Solid state NMR, IR and X-ray diffraction studies of the structure and motion of L-leucinamide

Abstract

Crystalline L-leucinamide has been studied by ¹³C CP/MAS NMR, FTIR spectroscopy and X-ray crystallography. The ¹³C CP/MAS spectrum of crystalline L-leucinamide has been assigned using the depolarisation–repolarisation method. Two magnetically non-equivalent molecules in the unit cell have been found from the ¹³C CP/MAS spectra. Non-equivalence is also observed in the FTIR spectrum. The molecular dynamics were studied by measuring ¹³C spin–lattice relaxation in both laboratory and rotating frames. Results show that the mobilities of the two molecules are very different. Spin–lattice relaxation times for the two α carbons differ by an order of magnitude. Determination of the crystal structure by X-ray crystallography confirmed the existence of two distinct molecules in the unit cell; these differed in the orientation of the amide groups and in their involvement in the hydrogen bonding scheme, which may account for the observed NMR behaviour.