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

Abstract

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. © 1995 Nature Publishing Groups. All Rights Reserved.