Computational Studies of Coinage Metal Anion M - + CH 3 X (X = F, Cl, Br, I) Reactions in Gas Phase.

We characterized the stationary points along the nucleophilic substitution (S N 2), oxidative insertion (OI), halogen abstraction (XA), and proton transfer (PT) product channels of M - + CH 3 X (M = Cu, Ag, Au; X = F, Cl, Br, I) reactions using the CCSD(T)/aug-cc-pVTZ level of theory. In general, the reaction energies follow the order of PT > XA > S N 2 > OI. The OI channel that results in oxidative insertion complex [CH 3 -M-X] - is most exothermic, and can be formed through a front-side attack of M on the C-X bond via a high transition state OxTS or through a S N 2-mediated halogen rearrangement path via a much lower transition state invTS. The order of OxTS > invTS is inverted when changing M - to Pd, a d 10 metal, because the symmetry of their HOMO orbital is different. The back-side attack S N 2 pathway proceeds via typical Walden-inversion transition state that connects to pre- and post-reaction complexes. For X = Cl/Br/I, the invS N 2-TS's are, in general, submerged. The shape of this M - + CH 3 X S N 2 PES is flatter as compared to that of a main-group base like F - + CH 3 X, whose PES has a double-well shape. When X = Br/I, a linear halogen-bonded complex [CH 3 -X∙··M] - can be formed as an intermediate upon the front-side attachment of M on the halogen atom X, and it either dissociates to CH 3 + MX - through halogen abstraction or bends the C-X-M angle to continue the back-side S N 2 path. Natural bond orbital analysis shows a polar covalent M-X bond is formed within oxidative insertion complex [CH 3 -M-X] - , whereas a noncovalent M-X halogen-bond interaction exists for the [CH 3 -X∙··M] - complex. This work explores competing channels of the M - + CH 3 X reaction in the gas phase and the potential energy surface is useful in understanding the dynamic behavior of the title and analogous reactions.