Ab Initio Calculation of the Rotational Spectrum of CH5+ and CD5+

Abstract

The highly flexible CH5+ molecular ion has been shown by ab initio calculations to have 120 symmetrically equivalent minima of Cs symmetry in its ground electronic state. Each minimum has the structure of a hydrogen molecule bound to the apex of a CH3+ pyramid, with the hydrogen molecule approximately perpendicular to the C3 axis. Complete proton rearrangement, making all minima accessible to each other, is possible as a result of two large-amplitude internal motions: an internal rotation about the C3 axis with an ab initio barrier of 30 cm-1 and an internal flip motion with an ab initio barrier of 300 cm-1 that exchanges protons between the H2 and CH3+ groups. We calculate the structure of the J = 2 ← 1, and 1 ← 0 rotational transitions for CH5+ and CD5+. The calculation proceeds in two stages. The first stage involves calculating rotation−torsion energies, and the second stage involves a matrix diagonalization to include the flip tunneling. In the first stage the rotation−torsion energies are calculate...