| Abstract
| - We present a quantum chemical ab initio study which demonstrates a new combined experimentaland theoretical approach, whereby a comparison of calculated and experimental 1H NMR chemical shifts allowsthe elucidation of structural arrangements in solid-state molecular ensembles, taking advantage of the markedsensitivity of the 1H chemical shift to intermolecular interactions. Recently, Brown et al. have shown that,under fast magic-angle spinning (MAS) at 35 kHz, the resolution in a 1H NMR spectrum of the solid phaseof an alkyl-substituted hexabenzocoronene (HBC) derivative is sufficient to observe the hitherto unexpectedresolution of three distinct aromatic resonances ( J. Am. Chem. Soc. 1999, 121, 6712). Exploiting the additionalinformation about proton proximities provided by 1H double-quantum (DQ) MAS NMR spectroscopy, it wasshown that the results are qualitatively consistent with the aromatic cores packing in a manner similar to thatin unsubstituted HBC. Using the HBC-C12 molecule as an example, we show here that the new combinedexperimental and theoretical approach allows the observed 1H chemical shifts to be related in a quantitativemanner to the intermolecular structure. In the quantum chemical calculations, a series of model systems ofstacked HBC oligomers are used. On account of the marked dependence of the 1H chemical shift to ringcurrents arising from nearby aromatic rings, the calculated 1H chemical shifts are found to be very sensitiveto the stacking arrangement of the HBC molecules. Moreover, the ring current effect is found to be particularlylong range, with a considerable influence of the second neighbor, at a distance of 700 pm, being observed.
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