Richard B. Maynard

Calculation of lanthanide-induced shifts from molecular structure. III. Other lanthanides☆

Abstract The NMR spectra of Ln(thd)3(sub)2 (where Ln = Pr, Sm, Dy, Ho, Er, Yb, and sub = 3-pic and 3,5-lut) in CS2 were observed at 30°C and the lanthanide-induced shifts for all protons, including those of the chelate, were calculated using the complete dipolar shift equation. The values for the magnetic anisotropy parameters D1 and D2 and the location of the x magnetic axis were obtained by a least-squares fit of the observed data. The calculated magnetic anisotropies were found to be in fair to good agreement with the single-crystal anisotropy data. It was found that the location of the x magnetic axis was the parameter most sensitive to errors in the procedure.

Thermodynamic parameters for the conformational equilibrium of [Eu(tmhd)3(3Me-py)2] from slow-exchange nuclear magnetic resonance spectra

The low-temperature 1H n.m.r. spectrum ( < 153 K) of [Eu(tmhd)3(3Me-py)2] has two peaks assigned to the picoline ortho-protons. This may be the result of either a Fermi contact interaction or the presence of unequally populated conformers related by rotation of the picoline ring about the Eu–N bond. The Fermi contact interaction is dismissed because the separation of the two ortho peaks of [Eu(tmhd)3(3Me-py)2] is solvent dependent, being 16 p.p.m. in CS2 but only 3 p.p.m. in CCl2F2 at 153 K. The equilibrium constant for the conformational equilibrium has been calculated from the observed deviation of the two ortho, the meta, and the methyl peaks from the positions calculated assuming a dipolar mechanism. Analysis of the temperature dependence of the equilibrium constant yields values of ΔH⊖= 1.0 ± 0.2 kcal mol–1 and ΔS⊖= 8 ± 1 cal K–1 mol–1 in CCl2F2 and ΔH⊖= 4.3 ± 0.5 kcal mol–1 and ΔS⊖= 26 ± 3 cal K–1 mol–1 in CS2. The large values for ΔH⊖ and ΔS⊖ observed in CS2 suggest that solvation plays a dominant role in the conformational equilibrium. The smaller values observed in CCl2F2 indicate that solvation is less important there, but the relatively large ΔS⊖ shows that it remains a significant factor.