Many wild molecules have been dreamed up by theoretical chemists in the spirit of challenging the conceptions of a chemical bond. Of the many ways chemists try to determine the presence of a bond, one has found quite a following: the bond critical point. This method, based on electron density has helped clarify whether a chemical bond is present or not. Recently, however, some unintuitive results of this methodology were realized. A recent report on phenanthrene (shown below) summarizes the conundrum (DOI:10.1002/anie.200805751).



The two hydrogens shown explicitly above have a bond critical point between them, and the energy of that bond was estimated to be approximately 10 kcal/mol. The authors of the recent report show that there is no attractive force between the two hydrogen atoms by using high level quantum chemical calculations and Raman spectroscopy (of the dideuterated phenanthrene).

This calls into question whether the use of a bond critical point is useful in cases that push the limits.