Verfeinerung der Pyren-Struktur

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Abstract
Using 1608 reflections (including 354 unobserved) the structure of pyrene, C16H10, has been refined. a = 13.649 ± 7, b = 9.253 ± 5, c = 8.470 ± 4 Å, β = 100.28 ± 3°, P21/a, Z = 4. Because of the great anisotropic vibrations of the rigid molecule, the isotropic refinement ended at R = 13.6% (14.9%), whereas by the anisotropic refinement R dropped to 4.4% for the 1254 observed reflections (R = 4.7% if 40 unobserved with Fc ≥ Fmin are included). During the refinement the shortest C–C distance increased from 1.311 to 1.341 Å. Within the limits of error the symmetry of the molecule is mmm. The vibrational behaviour of the carbon atoms can be explained completely by the vibrations of the rigid molecule (translational vibrations parallel to the molecule axes L, M, N: 0.255, 0.214 and 0.171 Å, oscillations around these axes: 4.35°, 3.2° and 4.75°). The hydrogen atoms seem to be librating additionally perpendicular to the C–H bonds. The reasons are discussed, why a former anisotropic refinement using only the strongest 550 reflections (Camerman and Trotter, 1965) ended at R = 11%, although these data together with the new parameters yield an R of 5.8%. Finally the bond lengths of pyrene are compared with those of related molecules and from quantum chemical calculations.