IS THE STEREOMUTATION OF METHANE POSSIBLE

Large basis set ab initio calculations at correlated levels including MP2, single reference, as well as multireference configuration interac tion, carried out on the methane potential energy surface, have locate d and characterized a transition structure for stereomutation (one ima ginary frequency). This structure is best described asa pyramidal comp lex between singlet methylene and a side-on hydrogen molecule with C-s symmetry. At the single reference CI level, it lies 105 kcal/mol abov e 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 po int energy (ZPE) corrections for all structures). Dissociation of the transition state into triplet methylene and hydrogen also is endotherm ic (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. Con firming 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 wi th 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; on e of these leads to the T-d global minimum and the other to the C-s (p arallel) stereomutation transition structure. Multireference CI calcul ations with a large atomic natural orbitals basis set produce similar results, with the electronic energy of the C-s stereomutation transiti on state 0.7 +/- 0.5 kcal/mol higher than that of CH3. + H-. dissociat ion products, and a ZPE-corrected energy which is 5 +/- 1 kcal/mol hig her. Also considered are photochemical pathways for stereomutation and the possible effects of nuclear spin, inversion tunneling, and the pa rity-violating weak nuclear interaction on the possibility of an exper imental detection of stereomutation in methane. (C) 1995 by John Wiley and Sons, Inc.