Questioning the Central Bond in [1.1.1]Propellane: Insights from Probability Density Analysis

In the 1980s, the investigation of [1.1.1]propellane started to attract a lot of attention. The key question is the nature of the stabilization of its singlet state relative to its triplet state, sparking an intense debate about the existence of a central bond. In this work, this case is revisited with orbital-independent probability density analysis (PDA), which is based on the measurable many-electron probability density. The most probable electron positions and their exchange paths are studied for singlet [1.1.1], [2.1.1], [2.2.1], and [2.2.2]propellane as well as the triplet state in [1.1.1]propellane. The results are compared to those in ethane, cyclopropane, bicyclo[1.1.0]butane, and bicyclo[1.1.1]pentane. By analyzing the differences between singlet and triplet [1.1.1]propellane in comparison with other molecules, we conclude that the notion of a central bond is not justified and the stabilization of singlet [1.1.1]propellane arises solely from unusual many-electron exchanges involving the wing bonds.