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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
Journal of the American Chemical Society
  • Rainer Glaser, Missouri University of Science and Technology
  • Godwin Sik Cheung Choy
  • Grace Shiahuy Chen
  • Hansjörg Gratzmacher
Abstract

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.

Department(s)
Chemistry
Keywords and Phrases
  • organic compound,
  • radical,
  • article,
  • drug stability,
  • drug structure,
  • electron spin resonance,
  • molecular dynamics
Document Type
Article - Journal
Document Version
Citation
File Type
text
Language(s)
English
Rights
© 1996 American Chemical Society (ACS), All rights reserved.
Publication Date
11-1-1996
Publication Date
01 Nov 1996
Disciplines
Citation Information
Rainer Glaser, Godwin Sik Cheung Choy, Grace Shiahuy Chen and Hansjörg Gratzmacher. "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" Journal of the American Chemical Society Vol. 118 Iss. 46 (1996) p. 11617 - 11628 ISSN: 0002-7863; 1520-5126
Available at: http://works.bepress.com/rainer-glaser/75/