J. Am. Chem. Soc. 1997, 119, 9287-9288
9287
The •CH2CH2O(H)CH3+ â-distonic radical cation (Dis•+) was
produced by fragmentation of ionized 1,2-dimethoxyethane9 in
the external ion source of a Bruker CMS-47-X FT-ICR
spectrometer. This ion reacts in the ICR cell with tert-butyl
alcohol (eq 1) to yield C3H10O2•+ through isobutene loss. This
ion appears to be a radical cation weakly bonded to a water
molecule, [Dis•+, H2O], since (i) upon collision, water elimina-
tion gives back an ion that reacts by regioselective addition to
Preparation and Reactivity of Solvated Distonic
Ions and Ionized Enols in the Gas Phase
V. Troude, G. van der Rest, P. Mourgues, and H. E. Audier*
Laboratoire des Me´canismes Re´actionnels, URA CNRS 1307
Ecole Polytechnique, F-91128 Palaiseau Cedex, France
ReceiVed March 7, 1997
•
+ 10
CH2O as does CH2CH2O(H)CH3
and (ii) in the presence
of H218O, rapid exchange of a water molecule takes place.
The study of solvated ions in the gas phase is of prime
importance in understanding the role of the solvent in ionic
chemical and biochemical reactions. Many studies deal with
the physicochemistry of clusters containing a great number (n
> 10) of solvent molecules;1 a case in point concerns the many
studies of water clusters to gain understanding of solvation and
nucleation phenomena.2 Chemical reactivity studies of organic
cations or anions in microscopic systems with a small number
of solvent molecules (n < 5) represent an increasing field of
interest. For instance, the influence of solvation upon proton
transfer or nucleophilic displacement reactions has been re-
ported.3,4 It has been shown also that solvation decreases the
reactivity of enolate anions.5 However, studies of small clusters
involving organic radical cations are relatively rare.4,6 In this
vein, reactions within clusters of ionized alkenes,4 as well as
nucleophilic substitution within [halobenzene•+, nNH3] ions,7
have been studied.
One of the major difficulties in this kind of study is to find,
for each system, a practical and specific method to produce the
desired solvated species. While it is often straightforward to
generate solvated cations by using a high-pressure ion source,8
this method is often not applicable for making solvated radical
cations, the preparation of which requires removal of an electron
from neutral clusters obtained in supersonic expansions of a
solvent gas containing the molecule to be ionized.4,6,7 However,
that method requires that the neutral species corresponding to
the radical cation is available, which is not the case when the
goal is to generate solvated distonic ions or solvated ionized
enols. A simple chemical method, reaction of tert-butyl alcohol
with an ion of interest, is described herein to prepare such
solvated species. Their structure and their reactivity are
discussed.
•CH2CH2O(H)CH3+ + t-C4H9OH f
+
[•CH2CH2O(H)CH3 , H2O] + iso-C4H8 (1)
The reactions of [Dis•+, H2O] demonstrate that the initial ion
has retained its â-distonic structure in the solvated species. For
instance, I• abstraction from allyl iodide indicates the existence
of a radical site in the solvated species11 (eq 2).
+
[•CH2CH2O(H)CH3 , H2O] + C3H5I f
+
•
[ ICH2CH2O(H)CH3 , H2O] + C3H5 (2)
With dimethyl ether, two competing, rapid reactions occur:
(i) substitution of the H2O moiety by CH3OCH3, which confirms
the existence of a weak bond between Dis•+ and H2O, and (ii)
•+
C2H4 transfer from the [Dis•+, H2O] ion to CH3OCH3. This
process is highly characteristic of ions with a â-distonic
structure.12 Consequently, the solvated species is [•CH2CH2O-
(H)CH3+, H2O]. Using ab initio calculations13 for the [•CH2-
CH2O(H)CH3+, H2O] systems, we find two minima, a and b
on the potential energy surface. Structure a, poorly stabilized
(10 kcal/mol) compared to the energy of separate partners, is
an electrostatically bound complex, while the more stable form
b is strongly stabilized by H-bonding (26 kcal/mol) (Figure 1).
The solvated species does not give all the reactions observed
for the naked ion: [•CH2CH2O(H)CH3+, H2O] reacts neither
with CH3OCH3 by H• abstraction nor with CH2O by addition-
elimination, whereas •CH2CH2O(H)CH3+ does.10 The reasons
for such differences in reactivity are currently under study.
Preliminary computational results show that the energy barrier
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27, 787. (c) The reaction of this distonic ion with tert-butyl alcohol at 2.8
× 10-8 mbar (and 10-7 mbar of argon) leading to the solvated species
occurs at about 80% of the collision rate.
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of conventional ethyl methyl ether radical cation: Troude, V. The`se de
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