Structure Determination of Organolithium Compounds by 7Li Residual Quadrupolar Couplings

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Nuclear magnetic resonance (NMR) spectroscopy is the most important analytical method for the characterization of organolithium and other organometallic reagents in solution, i.e. under conditions where these are typically employed in synthetic chemistry. Although classical solution NMR spectroscopy provides a wealth of information (isotropic 1H, 13C and 6/7Li chemical shifts, scalar couplings, nuclear Overhauser effects), it is blind to anisotropic interactions such as shielding anisotropies, dipolar and quadrupolar couplings. The proposed project aims at making these anisotropic NMR parameters visible under weakly aligned (oriented), nevertheless solution-like, conditions and accessible to the structure determination of organolithium and other organometallic/organoboron compounds. As alignment medium, crosslinked polystyrene, prepared under inert conditions and swollen to ~80% solvent content will be used and further optimized. In a first step towards structure determination, 7Li residual quadrupolar couplings (RQCs) will be measured in rigid and symmetric lithium compounds, where the alignment tensor is obtained from residual dipolar couplings (RDCs), and the electric field gradient (EFG) is calculated using density functional theory (DFT). In a further step, we aim at an improved understanding of the Li coordination sphere, using a combination of the experimental 7Li RQCs, solid-state NMR, charge densities from X-ray diffraction data and DFT calculations performed both in the gas phase and in the solid-state. Finally, RQC measurements will also be done on compounds containing other alkali metals (23Na, 87Rb, 133Cs) and Group 13 elements (11B, 27Al, 71Ga). This novel and promising approach strongly benefits from the combined expertise of the project group and its collaborators in the fields of anisotropic and solid-state NMR, X-ray diffraction, lithium, organometallic and quantum chemistry and will help to understand the interplay between solid-state and solution structures.