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 ( C _{3 h} or C _s ) and their rotamers 2 ( C _s ) 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 C _{3 v} models 6 indicate stronger S-C π-interactions in the cations 1 and 2 than in the dications. C _{3 h} - 1b undergoes a Jahn-Teller distortion to C _s - 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 CX ₃ ^{n +} 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 CH ₃ ⁺ . 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 Δ H _f ( 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.