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Understanding the Infrared Spectrum of Bare CH₅⁺
- Asvany, Oskar, P, Padma Kumar, Redlich, Britta, Hegemann, Ilka, Schlemmer, Stephan, Marx, Dominik
- Science 2005 v.309 no.5738 pp. 1219-1222
- carbon dioxide, freezing, hydrogen, methane, molecular dynamics
- Protonated methane, CH₅⁺, continues to elude definitive structural assignment, as large-amplitude vibrations and hydrogen scrambling challenge both theory and experiment. Here, the infrared spectrum of bare CH₅⁺ is presented, as detected by reaction with carbon dioxide gas after resonant excitation by the free electron laser at the FELIX facility in the Netherlands. Comparison of the experimental spectrum at [approximately]110 kelvin to finite-temperature infrared spectra, calculated by ab initio molecular dynamics, supports fluxionality of bare CH₅⁺ under experimental conditions and provides a dynamical mechanism for exchange of hydrogens between CH₃ tripod positions and the three-center bonded H₂ moiety, which eventually leads to full hydrogen scrambling. The possibility of artificially freezing out scrambling and internal rotation in the simulations allowed assignment of the infrared spectrum despite this pronounced fluxionality.