Experimental determination of the carboxylate oxygen electric-field-gradient and chemical shielding tensors in l-alanine and l-phenylalanine

✍ Scribed by Kazuhiko Yamada; Miwako Asanuma; Hisashi Honda; Takahiro Nemoto; Toshio Yamazaki; Hiroshi Hirota

• Publisher: Elsevier Science
• Year: 2007
• Tongue: English
• Weight: 669 KB
• Volume: 843
• Category: Article
• ISSN: 0022-2860
• DOI: 10.1016/j.molstruc.2006.12.034

✦ Synopsis

We report a solid-state 17 O NMR study of the 17 O electric-field-gradient (EFG) and chemical shielding (CS) tensors for each carboxylate group in polycrystalline L-alanine and L-phenylalanine. The magic angle spinning (MAS) and stationary 17 O NMR spectra of these compounds were obtained at 9.4, 14.1, and 16.4 T. Analyzes of these 17 O NMR spectra yielded reliable experimental NMR parameters including 17 O CS tensor components, 17 O quadrupole coupling parameters, and the relative orientations between the 17 O CS and EFG tensors. The extensive quantum chemical calculations at both the restricted Hartree-Fock and density-functional theories were carried out with various basis sets to evaluate the quality of quantum chemical calculations for the 17 O NMR tensors in L-alanine. For 17 O CS tensors, the calculations at the B3LYP/D95 ** level could reasonably reproduce 17 O CS tensors, but they still showed some discrepancies in the d 11 components by approximately 36 ppm. For 17 O EFG calculations, it was advantageous to use calibrated Q value to give acceptable C Q values. The calculated results also demonstrated that not only complete intermolecular hydrogen-bonding networks to target oxygen in L-alanine, but also intermolecular interactions around the NH 3 þ group were significant to reproduce the 17 O NMR tensors.