Raman spectroscopic study on the enthalpy of conformational change of N,N-dimethylpropionamide bound to calcium ion

Solvation structure of the calcium ion in N,N-dimethylpropionamide (DMPA) has been studied by titration Raman spectroscopy by varying molality of Ca(ClO4)2 at 298 K and by varying temperature. Raman spectra of the solutions show bands ascribable to the planar cis and non-planar staggered conformers of free DMPA in the bulk and bound DMPA in the coordination sphere of the metal ion. The solvation number of the calcium ion in DMPA is found to be six, which is appreciably smaller than that in water, indicating that a steric hindrance operates among DMPA molecules simultaneously bound to the metal ion. It is also indicated that the planar cis conformer is preferred to the non-planar staggered conformer in the bulk, while it is much less preferred around the calcium ion. The enthalpy for conformational change from the planar cis to the non-planar staggered ΔH° is then evaluated to be 0.6 kJ mol−1 for those bound to the calcium ion. The value is significantly larger (or more endothermic) than that of the corresponding conformational change (−11 kJ mol−1) around the manganese(II) ion, indicating that the enthalpy for the conformational change depends strongly on the metal ion. The ΔH° value seems to sensitively change depending on the solvation structure, particularly the ionic radius.