An ab initio study of geometry and vibrational spectrum of cyclic (B6C)−2: The out-of-plane bending problem

In this Letter, a comprehensive ab initio post-HF study was conducted on the geometry and vibrational spectrum of cyclic, hypercoordinate carbon containing (B6C)−2 dianion. The analysis reveals extraordinary sensitivity of out-of-plane bending mode of this molecule to the diffuse functions of Pople-type basis sets. Accordingly, this molecule seems to be another example of the set of recently described planar molecules that exhibit spurious (but artificial) ‘imaginary’ out-of-plane bending frequencies at post-HF levels. The analysis eventually comes to the conclusion that this anion is indeed stable with regard to nuclear distortions, but unstable in regard to electron auto-detachment.