CH5+: Chemistry's Chameleon Unmasked

Abstract

The nuclear vibrational wave function and zero-point vibrational energy of CH₅⁺ are calculated using quantum diffusion Monte Carlo techniques on an interpolated potential energy surface constructed from CCSD(T)/aug‘-cc-pVTZ ab initio data. From this multidimensional wave function, the vibrationally averaged rotational constants and radial distribution functions for atom−atom distances within the molecule are constructed. It is found that the distributions of all 10 H−H distances are bimodal and identical. The radial distribution functions obtained for the five C−H distances are also identical, but unimodal. The three rotational constants were found to be 3.78, 3.80, and 3.83 cm^-1. These values indicate that the ground state of CH₅⁺ is significantly more symmetric than its global minimum energy structure. We conclude that the zero-point motion of CH₅⁺ renders all five protons equivalent in the ground state and precludes the assignment of a unique structure to the molecule.