Magnetic Shielding Studies of C₂ and C₂H₂ Support Higher than Triple Bond Multiplicity in C₂

Abstract

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 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 ca. 224-227 ppm and ca. 123-125 ppm ranges for C 2 and C 2 H 2 , respectively.

C2: An Asymmetric Specie ?

“…Based on the results obtained here, as well as literature data [1][2][3][4][5][6][7][8][9], some conclusion can be pointed out:…”

Analysis of carbon‐carbon bonding in small hydrocarbons and dicarbon using dynamic orbital forces: Bond energies and sigma/pi partition. Comparison with sila compounds

“…The energy of this “inverted bond” was found of about 20 kcal/mol. This conclusion is supported by the compared magnetic shielding of C 2 and acetylene and also by a recent study using a symmetry‐broken wave function, yielding to an effective bond order of 3.36 …”

Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low‐Lying Electronic States of S2N2

“…In this paper we study bonding and aromaticity in the ground, first singlet excited and lowest triplet electronic states of S 2 N 2 by analysing the changes in the off‐nucleus magnetic shielding tensor, σ ( r ), within the space surrounding the molecule. Previous research on carbon–carbon bonds showed that the off‐nucleus isotropic magnetic shielding, , is capable of exposing the differences between carbon−carbon bonds of different multiplicities in much greater detail than is achievable using the total electronic density. One of our aims is to establish whether the variations in σ iso ( r ) provide similar insights into the nature of the S−N bond in S 2 N 2 .…”