Static solid-state 14N NMR and computational studies of nitrogen EFG tensors in some crystalline amino acids

Abstract

The recently reported direct enhancement of integer spin magnetization (DEISM) methodology for signal enhancement in solid-state NMR of integer spins has been used to obtain static ¹⁴N powder patterns from α-glycine, L-leucine and L-proline in relatively short experimental times at 9.4 T, allowing accurate determination of the quadrupolar parameters. Proton decoupling and deuteration of the nitrogen sites were used to reduce the ¹H–¹⁴N dipolar contribution to the transverse relaxation time allowing more echoes to be acquired per scan. In addition, ab initio calculations using molecular clusters (Gaussian 03) and the full crystal lattice (CASTEP) have been employed to confirm these results, to obtain the orientation of the electric field gradient (EFG) tensors in the molecular frame, and also to correctly assign the two sets of parameters for L-leucine. The ¹⁴N EFG tensor is shown to be highly sensitive to the surrounding environment, particularly to nearby hydrogen bonding.