Magnetic Shielding Studies of C₂ and C₂ H₂ Support Higher than Triple Bond Multiplicity in C₂

The carbon-carbon bonds in the ground states of C₂ and C₂ H₂ , at their equilibrium geometries, are compared by analysing the changes in the off-nucleus magnetic shielding tensor within the space surrounding each of these molecules. A wide range of quantum-chemical approaches, including full-valence CASSCF-GIAO, CCSD(T)-GIAO and CCSDT-GIAO, all with the cc-pVQZ basis set, as well as HF-GIAO and MP2-GIAO, with the cc-pVQZ, cc-pV5Z and cc-pV6Z basis sets, show that the surroundings of the carbon-carbon bond in C₂ are more shielded than those of the carbon-carbon bond in C₂ H₂ . The additional shielding of the carbon-carbon bond in C₂ is found to be due to a larger paramagnetic contribution to the component of the shielding tensor which is perpendicular to the molecular axis. The analysis of the off-nucleus shielding data indicates that the carbon-carbon bond in C₂ is "bulkier" and, therefore, of a higher multiplicity, but weaker than the corresponding bond in C₂ H₂ . According to the results of the shielding calculations, the carbon nuclei in C₂ should be much more shielded than those in C₂ H₂ , with ¹³ C isotropic magnetic shieldings in the ranges of around 224-227 ppm and around 123-125 ppm for C₂ and C₂ H₂ , respectively.