Structural quantum effects and three-centre two-electron bonding in CH+5

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HYPERCOORDINATE carbonium ions can be formed by protonating saturated hydrocarbons with superacids1–3. As this leaves a deficiency of bonding electrons, the resulting non-classical carbocations contain bonds in which two electrons are shared between three nuclei. Protonated methane, CH+5, might be seen as the prototype of such species1–3. But recent calculations4,5 have suggested that all five C–H bonds are effectively equivalent and exchange dynamically very rapidly. It was therefore concluded4 that CH+5 is a highly fluxional molecule without a definite structure, in which the representation in terms of three-centre two-electron bonding is misleading. Here we use a recently developed technique6 to perform ab initio electronic structure calculations that include quantum effects of the nuclei. We find that, although there are prominent quantum-mechanical effects on the structure, including fluxional-ity, pseudo-rotations and hydrogen scrambling, the quantum ground state is nevertheless dominated on average by configurations in which an H2 moiety is attached to a CH3 group forming a three-centre two-electron bond. To this extent, CH+5 should therefore resemble other carbonium ions.

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Marx, D., Parrinello, M. Structural quantum effects and three-centre two-electron bonding in CH+5. Nature 375, 216–218 (1995) doi:10.1038/375216a0

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