SN2 Reactions with an Ambident Nucleophile: A Benchmark Ab Initio Study of the CN-+ CH3Y [Y = F, Cl, Br, and I] Systems

We characterize the Walden-inversion, front-side attack, and double-inversion SN2 pathways leading to Y- + CH3CN/CH3NC and the product channels of proton abstraction (HCN/HNC + CH2Y-), hydride-ion substitution (H- + YH2CCN/YH2CNC), halogen abstraction (YCN-/YNC- + CH3 and YCN/YNC + CH3-), and YHCN-/YHNC- complex formation (YHCN-/YHNC- + 1CH2) of the CN- + CH3Y [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-SN2 electronic ground-state product channels are endothermic (ΔH0 = 31-92 kcal/mol). © 2022 The Authors. Published by American Chemical Society

Rethinking the X- + CH3Y [X = OH, SH, CN, NH2, PH2; Y = F, Cl, Br, I] S(N)2 reactions

Moving beyond the textbook mechanisms of bimolecular nucleophilic substitution (S(N)2) reactions, we characterize several novel stationary points and pathways for the reactions of X- [X = OH, SH, CN, NH2, PH2] nucleophiles with CH3Y [Y = F, Cl, Br, I] molecules using the high-level explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ(-PP) [n = D, T, Q] basis sets. Besides the not-always-existing traditional pre- and post-reaction ion-dipole complexes, X-H3CY and XCH3Y-, and the Walden-inversion transition state, [X-CH3-Y](-), we find hydrogen-bonded X-HCH2Y (X = OH, CN, NH2; Y F) and front-side H3CYX- (Y F) complexes in the entrance and hydrogen-bonded XH2CHY- (X = SH, CN, PH2) and H3CXY- (X = OH, SH, NH2) complexes in the exit channels depending on the nucleophile and leaving group as indicated in parentheses. Retention pathways via either a high-energy front-side attack barrier, XYCH3-, or a novel double-inversion transition state, XHCH2Y-, having lower energy for X = OH, CN, and NH2 and becoming submerged (barrier-less) for X = OH and Y = I as well as X = NH2 and Y = Cl, Br, and I, are also investigated

Unconventional S(N)2 retention pathways induced by complex formation: High-level dynamics investigation of the NH2- + CH3I polyatomic reaction

Investigations on the dynamics of chemical reactions have been a hot topic for experimental and theoretical studies over the last few decades. Here, we carry out the first high-level dynamical characterization for the polyatom-polyatom reaction between NH2- and CH3I. A global analytical potential energy surface is developed to describe the possible pathways with the quasi-classical trajectory method at several collision energies. In addition to S(N)2 and proton abstraction, a significant iodine abstraction is identified, leading to the CH3 + [NH2 center dot center dot center dot I](-) products. For S(N)2, our computations reveal an indirect character as well, promoting the formation of [CH3 center dot center dot center dot NH2] complexes. Two novel dominant SN2 retention pathways are uncovered induced by the rotation of the CH3 fragment in these latter [CH3 center dot center dot center dot NH2] complexes. Moreover, these uncommon routes turn out to be the most dominant retention paths for the NH2- + CH3I S(N)2 reaction. Published under an exclusive license by AIP Publishing