DOES CH5+ PREFER A C2V RATHER THAN A CS STRUCTURE

The closely related C(s) (1) and C2v (3) structures of CH5+ have been reinvestigated at many ab initio levels using MP2/6-31G** and MP2/6-311+ +G(2df,2pd) geometries. The largest basis sets employed were 6-311G(3df,2p), 6-311+ +G(3df,3pd), and the Dunning "correlation consistent" polarized triple-split valence basis set (cc-pVTZ). Electron correlation was probed at the MP4 level, but the QCISD method was also used with the largest basis sets. While electron correlation favors 3 over 1 by about 2 kcal/mol, the correlated relative energies with all basis sets employed range from 0.36-1.03 kcal/mol in favor of 1. The best estimate of this difference, 0.86 kcal/mol, is essentially identical with the (scaled) zero-point energy difference, 0.84 kcal/mol, favoring 3 over 1. These results indicate that 1 and 3 have almost exactly the same energy at 0 K. Our best value for the dissociation energy of CH5+ is 42.0 kcal/mol [QCISD(T)/6-311+ +G(3df,3pd)//MP2(fu)/6-311+ +G(2df,2pd), corrected to 298 K], which agrees very well with the experimental value.