A different kind of problem arises in the case of 2,4,6-octatriene, where the conformations of both outer double bonds are set but the conformation of the interior double bond is unknown (Figure ) . The use of PPBs along carboncarbon bonds artifactually sets the inner double bond conformation. In this case the problem can be solved by reifying the hydrogens attached to the interior double bond, then directing PPBs along the resulting carbon-hydrogen bonds. This is, however, not a general solution. It will fail, for example, in the case of a nitrogen-nitrogen bond due to the lack of reifiable hydrogens.