A PRELIMINARY-STUDY OF THE PROTON REARRANGEMENT ENERGY-LEVELS AND SPECTRUM OF CH5+

This theoretical paper is concerned with the proton rearrangement ener gy levels and spectrum of the CH5+ molecular ion, and it is based on t he ab initio results of P. R. Schreiner, S-J. Kim, H. F. Schaefer, and P. v. R. Schleyer [J. Chem. Phys. 99, 3716-3720 (1993)]. The ab initi o work predicts that the molecule should be considered as an H-2 molec ule bound with a dissociation energy of about 15 000 cm(-1) at the ape x of a pyramidal CH3+ group. At equilibrium the H-2 axis is nearly per pendicular to the C-3 axis of the CH3+ group, eclipsing a CH bond. The internal rotation of the H-2 about the C-3 axis has a barrier height of 30 cm(-1). There is also an internal ''flip'' motion through a C-2v structure, with a barrier of 300 cm(-1), that exchanges a CH3+ and an H-2 proton in the molecule and that makes all 120 symmetrically equiv alent minima accessible. Using the ab initio equilibrium structure and torsional barrier, the rotation-torsion energy levels and spectrum ar e calculated. The tilt of the C-3 axis of the CH3+ group away from the internal rotation axis of the H-2 has a very significant effect on th e energy levels. The tunneling resulting from the flip motion will pro duce splittings in the rotation-torsion energy levels and a spectrum t hat will have characteristic relative intensities because of the nucle ar spin statistical weights. These weights are calculated using the co mplete nuclear permutation inversion group G(240) = S-5 X {E, E}, all elements of which are feasible. There are many levels with zero nucle ar spin statistical weight, and this will make the spectrum simpler th an would otherwise be the case. (C) 1996 Academic Press, Inc.