Ronald Dubois

Comments on the sensitivity of lanthanide-induced shifts to the metal substrate distance

Abstract The agreement between calculated and measured lanthanide induced shifts for the 2:1 pyridine (py), 4-picoline (4-pic) and 3,5-lutidine (3,5-lut) adducts of Eu(thd) 3 at both ambient and low temperatures using the full two-term dipolar shift equation has been examined as a function of LnN distance. It is found that the agreement factor is of limited sensitivity as a function of the LnN distance. These results question the validity of LnS distances determined exclusively using the single term McConnell-Robertson equation for calculating LIS.

Barrier to rotation about the Eu–N bond in the 3-picoline diadduct of tris-(2,2,6,6-tetramethylheptane-3,5-dionato)europium(III), Eu(dpm)3(3-pic)2

The rotational barrier about the Eu–N bond in Eu(dpm)3(3-pic)2 is found to be at least 7·0 kcal/mol.

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.