Conformational studies of polymorphic N-octyl-D-gluconamide with 15N (labeled) 13C (natural abundance) REDOR spectroscopy

Magnetic dipolar couplings between the 15N atom (labeled) and neighboring 13C atoms (natural abundance) in three solid modifications of N-octyl-D-gluconamide are measured with rotational echo double resonance (REDOR). A unique spectral assignment of 13C resonances is possible by means of their dipolar dephasing. While in the monolayer crystal and in the fiber modification the assignment is amenable to the solution spectra, in the bilayer crystallites a different assignment is found. The dipolar couplings in the range 45 to 1220 Hz are converted into CN distances. These distances are employed in conjunction with the 13C chemical shieldings of the CP-MAS spectra to determine sets of possible torsion angles, which define the molecular conformation in the neighborhood of the amide group. In contrast to the monolayer crystal, for the fiber and bilayer crystallite modifications a gauche bend at the C2–C3 bond is found, giving the molecules the shape of a 2G sickle.