Taming CH_5~+, the 'enfant terrible' of chemical structures

摘要

Protonated methane, CH_5~+, is a quantum dynamical system that challenges our understanding of chemical bonding and structure. The bonding does not lead to a trigonal bipyramid. Instead, the five protons swarm around the central carbon, and this gives rise to an incredibly complex vibration-rotation-tunneling infrared spectrum (1), making it an "enfant terrible" for spectroscopists. Ab initio theory has found that "there is essentially no barrier to hydrogen scrambling" (2) and "the very concept of molecular structure becomes problematic for this molecule" (3). For its parent molecule, CH_4, each rotational level corresponds to one quantum state, but for CH_5~+ it corresponds to 2 × 5! = 240 states. However, on page 1346 of this issue, Asvany et al. (4) report combination differences (Co-Diffs) of the low-energy levels of CH_5~+, a first step at "taming" its spectrum.