Carbones – A Classification on the Magnetic Criterion

Carbones (carbodiphosphoranes, bent allenes and chalcogen-stabilized carbones) bear the same resonance contributor X⁺−C²⁻−Y⁺ (X⁺, Y⁺=PR₃⁺, CR₂⁺, SR₂⁺, SeR₂⁺, S⁺R₂=NR) and exhibit unique bonding and donating properties at the central carbon atom. A classification is given on basis of both the geometry and the magnetic properties (¹³C chemical shift of the central carbon atom and the spatial magnetic properties, through-space NMR shieldings (TS NMRSs), actually the anisotropy effect or the ring current effect of aromatic species). TS NMRS values have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The synergy of geometry (linear or bent, orthogonal or twisted structures) and NMR characteristics (extend of the high field shift of the central carbon atom, anisotropy effect of the allene-like C=C double bonds or the ball-like anisotropy effect of carbone-like central carbon atom) provides a comprehensive picture of the dominating resonance contributor.