NMR crystallography of oxybuprocaine hydrochloride, Modification II°

The ¹³C CPMAS spectrum is presented for the polymorph of oxybuprocaine hydrochloride which is stable at room temperature, i.e. Mod. II°. It shows crystallographic splittings arising from the fact that there are two molecules, with substantially different conformations, in the asymmetric unit. An INADEQUATE two-dimensional experiment was used to link signals for the same independent molecule. The chemical shifts are discussed in relation to the crystal structure. Of the four ethyl groups attached to NH⁺ nitrogens, one gives rise to unusually low chemical shifts, very different from those of the other three ethyl groups. This is attributed empirically to γ-gauche conformational effects, as is confirmed by shielding computations. These considerations allow ¹³C signals to be assigned to specific carbons in the two crystallographically inequivalent molecules in the crystal structure. Indeed, information about the conformations is inherent in the NMR spectrum, which thus provides data of crystallographic significance. A ¹³C/¹H HETCOR experiment enabled resolution to be obtained in the ¹H dimension and allowed ¹H and ¹³C signals for the same independent molecule to be linked.