Buntkowsky, G. ; Hoffmann, W. ; Kupka, T. ; Pasterna, G. ; Jaworska, M. ; Vieth, H. M. (1998)
Application of optical nuclear polarization enhanced C-13 NMR.
In: Journal of Physical Chemistry A, 102 (29)
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

A gain in detection sensitivity of more than 3 orders of magnitude has been achieved in high-resolution solid-state C-13 NMR of monocrystalline fluorene doped with acridine by applying optical nuclear polarization (ONP) via excited triplet states to protons and transferring this proton polarization to the C-13 nuclei. This sensitivity gain was utilized to measure the angular dependence (rotation pattern) of the C-13 NMR lines. In this way the principal values and orientations of all C-13 chemical shielding tensors were determined. While the C-13 shielding tensor of the bridging methylene group exhibits only a small anisotropy, at the aromatic carbon positions the typical strong anisotropy is observed. All tensors belonging to the same molecule have one principal axis, perpendicular to the molecular plane, in common, showing that in the crystal lattice the fluorene molecule is in a planar configuration. The experimental data are compared to ab initio calculations employing optimized geometries and gauge included atomic orbitals density functional theory (GIAO DFT). The orientations of all calculated tensors are in excellent agreement with the experimental data. On an absolute scale the calculated shielding parameters reproduce the experimental values reasonably well. A significant improvement of the calculated eigenvalues is achieved by shifting the tensors employing data from calculations of benzene and methane.

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