MRC - up to March 2008

Magnetic Resonance in Chemistry - up to March 2008

Magnetic Resonance in Chemistry
Volume 46, Issue 3 , Pages 206 - 214
A solid-state 53Cr NMR study of chromate and dichromate salts
Michelle A. M. Forgeron, Roderick E. Wasylishen *
Department of Chemistry, Gunning/Lemieux Chemistry Centre, University of Alberta, Edmonton, Alberta, T6G 2G2, Canada
Keywords solid-state NMR • low-gamma nuclei • quadrupolar nuclei • Cr-53 • electric field gradient (EFG) • magnetic shielding

Abstract
Solid-state 53Cr NMR spectra of a series of chromate (CrO42-) and dichromate (Cr2O72-) salts have been examined by employing the stepped-frequency quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) experiment and high applied magnetic field strengths, 11.75 and 18.8 T. Cr-53 nuclear quadrupolar coupling constants, CQ(53Cr), ranging from 1.23 to 5.01 MHz for the CrO42- salts and 7.25 to 8.14 MHz for the Cr2O72- salts have been measured. For the dichromate salts, this corresponds to central transition 53Cr NMR lineshapes of 200-250 kHz at 18.8 T. The use of hyperbolic secant (HS) pulses in combination with the Hahn-echo (HE) or QCPMG experiment results in significant sensitivity enhancements when acquiring 53Cr NMR spectra of magic-angle spinning (MAS) samples, provided the MAS rate is fast with respect to the second-order quadrupolar interaction. For the CrO42- and Cr2O72- salts, the anisotropic chromium magnetic shielding interaction is generally negligible compared to the second-order 53Cr nuclear quadrupolar interaction. No simple correlation between the structure of the CrO42- and Cr2O72- anions and the observed CQ(53Cr) values has been found.
Digital Object Identifier (DOI) 10.1002/mrc.2164 About DOI

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Magnetic Resonance in Chemistry
Volume 46, Issue 3 , Pages 226 - 234
A solid-state 17O NMR study of L -phenylalanine and L -valine hydrochlorides
Kazuhiko Yamada 1 2 *, Tadashi Shimizu 1, Shinobu Ohki 1, Toshio Yamazaki 2
1National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan
2Protein Research Group, Genomic Sciences Center, Yokohama Institute, RIKEN
Keywords NMR • 17O • electric-field-gradient tensor • chemical-shielding tensor • amino acid hydrochloride • hydrogen bonds

Abstract
We have presented an experimental investigation of the oxygen-17 chemical shielding (CS) and electric-field-gradient (EFG) tensors for -COOH groups in polycrystalline amino acid hydrochlorides. The 17O CS and EFG tensors including the relative orientations between the two NMR tensors are determined in [17O]-L-phenylalanine hydrochloride and [17O]-L-valine hydrochloride by the analysis of the 17O magic-angle-spinning (MAS) and stationary NMR spectra obtained at 9.4, 11.7, 16.4, and 21.8 T. The quadrupole coupling constants (CQ) and the span of the CS tensors are found to be 8.41-8.55 MHz and 7.35-7.41MHz, and 548-570 ppm and 225-231 ppm, for carbonyl and hydroxyl oxygen atoms, respectively. Extensive quantum chemical calculations using density functional theory (DFT) have been also carried out for a hydrogen-bonding model. It is demonstrated that the behavior of the dependence of hydrogen-bond distances on 17O NMR tensors for the halogen ions is different from those for the water molecule.
Digital Object Identifier (DOI) - 10.1002/mrc.2167 About DOI

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