Calculated NMR Parameters (Chemical Shifts and Coupling Constants) of Cyclic C4H2 and C4H4 Molecules Containing Carbene Centers, and of Some of their Boron Analoga, Using Density Functional Theory (DFT)

Singlet state structures of small, cyclic hydrocarbons which can result from the addition of molecular dicarbon (C2) to ethyne (HC≡CH) or ethene (H2C=CH2) have been calculated (B3LYP/6311+G(d,p) level of theory), and were found to contain carbene centres. Some structures of analogous boranes (replacement of the carbene centers by BH fragments) were also calculated. The computation of NMR parameters such as chemical shifts δ13C and δ11B, and coupling constants 1J(13C,1H), 1J(11B,1H), J(13C,13C) and J(13C,11B) shows that these data can be used for the discussion of the bonding situation. The presence of inverted carbene centers is clearly indicated by the increased 13C nuclear magnetic shielding. Scalar 13C-13C spin-spin coupling involving carbene centers are frequently dominated by spin-dipole and spin-orbital interactions.