Structure assignment in the solid state by the coupling of quantum chemical-calculations with NMR experiments: A columnar hexabenzocoronene derivative

We present a quantum chemical ab initio study which demonstrates a new comb ined experimental and theoretical approach, whereby a comparison of calcula ted and experimental H-1 NMR chemical shifts allows the elucidation of stru ctural arrangements;in solid-state molecular ensembles, taking advantage of the marked sensitivity of the H-1 chemical shift to intermolecular interac tions. Recently, Brown et al, have shown that, under fast magic-angle spinn ing (MAS) at 35 kHz, the resolution in a H-1 NMR spectrum of the solid phas e of an alkyl-substituted hexabenzocoronene (HBC) derivative is sufficient tb-observe the hitherto unexpected resolution of three distinct aromatic re sonances ( J. Am. Chem. Sec. 1999, 121, 6712. Exploiting the additional inf ormation about proton proximities provided by H-1 double-quantum (DQ) MAS N MR spectroscopy, it was shown that the results are qualitatively consistent with the aromatic cores packing in a manner similar to that in unsubstitut ed HBC. Using the HBC-C-12 molecule as an example, we show here that the ne w combined experimental and theoretical approach allows the observed H-1 ch emical shifts to be related in a quantitative manner to the intermolecular structure. In the quantum chemical calculations, a series of model systems of stacked HBC oligomers are used. On:account of the marked dependence of t he H-1 chemical shift to ring currents arising from nearby aromatic rings, the calculated H-1 chemical shifts are found to be very sensitive to the st acking arrangement of the HBC molecules. Moreover, the ring current effect is found to be particularly long range, with a considerable influence of th e second neighbor, at a distance of 700 pm, being observed.