Peter R. Schreiner
"Does CH5+ Have (a) 'Structure'? A Tough Test for
Experiment and Theory"
Angew Chem Int Ed Engl. 2000, 39(18), 3239-3241 [Only registered users see links. ]
Dominik Marx and Michele Parrinello*
"CH5+: The Cheshire Cat Smiles"
Science, 1999, 284(5411), 59-61.
Protonated methane, CH5+, is one of the Holy Grails of
rotational-vibrational molecular spectroscopy. The reasons
are manifold. First, adding a proton to the simple
methane molecule, CH4, generates the floppy or fluxional
molecule CH5+, to which the traditional concepts of
vibrational spectroscopy no longer apply. As a result,
bare CH5+ has resisted characterization by infrared
spectroscopy ever since its discovery by mass spectroscopy
in the early 1950s -- until now when Takeshi Oka and
his group at the University of Chicago have finally
managed to record its spectrum, as reported on page 135
of this issue. Second, the chemical bonding in CH5+
cannot be described by single Lewis structures as taught
in freshman chemistry, where a line between two atoms
denotes a chemical bond formed by two electrons. Rather,
in CH5+, three atoms are connected by two electrons.
Such three-center-two-electron (3c-2e) bonds (see Fig. 1A)
characterize hypercoordinated carbocations for which CH5+
is the prototype. These carbonium ions or nonclassical
carbocations, as they are also called, are crucial as
extremely reactive intermediates in hydrocarbon reactions
catalyzed by very strong so-called magic acids and are at
the heart of the electrophilic substitution chemistry of
aliphates developed by George Olah and his school.
Finally, there is substantial astrochemical interest in
CH5+, which is implicated in reactions that form part of
the intricate synthesis of polyatomic species in cold