1508 J. Phys. Chem. A, Vol. 101, No. 8, 1997
Sahlstrom et al.
changes associated with each of the two steps have been
determined and place the energy of the SN2 transition state for
-
1
this reaction at 1.6 kcal mol above that of the reactants.
The experimental method demonstrated here for the study
of gas phase ion-molecule reactions is made additionally
significant by recent theoretical treatments of gas phase SN2
displacement reactions in which the association complexes
25
resulting from the attachment of chloride ion to methyl chloride
2
6
and to methyl bromide were predicted to behave in very
complex and distinctly non-statistical manners. Such behavior
would greatly complicate all attempts to interpret rate constants
for ion-molecule reactions of this type if the rate constants
were measured under conditions of relatively low pressure where
the association complexes are isolated during their lifetimes.
Therefore, techniques such as described here, where very high
buffer gas pressure causes all reaction intermediates to be
brought into a state of thermal equilibrium with the medium,
might provide the only means of obtaining macroscopic rate
constants that can be reliably interpreted in terms of the potential
energy surfaces for dynamically complex ion-molecule reac-
tions of this type.
-
Figure 12. Principal energy changes along the Cl /i-PrBr reaction
coordinate. The first three magnitudes indicated in the figure have been
determined here by IMS measurements of rate constants, k
equilibrium constants, K . These measurements place the S 2 transition
state for this reaction at 1.6 kcal mol above the energy of the reactants.
The enthalpy of the final products has been calculated from known
thermochemical data.
1
, and
1
N
-1
83.4 kcal mol-1 22) and bromine (77.6 kcal mol-1-1 22) atoms
and the known C-Cl and C-Br bond energies of i-PrCl (81
(
-1-1 23
-1-1 23
kcal mol
) and i-PrBr (68 kcal mol
) while assuming
Acknowledgment. The authors thank D. A. Williamson for
providing PHPMS measurements. This work was supported
by a grant from the Chemistry Division of the National Science
Foundation (CHE-9509330).
that all C-H and C-C bonds within i-PrCl and i-PrBr are the
same. From the present IMS measurements, it is therefore
concluded that the energy of the SN2 transition state for the
reaction of chloride with isopropyl bromide lies about 1.6 kcal
mol above that of the reactants. Corresponding IMS mea-
surements made previously in our laboratory for the reactions
of chloride ion with methyl bromide, ethyl bromide, and n-butyl
bromide placed the SN2 transition states for those reactions at
-
1
References and Notes
(
1) Knighton, W. B.; Grimsrud, E. P. In AdVances in Gas Phase Ion
Chemistry; Adams, N. G., Babcock, L. M., Eds.; JAI Press: Greenwich,
CN, 1996; Vol. 2, pp 219-256.
-
1
2
.2, 0.0, and 1.3 kcal mol , respectively, below the energies
(2) Cacace, F. Acc. Chem. Res. 1988, 21, 215-222.
(3) Cacace, F. Science 1990, 250, 392-399.
of the reactants. The higher transistion state energy observed
here for the reaction of chloride with isopropyl bromide can be
attributed to increased steric hindrance to backside attack for
this secondary alkyl bromide.
(4) Speranza, M. Mass Spectrom. ReV. 1992, 11, 73-117.
(5) Speranza, M. Int. J. Mass Spectrom. Ion Processes 1992, 118/119,
95-447.
3
(
(
(
6) Giles, K.; Grimsrud, E. J. Phys. Chem. 1992, 96, 6680-6687.
7) Giles, K.; Grimsrud, E. P. J. Phys. Chem. 1993, 97, 1318-1323.
8) Knighton, W. B.; Bognar, J. A.; O’Connor, P. M.; Grimsrud, E. P.
Conclusions
In this study, the relatively complex nucleophilic SN2
displacement reaction of the chloride ion with isopropyl bromide
has been studied in nitrogen buffer gas at 640 Torr between 20
and 175 °C by ion mobility spectrometry. Under the conditions
existing within the IMS, a prior clustering reaction between the
two reactants occurs sufficiently rapidly that a state near that
of the true clustering equilibrium condition is continuously
maintained. This allows the energetics of this process to be
accurately characterized while a slower SN2 displacement
reaction is simultaneously occurring. As a result, both processes
can be quantitatively characterized from reaction-modified IMS
spectra. Analysis of the present IMS results are greatly
facilitated by the fact that reaction intermediates, as well as
reactants, tend to be maintained in a state of physical equilibrium
with the buffer gas medium when unusually high buffer gas
pressures are used.
J. Am. Chem. Soc. 1993, 115, 12079-12084.
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(
(
12) Henchman, M.; Hierl, P. M.; Paulson, J. F. AdV. Chem. Ser. 1987,
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(
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(19) Laidler, K. J. Chemical Kinetics; McGraw-Hill Book Co.: New
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It has been shown that under these conditions the SN2
nucleophilic displacement reaction probably occurs by the
following distinctly two-step mechanism
(
20) Su, T.; Bowers, M. T. Gas Phase Ion Chemistry; Bowers, M. T.,
Ed.; Academic Press: New York, 1979; p 83.
21) Laidler, K. J. Chemical Kinetics; McGraw-Hill Book Co.: New
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(
K1
k1
-
-
-
Cl + i-PrBr y
\
z Cl (i-PrBr)
98 Br + i-PrCl (25)
(
-
in which a thermalized ion complex, Cl (i-PrBr), is maintained
in a state of chemical equilibrium with the reactants. An SN2
displacement reaction then occurs within the cluster ion by its
(
(
(
(
24) Dougherty, R. C. Org. Mass Spectrom. 1974, 8, 85.
25) Vande Linde, S. R.; Hase, W. L. J. Phys. Chem. 1990, 94, 6148.
26) Wang, H.; Peslherbe, G. H.; Hase, W. L. J. Am. Chem. Soc. 1994,
-
unimolecular decomposition to Br -containing product ions. By
measurements of the associated equilibrium constants, K1, and
rate constants, k1, as a function of temperature, the enthalpy
116, 9644.