Alexis Delgado: Unusually Long C-C Bonds

For decades chemists have attempted to create a molecule with the longest covalent C-C bond possible. This has typically been done through steric effects or molecular strain. More recently the diamino-o-carborane analog, di-N,N-dimethylamino-o-carborane, has been observed to have a unusually long C-C bond which has been attributed to negative hyperconjugation effects (i.e.charge transfers). In our work we computationally analyzed the C-C bonds for a unique set of 53 molecules including clamped bonds, highly sterically strained complexes, electron deficient species, and the di-N,N-dimethylamino-o-carborane molecule in order to consider all routes for obtaining the longest C-C bond. We derive local vibrational stretching force constants for targeted C-C bonds to quantify the intrinsic strengths of these bonds. Our systematic study quantifies for the first time that whereas steric hindrance and/or strain definitely elongate a C-C bond, electronic effects can lead to even longer and weaker C-C bonds. Within our set of molecules the electron deficient ethane radical cation, in D3d symmetry, acquires the longest C-C bond with a length of 1.935 angstroms followed by di-N,N-dimethylamino-o-carborane with a bond length of 1.930 angstroms. However, the C-C bond in di-N,N-dimethylamino-o-carborane is the weakest compared the ethane radical cation; revealing that the longer bond is n