A preliminary study of the proton rearrangement energy levels and spectrum of CH5+

This theoretical paper is concerned with the proton rearrangement energy levels and spectrum of the CH5+ molecular ion, and it is based on the 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 initio work predicts that the molecule should be considered as an H-2 molecule bound with a dissociation energy of about 15 000 cm(-1) at the apex of a pyramidal CH3+ group. At equilibrium the H-2 axis is nearly perpendicular 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 equivalent minima accessible. Using the ab initio equilibrium structure and torsional barrier, the rotation-torsion energy levels and spectrum are 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 the energy levels. The tunneling resulting from the flip motion will produce splittings in the rotation-torsion energy levels and a spectrum that will have characteristic relative intensities because of the nuclear spin statistical weights. These weights are calculated using the complete nuclear permutation inversion group G(240) = S-5 X {E, E*}, all elements of which are feasible. There are many levels with zero nuclear spin statistical weight, and this will make the spectrum simpler than would otherwise be the case. (C) 1996 Academic Press, Inc.