Molecular vibrations and structure of the light-induced metastable state of [Fe(CN)5NO]2−

The molecular structure of the metastable state (MS1) of sodium nitroprusside was studied by polarized Raman spectroscopy and the normal coordinate analysis on 14NO- and 15NO-substituted crystals. The downshift of the bending mode of the Fe–(NO) bond upon 15NO substitution is significantly smaller in MS1 than in the ground state. The normal coordinate analysis indicates that the effect of the N-atom mass on the mode is large in the Fe–N–O linkage but small in the Fe–O–N linkage. The small isotope shift observed in MS1 suggests the Fe–O–N linkage that was proposed in a recent X-ray diffraction study.