Dodecahedrane: Symmetry and dynamics in the solid state
Sprache des Vortragstitels:
Englisch
Original Tagungtitel:
3rd Workshop on NMR and MRI relaxometry in chemistry and bio-medicine
Sprache des Tagungstitel:
Englisch
Original Kurzfassung:
Objects with five-fold axes of symmetry are forbidden to express this symmetry in ordered periodic arrays (e.g. crystal lattices) due to their inability for proper space filling without gaps or voids. This restriction is lifted for objects of two-, three-, four-, and six-fold rotational symmetry. It is predictable that dodecahedrane's five membered rings must all undergo a small distortion upon crystallization. This distortion from the ideal is consistent with eight carbons forming the vertices of an ideal cube within the lattice. Six sets of two adjacent carbons form ethano bridges above this cube's six faces. The X-ray crystallographer finds crystalline dodecahedrane's twenty carbons to reside in two diastereotopic sets in the ratio of eight on-vertex carbons to twelve off-vertex partners of a Th-symmetry solid skeleton.
The lack of appendages in crystalline spherical molecules enables them to undergo reorientation about their molecular axes which is readily observable by solid-state NMR. Thus, at ambient temperature, both S6-symmetry crystalline cubane and Th-symmetry crystalline dodecahedrane exhibit 13C isochrony due to fast topomerization of their diastereotopic nuclei. We herein present {1H,13C} CP-MAS NMR build-up rates attesting the mobility differences of e.g. lighter cubane versus the heavier dodecahedrane. T1 relaxation rates and chemical shielding powder patterns are measured at ambient conditions down to 100 K. These 13C MAS spectra display line-narrowing with decreasing temperature until five relatively narrow signals are observed below 224K. A observation that is not consistent with a static Th-symmetry of dodecahedrane geometry. Density functional theory calculations of NMR chemical shieldings are used to compare these experimental results with different models of dynamical / structural disorder or exchange.
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