Large amplitude quantum mechanics in polyatomic hydrides. I. A particles-on-a-sphere model for XHn

Exactly solvable 1D model explains the low-energy vibrational level structure of protonated methane

A new one-dimensional model is proposed for the low-energy vibrational quantum dynamics of CH5+ based on the motion of an effective particle confined to a 60-vertex graph Γ60 with a single edge length parameter. Within this model, the quantum states of CH5+ are obtained in analytic form and are related to combinatorial properties of Γ60. The bipartite structure of Γ60 gives a simple explanation for curious symmetries observed in numerically exact variational calculations on CH5+.

Quantum Microsolvation of Protonated Methane with ^{4}He: Large-Amplitude Motion Heavily Influences Bosonic Exchange

Quantum simulations of small CH_{5}^{+}·^{4}He_{n} complexes disclose significant and antagonistic impact of small-amplitude local vibrational motion vs large-amplitude global fluxional motion within the CH_{5}^{+} impurity on helium in real and permutation space. While the former significantly enhances bosonic exchange in the surrounding ^{4}He microsolvation shell compared to the rigid-body reference, the latter greatly suppresses long permutation cycles, which is traced back to the different nature of these quantum fluctuations. Therefore, it is expected that the resulting impact on local superfluidity is generic for fluctuating impurities in bosonic environments.

Quantum Deconstruction of the Infrared Spectrum of CH5+

We present two quantum calculations of the infrared spectrum of protonated methane (CH5+) using full-dimensional, ab initio-based potential energy and dipole moment surfaces. The calculated spectra compare well with a low-resolution experimental spectrum except below 1000 cm(-1), where the experimental spectrum shows no absorption. The present calculations find substantial absorption features below 1000 cm(-1), in qualitative agreement with earlier classical calculations of the spectrum. The major spectral bands are analyzed in terms of the molecular motions. Of particular interest is an intense feature at 200 cm(-1), which is due to an isomerization mode that connects two equivalent minima. Very recent high-resolution jet-cooled spectra in the CH stretch region (2825 to 3050 cm(-1)) are also reported, and assignments of the band origins are made, based on the present quantum calculations.

Understanding hydrogen scrambling and infrared spectrum of bare CH5+based on ab initio simulations

A comprehensive study of the properties of protonated methane obtained from ab initio molecular dynamics simulations is presented. Comparing computed infrared spectra to the measured one gives further support to the high fluxionality of bare CH(5)(+). The computational trick to partially freezing out large-amplitude motion, in particular hydrogen scrambling and internal rotation of the H(2) moiety, leads to an understanding of the measured IR spectrum despite the underlying rapid hydrogen scrambling motion that interconverts dynamically structures of different symmetry and chemical bonding pattern. In particular, the fact that C-H stretching modes involving the carbon nucleus and those protons that form the H(2) moiety and the CH(3) tripod, respectively, result in distinct peaks is arguably experimental support for three-center two-electron bonding being operative at experimental conditions. It is proposed that hydrogen scrambling is associated with the softening of a mode that involves the bending of the H(2) moiety relative to the CH(3) tripod, which characterizes the C(s) ground-state structure. The potential energy surface that is mapped on to a two dimensional subspace of internal coordinates provides insight into the dynamical mechanism for exchange of hydrogens between CH(3) tripod and the three-center bonded H(2) moiety that eventually leads to full hydrogen scrambling.