The Nature of the Fourth Bond in the Ground State of C-2 : The Quadruple Bond Conundrum
Hiberty, Philippe C.
Rzepa, Henry S.
- 化学化工－已发表论文 
Does, or doesn't C-2 break the glass ceiling of triple bonding? This work provides an overview on the bonding conundrum in C-2 and on the recent discussions regarding our proposal that it possesses a quadruple bond. As such, we focus herein on the main point of contention, the 4thbond of C-2, and discuss the main views. We present new data and an overview of the nature of the 4thbondits proposed antiferromagnetically coupled nature, its strength, and a derivation of its bond energy from experimentally based thermochemical data. We address the bond-order conundrum of C-2 arising from generalized VB (GVB) calculations by comparing it to HCCH, and showing that the two molecules behave very similarly, and C-2 is in no way an exception. We analyse the root cause of the deviation of C-2 from the Badger Rule, and demonstrate that the reason for the smaller force constant (FC) of C-2 relative to HCCH has nothing to do with the bond energies, or with the number of bonds in the two molecules. The FC is determined primarily by the bond length, which is set by the balance between the bond length preferences of the sigma- versus -bonds in the two molecules. This interplay in the case of C-2 clearly shows the fingerprints of the 4thbond. Our discussion resolves the points of contention and shows that the arguments used to dismiss the quadruple bond nature of C-2 are not well founded.