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

Abstract

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.