Infrared Spectra and Structure of the Crystalline Sodium Acetate Complexes of U(VI), Np(VI), Pu(VI), and Am(VI). A Comparison of Metal-Oxygen Bond Distance and Bond Force Constant in this Series

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Abstract
Infrared spectra of solid NaXO2(Ac)3, with X=U, Np, Pu, and Am, have been observed. From the symmetric and asymmetric stretching frequencies of the O–X–O groups, approximate X–O force constants have been calculated and were found to decrease in the order kU—O>kNp—O>kPu—O> kAm—O, the respective values being about 0.705, 0.698, 0.675, and 0.612 megadyne/cm. From the cell constants for NaXO2(Ac)3 it is apparent that the X–O bond distance decreases in the same order—RU—O>RNp—O>RPu—O>RAm—O. Thus, a decrease in bond distance appears to be accompanied by a decrease in force constant, probably because the bond, though shortened by contraction of the electron shells of the metal, is weakened by interaction with the extra valence shell electrons.