CH+5: A Molecule without Shape?

Our title question was recently brought to widespread attention and tentatively answered based on high-level
quantum chemical calculations, whereas state-of-the-art experiments remain inconclusive. Theory predicts that the conventional representation of CH+(5) (eclipsed C(s)) with three-center, two-electron bonding is misleading, and that CH+(5) is highly fluxional without a definite structure. Accordingly, the standard textbook perception of CH+(5) as being the prototype of nonclassical carbocations - a major pillar of ``hypercarbon chemistry'' - should be revised. We reinvestigate CH+(5) using a novel technique \to perform ab initio electronic structure calculations simultaneously including quantum effects on the nuclei. This reveals dramatic structural quantum effects, such
as pronounced fluxionality, pseudorotations, and hydrogen scrambling. Nevertheless the quantum ground state is dominated by three-center, two-electron bonded C(s)-like structures. Consequently, and in agreement with
experience, the chemical behavior of CH+(5) is expected to be of the hypercoordinate carbonium type, whereas its spectroscopic properties will be elusive owing to its rather complex dynamics.

By: D. Marx, M. Parrinello (Max-Planck Inst., Germ.)

Published in: Nature, volume 375, (no ), pages 216-8 in 1995

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