IS THE STEREOMUTATION OF METHANE POSSIBLE

Large basis set ab initio calculations at correlated levels including MP2, single reference, as well as multireference configuration interaction, carried out on the methane potential energy surface, have located and characterized a transition structure for stereomutation (one imaginary frequency). This structure is best described asa pyramidal complex between singlet methylene and a side-on hydrogen molecule with C-s symmetry. At the single reference CI level, it lies 105 kcal/mol above the methane T-d-ground state but is stable relative to dissociation into CH2((1)A(1)) and H-2 by 13 kcal/mol at 0 K (with harmonic zero point energy (ZPE) corrections for all structures). Dissociation of the transition state into triplet methylene and hydrogen also is endothermic (by 4 kcal/mol), but single bond rupture to give CH3. and H-. is 3 kcal(mol exothermic. Thus, it does not appear likely that methane can undergo stereomutation classically beneath the dissociation limit. Confirming earlier conclusions, side-on insertion of (1)A(1) CH2 into H-2 in a perpendicular geometry occurs without activation energy. Planar (D-4h) methane (130.5 kcal/mol) has four imaginary frequencies. Two of these are degenerate and lead to equivalent planar C(2v)structures with one three-center, two-electron bond and two two-electron bonds and two imaginary frequencies. The remaining imaginary frequencies of the D-4h form lead to tetrahedral (T-d) and pyramidal (C-4v) methane. The latter has three negative eigenvalues in the force-constant matrix; one of these leads to the T-d global minimum and the other to the C-s (parallel) stereomutation transition structure. Multireference CI calculations with a large atomic natural orbitals basis set produce similar results, with the electronic energy of the C-s stereomutation transition state 0.7 +/- 0.5 kcal/mol higher than that of CH3. + H-. dissociation products, and a ZPE-corrected energy which is 5 +/- 1 kcal/mol higher. Also considered are photochemical pathways for stereomutation and the possible effects of nuclear spin, inversion tunneling, and the parity-violating weak nuclear interaction on the possibility of an experimental detection of stereomutation in methane. (C) 1995 by John Wiley and Sons, Inc.