Determination of the 17O Quadrupolar Coupling Constant and of the 13C Chemical Shielding Tensor Anisotropy of the CO Groups of Pentane-2,4-dione and β-Diketonate Complexes in Solution. NMR Relaxation Study

Abstract

¹³C NMR relaxation times T₁ of the carbonyl groups of pentane-2,4-dione and β-diketonate complexes Al(acac)₃ and Zr(acac)₄ (acac: pentanedionate anion) were measured for various magnetic field strengths, allowing a determination of the contribution of the chemical shift anisotropy mechanism to the total relaxation. NOE and T₁ measurements for the ¹³C nucleus of the central methine carbon furnished the correlation time τ_c for the reorientation of theses species. The chemical shift tensor anisotropy Δσ could be deduced and compared to the values obtained in the solid state. The quadrupolar coupling constant (QCC) of the ¹⁷O nucleus could also be determined by measuring the line width of the ¹⁷O NMR signal and using the τ_c value. QCC values for the complexes are in the same range as for the pentane-2,4-dione molecule, indicating similar electronic distribution and symmetry around the oxygen atom of these different species. Δσ for the complexes are close together, and the values obtained in solution are approximately those obtained in the solid state. They are close to the value reported in the literature for tetraacetylethane, which can be considered as a dimer of a β-diketone, but slight differences are observed for the individual components of the chemical shielding tensor.