APPLICATION OF OPTICAL NUCLEAR-POLARIZATION ENHANCED C-13 NMR

A gain in detection sensitivity of more than 3 orders of magnitude has been achieved in high-resolution solid-state C-13 NMR of monocrystall ine fluorene doped with acridine by applying optical nuclear polarizat ion (ONP) via excited triplet states to protons and transferring this proton polarization to the C-13 nuclei. This sensitivity gain was util ized 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 shieldin g tensor of the bridging methylene group exhibits only a small anisotr opy, at the aromatic carbon positions the typical strong anisotropy is observed. All tensors belonging to the same molecule have one princip al axis, perpendicular to the molecular plane, in common, showing that in the crystal lattice the fluorene molecule is in a planar configura tion. The experimental data are compared to ab initio calculations emp loying optimized geometries and gauge included atomic orbitals density functional theory (GIAO DFT). The orientations of all calculated tens ors are in excellent agreement with the experimental data. On an absol ute scale the calculated shielding parameters reproduce the experiment al values reasonably well. A significant improvement of the calculated eigenvalues is achieved by shifting the tensors employing data from c alculations of benzene and methane.