S N 2 Reactions with an Ambident Nucleophile: A Benchmark Ab Initio Study of the CN - + CH 3 Y [Y = F, Cl, Br, and I] Systems.

We characterize the Walden-inversion, front-side attack, and double-inversion S N 2 pathways leading to Y - + CH 3 CN/CH 3 NC and the product channels of proton abstraction (HCN/HNC + CH 2 Y - ), hydride-ion substitution (H - + YH 2 CCN/YH 2 CNC), halogen abstraction (YCN - /YNC - + CH 3 and YCN/YNC + CH 3 - ), and YHCN - /YHNC - complex formation (YHCN - /YHNC - + 1 CH 2 ) of the CN - + CH 3 Y [Y = F, Cl, Br, and I] reactions. Benchmark structures and frequencies are computed at the CCSD(T)-F12b/aug-cc-pVTZ level of theory, and a composite approach is employed to obtain relative energies with sub-chemical accuracy considering (a) basis-set effects up to aug-cc-pVQZ, (b) post-CCSD(T) correlation up to CCSDT(Q), (c) core correlation, (d) relativistic effects, and (e) zero-point energy corrections. C-C bond formation is both thermodynamically and kinetically more preferred than N-C bond formation, though the kinetic preference is less significant. Walden inversion proceeds via low or submerged barriers (12.1/17.9(F), 0.0/4.3(Cl), -3.9/0.1(Br), and -5.8/-1.8(I) kcal/mol for C-C/N-C bond formation), front-side attack and double inversion have high barriers (30-64 kcal/mol), the latter is the lower-energy retention pathway, and the non-S N 2 electronic ground-state product channels are endothermic (Δ H 0 = 31-92 kcal/mol).