Inductive and Conjugative S → C Polarizations in "Trithiocarbenium Ions" [C(SH)3]+ and [C(SH)3]·,2+. Potential Energy Surface Analysis, Electronic Structure Motif, and Spin Density Distribution


The formation of [C(SH)3]+ (a) by hydride abstraction from HC(SH)3 and its oxidation to the radical dication [C(SH)3],2+ (b) were studied to examine the potential of stabilizing carbenium ions via trithio substitution. Potential energy surfaces (PES) were explored at the HF/6-31G* level and energies were refined at the (P)MP4(full,sdtq)/6-31G* level without and with annihilation of spin contaminations. The unpaired π-electron in the radical lies well below the Fermi level and spin polarization and dynamic electron correlation become important. Open Y-conjugated structures 1 (C3h or Cs) and their rotamers 2 (Cs) are favored. Four cyclic, S-S connected, distonic, chiral stereoisomers 3b are local minima for the radical dication. The C-S rotational barriers to isomerization via 4 and automerization via 5 (two isomeric TSs) and the high energies of C3v models 6 indicate stronger S-C π-interactions in the cations 1 and 2 than in the dications. C3h-1b undergoes a Jahn-Teller distortion to Cs-1b' but pseudorotation is facile. The PES analyses suggest two strategies to achieve pyramidalization of the trivalent carbon in heteroatom-substituted carbenium ions via X-X interactions in CX3n+ or via face-preferential hyperconjugation. The basic approach was found to be successful:  The computed hydride affinity of 1a is ΔHA = 95.5 kcal/mol lower than for CH3+. ΔHA was partitioned into a methane destabilization of 32.0 kcal/mol and a carbenium ion stabilization of 63.5 kcal/mol. Our best estimate for the ionization energy of 1a is IP(1a) = 343.8 kcal/mol (14.9 eV) and results in ΔHf(1b) = 541.5 kcal/mol. The cations [C(SH)3]+ and [C(SH)3],2+ show the same unexpected electronic motif. Strong S→C donations occur in the π- and σ-systems and, instead of charge dispersal, large positive SH charges are arranged around a negative C center. The stabilization mechanisms in the S-containing ions and the lighter O homologues are fundamentally different due to the umpolung of the C-X bonds. Oxidation of [C(SH)3]+ removes S-π-electron density and increases the π-acidity of the C atom. The α-spin density is concentrated on the S atoms and carbon is β-spin polarized.



Keywords and Phrases

organic compound; radical; article; drug stability; drug structure; electron spin resonance; molecular dynamics

International Standard Serial Number (ISSN)

0002-7863; 1520-5126

Document Type

Article - Journal

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© 1996 American Chemical Society (ACS), All rights reserved.