Durene, crystal symmetry

Calculations of triplet excitonic bands for a number of crystals have been performed by Sternlicht and McConnell (26) and they have shown that the resonant interaction, proper for a crystal, can substantially change the EPR spectra when molecules aggregate to a crystal. An interesting effect related to the resonant interaction has been predicted for the naphthalene crystal (26). The EPR spectra of naphthalene molecules embedded as a solid solution into a durene crystal show four lines (34). This crystal, similar to the naphthalene crystal, contains two molecules in its unit cell. Thus, inside the crystal a naphthalene molecule can have two orientations. Each of these orientations provides two lines in the EPR spectra,13 and only in such cases, when the direction of the applied magnetic field is invariant with respect to symmetry operations of the durene crystal, four EPR lines conglomerate into two lines. 

The more symmetrical the isomer, the closer the molecules are packed in the crystal and the higher is the melting point. The order of decreasing symmetry, durene > prehnitene, corresponds to that of their respective melting points, +80°O-6.5°C. 

Fig. 143. Crystal structure of durene, showing alternative unib ceU a c having symmetry P2ljn.

Fig. 8. The c axis polarized second singlet-singlet absorption of As naphthalene in a durene host crystal. The center of the 0-0 band is near 34,550 cm 1 and that of the a (9) addition is near 35,050 cm 1. The fine structure, as narrow as 1 cm"1 in some crystals, is attributed to interference between the second excited singlet state and vibrational additions to the first which have coupling symmetry. The isotopes 1,4, 5,8 — dt, 2,3, 6,7 — d4, and dg-naphthalene produce entirely different structures. For a given isotope, the major structural features change as the separation between the two excited singlets is varied by host-induced shifts, but remain rather similar for hosts that produce the same separation.

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