The Chemical Bond in C-2

摘要

Quantum chemical calculations using the complete active space of the valence orbitals have been carried out for HnCCHn (n=0-3) and N-2. The quadratic force constants and the stretching potentials of HnCCHn have been calculated at the CASSCF/cc-pVTZ level. The bond dissociation energies of the C-C bonds of C-2 and HCCH were computed using explicitly correlated CASPT2-F12/cc-pVTZ-F12 wave functions. The bond dissociation energies and the force constants suggest that C-2 has a weaker C-C bond than acetylene. The analysis of the CASSCF wavefunctions in conjunction with the effective bond orders of the multiple bonds shows that there are four bonding components in C-2, while there are only three in acetylene and in N-2. The bonding components in C-2 consist of two weakly bonding sigma bonds and two electron-sharing bonds. The bonding situation in C-2 can be described with the sigma bonds in Be-2 that are enforced by two bonds. There is no single Lewis structure that adequately depicts the bonding situation in C-2. The assignment of quadruple bonding in C-2 is misleading, because the bond is weaker than the triple bond in HCCH.