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Abstract
The E.P.R. spectra of the lowest triplet state of each of 1,3- and 1,7-diazanaphthalene in solid solution in durene single crystals at 97 K are reported. They reveal deviations from perfect guest substitution. Detailed crystal packing calculations using atom-atom potentials and allowing for host lattice relaxation confirm that guest rotations about the normal axis are to be expected and could be several tens of degrees away from perfect substitution. Furthermore, guest centre of mass positions can be up to 1 Å away from the inversion centre of durene. The calculated potential curves with respect to this rotation show shallow potential minima corresponding to metastable sites but there is a considerable sensitivity to the potential set used. The broad, shallow features of the potential wells suggest that measurements at liquid nitrogen temperatures, representing an orientation which is time-averaged over several quanta of the lowest frequency librational mode, may well differ from those recorded at liquid helium temperatures.