CH5+: chemistry's chameleon unmasked

Area of Science:

• Quantum chemistry
• Computational physics
• Molecular spectroscopy

Background:

• The methyl hydride cation (CH5(+)) is a key intermediate in interstellar chemistry and combustion processes.
• Understanding its structure and dynamics is crucial for accurate modeling of these phenomena.
• Previous studies suggest a highly fluxional nature for CH5(+).

Purpose of the Study:

• To accurately determine the nuclear vibrational wave function and zero-point vibrational energy of CH5(+).
• To investigate the structural implications of zero-point motion on the molecule's symmetry.
• To provide a detailed analysis of interatomic distance distributions and rotational constants.

Main Methods:

• Quantum diffusion Monte Carlo (QDMC) techniques were employed.
• An interpolated potential energy surface was constructed using high-level ab initio (CCSD(T)/aug'-cc-pVTZ) data.
• Vibrationally averaged properties, including rotational constants and radial distribution functions, were calculated.

Main Results:

• All ten H-H distances exhibit identical, bimodal distributions.
• All five C-H distances show identical, unimodal distributions.
• The calculated rotational constants are 3.78, 3.80, and 3.83 cm(-1), indicating high symmetry.

Conclusions:

• The extensive zero-point motion of CH5(+) renders all five protons dynamically equivalent in the ground state.
• The molecule's effective symmetry in the ground state surpasses that of its minimum energy structure.
• A unique, static structure cannot be assigned to CH5(+) due to its inherent fluxionality.