Experimental ground-state combination differences of CH5+

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Science  20 Mar 2015:
Vol. 347, Issue 6228, pp. 1346-1349
DOI: 10.1126/science.aaa3304

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Getting a handle on the CH5+ spectrum

Protonated methane, CH5+, fascinates chemists because it seems to break the rules. There's no obvious place for the fifth hydrogen to bind, and so what happens is that all five hydrogens shuffle about like participants in an endless round of a musical-chairs game. And yet, the molecule has a vibrational spectrum that suggests some semblance of tighter ordering. Asvany et al. have now measured high-resolution vibrational spectra at two low temperatures (10 and 4 Kelvin). Their accompanying analysis makes headway on assigning the peaks and enhancing understanding of the molecule's dynamic structure.

Science, this issue p. 1346


Protonation of methane (CH4), a rather rigid molecule well described by quantum mechanics, produces CH5+, a prototypical floppy molecule that has eluded definitive spectroscopic description. Experimental measurement of high-resolution spectra of pure CH5+ samples poses a formidable challenge. By applying two types of action spectroscopy predicated on photoinduced reaction with CO2 and photoinhibition of helium cluster growth, we obtained low-temperature, high-resolution spectra of mass-selected CH5+. On the basis of the very high accuracy of the line positions, we determined a spectrum of combination differences. Analysis of this spectrum enabled derivation of equally accurate ground state–level schemes of the corresponding nuclear spin isomers of CH5+, as well as tentative quantum number assignment of this enfant terrible of molecular spectroscopy.

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