4600 J. Phys. Chem. A, Vol. 101, No. 25, 1997
Seeley et al.
pressure of 0.38 Torr. While production of CH3F + Br-(H2O)2
is the most exothermic channel, it was not observed to occur.
The n ) 3 reaction was studied at 240 and 190 K. At both
temperatures, the reaction is so slow that products could not be
observed and only an upper limit of 1 × 10-12 cm3 s-1 could
be placed on the rate constant A literature comparison is not
possible since there have been no previous measurements of
the rate constants for n ) 3 and higher. The n ) 4 and 5
reactions are faster than the n ) 3 reaction. The n ) 4 reaction
was studied at 213 K and n ) 5 reaction was studied at 190 K.
Both reactions have rate constants of approximately 4 × 10-12
of the increased central barrier height: Magnera and Kerbarle28
have performed a RRKM simulation on the SN2 reaction of Cl-
+ n-butyl chloride for a variety of central barrier heights. They
found that when the central barrier height (as referenced from
the potential energy of the free reactants) is less than -17 kJ
mol-1, the temperature dependence becomes more negative as
the central barrier height is increased. The central barrier height
for the F- + CH3Br reaction has been calculated to be -67 kJ
mol-1. While the barrier height for the F-(H2O) + CH3Br
reaction is not known, it is likely to be greater than that of the
F- + CH3Br reaction and within the range necessary the lead
to a slight increase in the negative temperature dependence. The
calculations of Magnera and Kerbarle show that for barrier
heights greater than -17 kJ mol-1 the temperature dependence
becomes more positive as the barrier height is increased. At
this stage the nucleophilic displacement reaction would be very
slow, perhaps too slow for accurate measurement. However,
we have observed a slight positive temperature dependence in
the reaction of OH-(H2O)2 with CH3Br, at temperatures of 300
K and higher.2
cm3 s-1. The only observed ionic products were F-(H2O)n-1
.
We have observed similar behavior in the reactions of Cl-(H2O)n
with CH3Br.3 Based on those results, we postulate the following
mechanism: The first step is ligand switching,
F-(H2O)n + CH3Br f F-(H2O)n-1(CH3Br) + H2O (4)
which is followed by thermal decomposition,
F-(H2O)n-1(CH3Br) + He f F-(H2O)n-1 + (CH3Br) + He
When the nucleophilic displacement reaction channel be-
comes inefficient, association or endothermic ligand switching
reaction channels can become important. For n ) 2 we observe
a significant amount of association to form F-(H2O)2CH3Br,
while for n ) 4 and 5 we observe ligand switching. We have
previously observed association for the OH-(H2O)n + CH3Br
reaction at n ) 2 and 3, and ligand switching for the Cl-(D2O)n
+ CH3Br reaction at n g 1. It is somewhat surprising that F-
hydrates ligand switch with CH3Br while OH- hydrates do not.
One possible explanation is that ligand switching is more
endothermic for the OH- case because the hydration energies
of OH- are greater than those of F- and/or because the methyl
bromide complexation energy of OH- is less than that of F-.
Small energy differences, on the order of 5 kJ mol-1, would be
sufficient to cause ligand switching to be observed for F- and
not for OH-. The existing experimental22,29 and theoretical
data,30-34 while indicating that the relevant energies for F- and
OH- are similar, are not sufficiently accurate to test this
explanation.
(5)
The rate determining step is likely to be the ligand-switching
step, which would be slightly endothermic thereby resulting in
the small rate constant. The thermal decomposition step is fast
enough to keep the concentration of F-(H2O)n-1(CH3Br) at
undetectable levels.
Discussion
The dynamics of gas phase SN2 reactions have previously
been explained in terms of a double-well potential energy
surface.4,25,27 This model maintains that an SN2 reaction of
X-(H2O)n + CH3Y proceeds by forming a collision complex,
(H2O)nX-(CH3Y), which then passes through a Walden inver-
sion transition state to form an exit-channel complex,
(XCH3)Y-(H2O)n. The exit-channel complex then dissociates
to form the products CH3X + Y-(H2O)n. Hydration stabilizes
the reactants more than it does the transition state because the
charge in the transition state is less localized. As a result, the
addition of each water molecule leads to a higher central barrier
and hence a decreased reaction rate. This qualitative model
has been supported by subsequent experimental1-9 and theoreti-
cal studies;10-14 however, it is now known that transfer of water
molecules to the leaving ion is inefficient, and thus highly
hydrated product ions are not observed.1-3,9
Conclusions
We have now used the SIFT technique to study the effect of
hydration and temperature on three SN2 reactions: F-(H2O)n
+ CH3Br, Cl-(D2O)n + CH3Br, and OH-(H2O)n + CH3Br.
These results along with those from other laboratories can be
explained in terms of a potential energy surface with a transition
state that is not as efficiently stabilized by hydration as the
reactants, resulting in reaction rates which decrease with
increasing hydration levels. In the absence of an efficient SN2
reaction channel, other mechanisms become important such as
association and endothermic ligand switching.
Our results for the F-(H2O)n + CH3Br reaction are also
consistent with the qualitative model. In the case of F-(H2O)n
+ CH3Br, the addition of one water molecule to the bare fluoride
ion decreases the reaction rate by a factor of 3. The addition
of the second water decreases the rate by greater than a factor
of 40, and the third water molecule makes the rate immeasurably
slow. This decrease in SN2 reactivity is similar to that observed
for OH-(H2O)n + CH3Br.
Acknowledgment. We would like to thank J. Williamson
and P. Mundis for technical support and A. P. Scott and L.
Radom for calculating the methyl bromide complexation energy
of OH-. This research was supported by the Air Force Office
of Scientific Research under task 2303EP4.
For the n ) 0-3 reactions we find Br- to be the major ion
product, with the amount of Br-(H2O) formed always less than
25% and decreasing with increasing temperature. Once again,
this result is similar to that found for OH-(H2O)n + CH3Br
with the exception that the fraction of Br-(H2O) production was
independent of temperature for the OH- case. The reason for
this difference is uncertain.
Increasing hydration level from n ) 0 to n ) 1 increases the
magnitude of the negative temperature dependence. A similar
result has been observed for the cases of OH-(H2O)n + CH3Br
and OD-(D2O)n + CH3Cl.1,2 This effect is likely to be the result
References and Notes
(1) Hierl, P. M.; Ahrens, A. F.; Henchman, M.; Viggiano, A. A.;
Paulson, J. F.; Clary, D. C. J. Am. Chem. Soc. 1986, 108, 3142.
(2) Viggiano, A. A.; Arnold, S. T.; Morris, R. A.; Ahrens, A. F.; Hierl,
P. M. J. Phys. Chem. 1996, 100, 14397.
(3) Seeley, J. V.; Morris, R. A.; Viggiano, A. A.; Wang, H.; Hase, W.
L. J. Am. Chem. Soc. 1997, 119, 577.
(4) Bohme, D. K.; Mackay, G. I. J. Am. Chem. Soc. 1981, 103, 978.