Dissociative Electron Transfer, Substitution, and Borderline Mechanisms in Reactions of Ketyl Radical Anions. Differences and Difficulties in Their Reaction Paths

Computational studies on ketyl anion radicals with methyl chloride and on ω-chloroalkanal radical anions, Cl(CH2)nC(H)O-• (n = 2, 3), find competing mechanisms: a dissociative electron transfer (ET) mechanism and a substitution (SUB(C)) mechanism leading to a C-alkylation product. H(CN)CO-•/CH3Cl proceeds unequivocally via the SUB(C) mechanism, and ω-chloroalkanal radical anions proceed by the ET mechanism, but the interpretation of the mechanism for H2CO-•/CH3Cl depends on the coordinate system used to explore the path. The steepest descent path in Z-matrix internal coordinates leads to the ET product at both the ROHF/6-31G* and UHF/6-31G* levels. The mass-weighted path leads to the ET product on the restricted open-shell Hartree−Fock (ROHF) surface but to the SUB(C) product on the unrestricted Hartree−Fock (UHF) surface. A valley−ridge inflection point heading in the direction of ET products was located on the mass-weighted UHF path, indicating that the potential energy surface branches toward ET produ...