The
isomerization of aliphatic amine radical cations via intermediate
[cyclopropane–NH3]
+•
and [cyclopropane–amine]
+•
ion-neutral complexes was studied experimentally with double-focusing
mass spectrometers and computationally with composite ab initio methods.
The results examine and extend Audier’s suggestion that primary
amine radical cations with alkyl substituents at the β- and/or
γ-carbon atoms isomerize via transient complexes of NH3 and alkyl cyclopropanes; these are formed by ring closure of the
easily accessible γ-distonic isomers. Ionized amines with substituents
at the α-carbon may react analogously when trialkyl cyclopropane
complexes can be formed. Isomerization via complex intermediates is
a major reaction pathway when the internal energy of the amine radical
cation is less than that required for simple CC-bond cleavage. Complexes
of unsubstituted or monosubstituted ionized cyclopropanes rarely contribute
to the isomerization reactions. Secondary and tertiary amine radical
cations do not undergo isomerization via cyclopropane intermediates,
whereas aliphatic ether radical cations do.
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