INDUCTIVE AND CONJUGATIVE S-]C POLARIZATIONS IN TRITHIOCARBENIUM IONS [C(SH)(3)](+) AND [C(SH)(3)](CENTER-DOT-2,+) - POTENTIAL ENERGY SURFACE ANALYSIS, ELECTRONIC STRUCTURE MOTIF, AND SPIN DENSITY DISTRIBUTION

摘要

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 pi-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-3h 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-3v models 6 indicate stronger S-C pi-interactions in the cations 1 and 2 than in the dications. C-3h-1b undergoes a Jahn-Teller distortion to C-5-1b' 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 la is Delta HA = 95.5 kca/mol lower than for CH3+. Delta HA 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 la is IP(la) = 343.8 kcal/mol (14.9 eV) and results in Delta 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 pi- and sigma-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-pi-electron density and increases the pi-acidity of the C atom. The alpha-spin density is concentrated on the S atoms and carbon is beta-spin polarized.