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

[Image: see text] 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 (ΔH(0) = 31–92 kcal/mol).