Does CH 5 + prefer a C 2v rather than a C s structure?

The closely related C s ( 1 ) and C 2v ( 3 ) structures of CH   5 +have been reinvestigated at many ab initio levels using MP2/6‐31G** and MP2/6‐311 + + G(2 df , 2 pd ) geometries. The largest basis sets employed were 6‐311G(3 df , 2 p ), 6‐311 + + G(3 df , 3 pd ), and the Dunning “correlation consistent” polarized triple‐split valence basis set (cc‐ p VTZ). 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 CH   5 +is 42.0 kcal/mol [QCISD(T)/6‐311 + + G(3 df , 3 pd )//MP2(fu)/6‐311 + + G(2 df , 2 pd ), corrected to 298 K], which agrees very well with the experimental value. © 1992 by John Wiley & Sons, Inc.