Spectroscopic evidence of vibrational dipole–dipole coupling between nitrosyl groups in some nitroprussides

Abstract

The nature and magnitude of the intermolecular vibrational coupling between nitrosyl groups in Sr[Fe(CN)₅NO]⋅4H₂O and Ba[Fe(CN)₅NO]⋅2H₂O have been studied quantitatively by IR spectroscopy. The isotopic dilution technique has been used by incorporating in the first compound up to 40% of ¹⁸O as N¹⁸O ligand in the crystal. The results can be interpreted by postulating a dynamic electric dipole–dipole interaction which operates between the polar NO stretching vibrations. The main features of the fine structure displayed by the NO stretching band in the infrared spectra of polycrystalline Sr[Fe(CN)₅N(¹⁶O/¹⁸O)]⋅4H₂O, and in single crystals of Ba[Fe(CN)₅NO]⋅2H₂O can be explained in terms of a linear‐chain model of nitrosyls vibrationally coupled through transition dipole–dipole forces, with interactions between nearest neighbors. The value ‖(∂m/∂S)₀‖=13 D/Å calculated on the basis of this model for the NO dipole‐moment derivative agrees satisfactorily with the value reported from integrated IR absorption intensity measurements. The coupling force constants between neighboring nitrosyls f₁=+0.172 mdyn/Å obtained for barium nitroprusside, and the average f̄₁=+0.189 mdyn/Å value obtained for the strontium salt, show that these interactions are the strongest thus far reported for ionic solids.