Nuclear inelastic resonant scattering of synchrotron radiation was applied
to the study solutions of Fe-57 complexes. In order to reveal different ine
lastic contributions solutions of two different Fe-57 complexes of differen
t molecular dimensions with solvents of substantially different viscosities
were studied. We argue that the only former experiment available in the li
terature overestimates the role of the diffusivity in affecting the spectru
m. The first direct observation of an intramolecular vibrational transition
assisting the nuclear resonance absorption in a liquid is reported.