Application of Optical Nuclear Polarization Enhanced 13C NMR

Abstract

A gain in detection sensitivity of more than 3 orders of magnitude has been achieved in high-resolution solid-state 13C 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 13C nuclei. This sensitivity gain was utilized to measure the angular dependence (rotation pattern) of the 13C NMR lines. In this way the principal values and orientations of all 13C chemical shielding tensors were determined. While the 13C 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 include...