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Magnetic Shielding Studies of C 2 and C 2 H 2 Support Higher than Triple Bond Multiplicity in C 2
Author(s) -
Karadakov Peter B.,
Kirsopp Josh
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201703051
Subject(s) - electromagnetic shielding , chemistry , carbon 13 , multiplicity (mathematics) , triple bond , electronic structure , bond length , molecular geometry , atomic physics , molecule , computational chemistry , crystallography , physics , double bond , nuclear physics , crystal structure , geometry , quantum mechanics , mathematics , organic chemistry , polymer chemistry
The carbon–carbon bonds in the ground states of C 2 and C 2 H 2 , 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 C 2 are more shielded than those of the carbon–carbon bond in C 2 H 2 . The additional shielding of the carbon–carbon bond in C 2 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 C 2 is “bulkier” and, therefore, of a higher multiplicity, but weaker than the corresponding bond in C 2 H 2 . According to the results of the shielding calculations, the carbon nuclei in C 2 should be much more shielded than those in C 2 H 2 , with 13 C isotropic magnetic shieldings in the ranges of around 224–227 ppm and around 123–125 ppm for C 2 and C 2 H 2 , respectively.