POTENTIAL BARRIERS, TUNNELING SPLITTINGS, AND THE PREDICTED J=1[-O SPECTRUM OF CH5+

It has been established from earlier ab initio calculations that in it s equilibrium configuration the CH5+ molecular ion consists of an H-2 moiety bound to the apex of a pyramidal CH3+ group; the H-2 group is a pproximately perpendicular to the C-3 axis of the CH3+ group, and the binding energy is about 15000 cm(-1). Two internal motions, the torsio n and the flip, provide connections with low barriers between all 120 symmetrically equivalent minima on the potential energy surface so tha t all proton permutations are feasible. We present the results of new high level ab initio calculations of the parts of the potential energy surface associated with these two motions, and in particular we deter mine the continually optimized structure, and associated electronic en ergy, for the CH5+ molecular ion as it undergoes the flip motion. For the flip motion we numerically integrate the one-dimensional Schroding er equation for the tunneling to determine the splitting. Since this s plitting is small (1.4 cm(-1)) we can incorporate it into a 120x120 ma trix treatment of the simultaneous torsion-flip dynamics to determine the energy level splitting pattern in the J=0 and 1 states, in the app roximation of neglecting other tunneling pathways, and we calculate th e positions of the lines in the J=1<--0 millimeter wave spectrum. (C) 1996 American Institute of Physics.