Assessing the Impact of Tagging Species on the Fluxional Behavior of Protonated Methane

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Protonated methane is considered to be an archetypal fluxional molecule since pronounced large-amplitude motion leads to so-called hydrogen scrambling. Given the fundamental importance of this parent of non-classical carbocations, it has been intensely studied since decades. What remains essentially unexplored, however, are the putative interaction-induced perturbations of large-amplitude motion and scrambling due to attaching weakly interacting light species, which is the exclusive focus of this project. Indeed, protonated methane has been tagged using hydrogen molecules in pioneering messenger action spectroscopy work of Y. T. Lee. Moreover, helium atoms are another ideal tagging species that has been widely used for other molecules, in particular quasi-rigid ones. This project has two different components which are enabled by using two distinct ab initio path integral-based techniques as developed recently. The first focus is the approximate computation of broadband infrared spectra at temperatures on the order of 100 K down to the region of low frequencies to elucidate the interactions of protonated methane with a few hydrogen molecules in terms of vibrational spectroscopy. The second focus is on the impact of only a few helium-four atoms in conjunction with Bose-Einstein statistics on large-amplitude motion and scrambling of protonated methane at temperatures on the order of 1 K in terms of its molecular structure.