Conformation of Solvent N,N-Dimethylpropionamide in the Coordination Sphere of the Zinc(II) Ion Studied by Raman Spectroscopy and DFT Calculations

Solvation structure of the zinc(II) ion in N,N-dimethylpropionamide (DMPA) was studied by Raman spectroscopy at varying temperature and by quantum mechanical calculations. No significant ion-pair formation was found for the Zn(ClO4)2 solution in the molality range m Zn < 1.5 mol kg-1, and the solvation number of the zinc(II) ion was determined to be 4, indicating that 6-coordination of DMPA is sterically hindered. Interestingly, DMPA molecules are under equilibrium between planar cis and nonplanar staggered conformers, and the latter is more preferred in the coordination sphere, while the reverse is the case in the bulk. The ΔG°, ΔH°, and TΔS° values of conformational change from planar cis to nonplanar staggered in the coordination sphere were obtained to be −0.9, −8.5, and −7.5 kJ mol-1, respectively. Density functional theory (DFT) calculations show that the planar cis conformer is more favorable than the nonplanar staggered one in the 1:2 cluster, as is the case for a single DMPA molecule and H(DMPA)+, indicating that there hardly occurs solvent−solvent interaction through the metal ion in the Zn2+−DMPA 1:2 cluster. On the other hand, the SCF energy of [Zn(planar cis-DMPA)4 - n (nonplanar staggered DMPA) n ]2+ (n = 0−4) decreases with increasing n, implying that the nonplanar staggered conformer is preferred in the solvate ion. It is thus concluded that solvent−solvent interaction through space, or solvation steric effect, plays a crucial role in the conformational equilibrium in the coordination sphere of the four-solvate metal ion.