Determination of the orientations for the 17O NMR tensors in a polycrystalline l-alanine hydrochloride

Abstract

We report a solid-state 17O NMR study of the 17O electric-field-gradient (EFG) and chemical shielding (CS) tensors for the carboxyl oxygen in an l-alanine hydrochloride. Using [ 17O]– and [ 13C, 17O]– l-alanine hydrochlorides, both the magnitudes and the orientations in the molecular frame of the 17O EFG and CS tensors could be determined by the analysis of the 17O magic-angle spinning (MAS) and stationary NMR spectra. For the carbonyl oxygen, the smallest EFG tensor component, V XX , and the largest EFG component, V ZZ , roughly lies in the carboxyl molecular plane and the direction of V XX is parallel to the dipolar vector between 13C and 17O, that is, the direction of C O bond. The angles between the intermediate EFG component, V YY , and δ 33 component, and between δ 22 component and V ZZ are found to be approximately 10° and 35°, respectively. We also present the results of the quantum chemical calculations for a theoretical hydrogen-bonding model, indicating that hydrogen-bonding strengths make it possible to vary both magnitudes and orientations of the carbonyl 17O EFG tensors in amino acid hydrochlorides.