Conformational equilibria of solventN,N-dimethylpropionamide in the bulk and in the coordination sphere of the manganese(ii) ion

Abstract

The methyl group and oxygen in the propionyl group CH₃CH₂CO– of N,N-dimethylpropionamide (DMPA) have been previously shown to be planar cis. The present measurements of Raman spectra at varying temperature indicate the presence of another isomer in equilibrium with the planar cis one. It is suggested that, according to our ab initio MO calculation, it is a non-planar staggered conformer with a C–C–C–O dihedral angle of ca. 90°, i.e. the methyl group of the propionyl group is located above the amide O–C–N plane. The equilibrium shifts to the non-planar staggered conformer with increasing temperature. The ΔH° value for the conformational change from the planar cis to non-planar staggered conformer in the bulk is 5.0 kJ mol⁻¹, which is in good agreement with the theoretical ones (6.6 kJ mol⁻¹). The manganese(II) ion is five-solvated in DMPA, and it is elucidated that the non-planar staggered conformer is preferred to the planar cis conformer for DMPA molecules bound to the manganese(II) ion. This is ascribed to the solvation steric effect among solvent molecules bound to the metal ion. The ΔH° value for the conformational change of the bound DMPA from the planar cis to non-planar staggered conformation is −11 kJ mol⁻¹.