Knowledge Bank

The low temperature doublet structure in the electronic spectra of the phenanthrene---$h10$ and $d10$ (guests) in biphenyl (host) mixed crystal systems is attributed to the existence of two environmentally inequivalent guest sites which the phenanthrene molecules occupy. Intermolecular H-H repulsion interactions are shown to largely determine the orientations of phenanthrene in biphenyl Results of temperature dependent studies $(1.8\circ -30.0\circ K)$ on the relative populations of the two inequivalent sites are discussed in relation to calculated intermolecular potential surfaces.