A theoretical study of the millimeterwave spectrum of CH5þ

This is a continuation of our earlier work aimed at predicting the millimeterwave spectrum of protonated methane CH5þ. As for protonated acetylene C2H3þ, it is the millimeterwave spectrum that will most directly provide the experimental information needed to understand the large amplitude motion of the molecule. Literature ab initio calculations show that the large amplitude motion of the five protons around the central carbon nucleus in CH5þ is not completely free, but is restricted by potential barriers at the trigonal bipyramid ðD3hÞ; square pyramid ðC4vÞ and end-on-H2 ðC3vÞ forms. Thus, the large amplitude motion proceeds mostly in the coordinate space that connects the structures called CsðIÞ; CsðIIÞ and C2v: These structures have essentially identical electronic energies and very similar rotational constants, as has already been shown in the literature. We calculate that they also have very similar dipole moments. The topology of the space of the large amplitude motion that connects the 120 versions of the CsðIÞ structure, the 120 versions of the CsðIIÞ structure, and the 60 versions of the C2v structure is considered here. The spectral signature of this large amplitude motion in the rotational spectrum is calculated with absolute intensities. It is hoped that these results will aid and stimulate attempts to see and assign the high resolution gas phase millimeterwave absorption spectrum of CH5þ. The J ¼ 1ˆ0 spectrum is predicted to be centered in the region 220–235 GHz, and if all the large amplitude motion splittings of this line are resolved, the strongest component (the Ki ¼ 0ˆ0 line) is predicted to have an integrated absorption intensity of 13 m/mol at 77 K. q 2003 Elsevier B.V. All rights reserved.