Dissociative electron transfer, substitution, and borderline mechanisms in reactions of ketyl radical anions. Differences and difficulties in their reaction paths

Abstract

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)C=O-•/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 H₂C=O^-•/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 products. Closer examination of the two-dimensional portion of the surface shows that the potential energy surface for this reaction descends from the transition state to a broad saddle point region and branches into two valleys:  one leading to the ET product and the other to the SUB(C) product. The ridge and saddle point region on the UHF surface are at lower energy and longer C−C and C−Cl bond lengths than on the ROHF surface, allowing the UHF mass-weighted reaction path to traverse the ridge into the SUB(C) valley. On the ROHF surface as the path descends from the transition state, the H₂C=O^-• moiety continues to approach the methyl chloride while the C−Cl bond lengthens, but then recoils to give the ET products. Cross-sections of the surface calculated at the UQCISD(T)/6-31G* level resemble the UHF cross-sections, indicating that the branching of the potential surface into two mechanisms is expected at this level, too. Thus, whereas from inspection of the surface in internal coordinates, the OCH₂C−CH₃−Cl^- transition state connects to the ET product, the mass-weighted path can cross the broad and shallow ridge and bifurcate thereafter to ET and SUB(C) products. Our study reveals a scenario where a group of isostructural transition states define a mechanistic family consisting of substitution, electron transfer, and borderline situations. Molecular dynamics studies may be necessary to explore the borderline situations.