Theoretical investigations of mode mixing in vibrationally excited states of CH5+.

Using diffusion Monte Carlo, five vibrational excited states of CH(5)(+) and CD(5)(+) are evaluated and analyzed. Here, we focus on the fundamentals in the five modes that are generated by requiring that the wave functions change sign at specified values of the five symmetry-adapted linear combinations (SALCs) of the CH or CD bond lengths. Even though the definitions of these modes are based on displacements of the CH or CD bond lengths, the frequencies are found to be low compared to previously calculated CH vibrational frequencies in this molecule. The totally symmetric mode, with A(1)(+) symmetry, has a calculated frequency of 2164 and 1551 cm(-1) for CH(5)(+) and CD(5)(+). The frequencies of the four fundamentals that arise from excitation of the four SALCs that transform under G(1)(+) symmetry have frequencies that range from 1039 to 1383 cm(-1) and from 628 to 893 cm(-1) in CH(5)(+) and CD(5)(+), respectively. The origins of the broken degeneracy are investigated and explained to reflect extensive coupling to the two low-frequency modes that lead to isomerization of CH(5)(+).