Near IR spectroscopic studies on the interaction between acetamide and Ln(dpm)(3) in carbon tetrachloride

The interactions between Ln(dpm)(3) [Ln(3+)=Eu3+, Yb3+, Pr3+, Sm3+, Tb3+ and Ho3+; dpm=tris 2,2,6,6-tetramethylheptane-3,5-dionate] and acetamide in CCl4 solution were investigated using near IR absorption spectroscopy. From the measured 2 nu(C=O+) amide III combination band of acetamide in the region of 2130-2180 nm (4695-4587 cm(-1)), it was found that acetamide is coordinating through its carbonyl oxygen atom to Ln(dpm)(3) [C=O ... Ln(dpm)(3)] and forms only 1:1 stoichiometric Ln(dpm)(3)- acetamide complex. The Delta H-0 values for the formation of Ln(dpm)(3)-acetamide obtained from the temperature studies are -39.1, -28.4, -25.5, -24.7, -21.1 and -17.7 kJ mol(-1) for Eu(dpm)(3), Yb(dpm)(3), Pr(dpm)(3), Sm(dpm)(3), Tb(dpm)(3) and Ho(dpm)(3), respectively, which are larger than those of the hydrogen bond between amide and various hydrogen accepters. Except Eu(dpm)(3) and Yb(dpm)(3), -Delta H-0 value increases as the ionic size increases.