Equatorial and axial hydrogens in heterocyclic six-membered rings: the rotational spectrum of piperazine

The rotational spectrum of piperazine has been investigated by free jet absorption millimeter wave spectroscopy. The spectrum of the polar conformer with axial–equatorial orientations of the two amino hydrogens was only observed. This assignment was confirmed by the spectra of the two monodeuterated and the bideuterated species. Ab initio and density functional calculations predict the observed conformer to have an energy intermediate between the equatorial–equatorial and axial–axial non-polar forms, the former species being the global minimum.