A Preliminary Study of the Proton Rearrangement Energy Levels and Spectrum of CH+5

Abstract

This theoretical paper is concerned with the proton rearrangement energy levels and spectrum of the CH+5molecular ion, and it is based on theab initioresults of P. R. Schreiner, S-J. Kim, H. F. Schaefer, and P. v. R. Schleyer [J. Chem. Phys.99,3716–3720 (1993)]. Theab initiowork predicts that the molecule should be considered as an H2molecule bound with a dissociation energy of about 15000 cm−1at the apex of a pyramidal CH+3group. At equilibrium the H2axis is nearly perpendicular to theC3axis of the CH+3group, eclipsing a CH bond. The internal rotation of the H2about theC3axis has a barrier height of 30 cm−1. There is also an internal “flip” motion through aC2vstructure, with a barrier of 300 cm−1, that exchanges a CH+3and an H2proton in the molecule and that makes all 120 symmetrically equivalent minima accessible. Using theab initioequilibrium structure and torsional barrier, the rotation–torsion energy levels and spectrum are calculated. The tilt of theC3axis of the CH+3group away from the internal rotation axis of the H2has 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 groupG240= S5× {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.