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Is the stereomutation of methane possible?
Author(s) -
Pepper Melanie J. M.,
Shavitt Isaiah,
Schleyer Paul Von Ragué,
Glukhovtsev Mikhail N.,
Janoschek Rudolf,
Quack Martin
Publication year - 1995
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540160208
Subject(s) - chemistry , potential energy surface , ground state , ab initio , methylene , bond dissociation energy , singlet state , multireference configuration interaction , dissociation (chemistry) , methane , zero point energy , atomic physics , basis set , computational chemistry , excited state , physics , density functional theory , quantum mechanics , organic chemistry
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 as a 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 CH 2 ( 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 CH 3 .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 CH 2 into H 2 in a perpendicular geometry occurs without activation energy. Planar (D 4 h ) methane (130.5 kcal/mol) has four imaginary frequencies. Two of these are degenerate and lead to equivalent planar C 2 v structures with one three‐center, two‐electron bond and two two‐electron bonds and two imaginary frequencies. The remaining imaginary frequencies of the D 4 h form lead to tetrahedral (T d ) and pyramidal (C 4 v ) 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 CH 3 .+ 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. © 1995 by John Wiley & Sons, Inc.