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

Abstract

HYPERCOORDINATE carbonium ions can be formed by protonating saturated hydrocarbons with superacids^1–3. 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 species^1–3. But recent calculations^4,5 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.