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| > >(CH5)+ is perfectly stable species in my CI 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. ] 73500475/PDFSTART Dominik Marx and Michele Parrinello* "CH5+: The Cheshire Cat Smiles" Science, 1999, 284(5411), 59-61. DOI: 10.1126/science.284.5411.59 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 interstellar clouds. |
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