An investigation of pi-pi packing in a columnar hexabenzocoronene by fast magic-angle spinning and double-quantum H-1 solid-state NMR spectroscopy

H-1 NMR methods employing very fast magic-angle spinning (MAS) are applied to the investigation of both the structure and dynamics of an alkyl-substit uted hexa-peri-hexabenzocoronene (HBC), which is known to form a columnar m esophase with a very high one-dimensional charge carrier mobility. For the crystalline phase, three distinct aromatic resonances are identified in the single-quantum MAS spectrum. The observation of these distinct aromatic re sonances is explained in terms of the differing degrees to which the aromat ic protons experience the ring current of adjacent layers. Using the rotor- synchronized double-quantum MAS method, definite proton-proton proximities are identified, which are shown to be in agreement with the known crystal s tructure of unsubstituted HBC. In the liquid-crystalline phase, axial motio n of the HBC disks leads to the averaging of the three sites into a single resonance. Furthermore, the reduction of the dipolar coupling caused by thi s motion is quantitatively investigated by an analysis of double-quantum MA S spinning sideband patterns. For all spectra, the resolution is significan tly improved for a compound where the cw-carbon positions have been deutera ted; the synthesis of this deuterated compound, in particular the final cyc lodehydrogenation step, is described.