum
mechanical treatment can significantly improve the theoretical description of
some condensed-phase characteristics.”
2.Hydrolysis of CH 3 Cl with 13 explicit water molecules.Yamataka and Aida used
ab initio calculations to study the reaction of chloromethane (methyl chloride)
with water, solvating the reactants with up to 13 water molecules [ 15 ]. With
three or with 13 solvent molecules “three important stationary points” were
located: two “complexes” and a transition state. These were a solvated CH 3 Cl
molecule (complex 1), the transition state, and solvated products (complex 2),
the latter being methanol and HCl (when 3 H 2 O were used) or methanol,
chloride ion and H 3 O+(when 13 H 2 O were used). Note that these so-called
complexes are not the same kind of species called complexes in the gas-phase
reaction (see Continuum solvation, below). With 13 water molecules the
transition state was surrounded by all the solvent molecules with, apparently,
no vacant spaces, and the reaction energetics and secondary deuterium effects
were reproduced well. Compared to the two “complexes”, the transition state
was strongly stabilized by solvation: with 13 H 2 O the relative energies were
complex 1, transition state, complex 2¼0, 24.04,"1.59 kcal mol"^1 , i.e. 0, 101
kJ,"6.7 kJ mol"^1. The authors point out that the stationary points they found
are probably not unique: various configurations of reacting species, starting
with CH 3 Cl and water and ending with CH 3 OH, Cl"and H 3 O+, may lie along
the reaction pathway.
An important feature of this reaction is that a bond to the solvent is made: in
forming CH 3 OH a proton is transferred from the oxygen that bonds to carbon, onto
a water molecule, giving H 3 O+. This is nicely reproduced with 13 H 2 O, but cannot
be modelled with continuum methods since these essentially adjust the electron
distribution in a cavity-ensconced molecule without breaking or making bonds. The
authors concluded that “apparently the 13 water system produced a reasonable
picture of the hydrolysis.”
Continuum solvation, implicit solvation. This is called implicit because a con-
tinuous medium, a continuum, is used to “imply” the presence of individual solvent
molecules. The algorithm places the solute in a cavity in a solvent medium, and
the interaction between the solute and the cavity is calculated. Using a continuum
instead of individual solvent molecules is, at its best, a way of averaging out the
effect of a large number of solvent molecules; indeed, ifmicrosolvation(above) is
used to calculate thermodynamic properties, then several calculations, best done
with molecular dynamics, would be needed, followed by the calculation of a
Boltzmann average. This is because there are several minimum-energy arrange-
ments of molecules around a solute (as hinted at in [ 15 ]). Although microsolvation
studies are needed if one wishes to computationally pinpoint the effect of molecules
of solvent on specific processes, as in the E2/SN2 studies above [ 14 ], continuum
524 8 Some “Special” Topics