Resonance Raman Enhancement of Imidazole Vibrations via Charge-Transfer Transitions of Pentacyanoiron(III) Imidazole and Imidazolate Complexes

Modest enhancement is observed for Raman modes of the bound imidazole (ImH) in [(ImH)Fe(CN)₅]^2- upon laser excitation in a broad, weak absorption band at ~480 nm, which is assignable, on the basis of excitation profiles, to overlapping charge-transfer transitions from the two highest ImH π Orbitals to the Fe^III d_π vacancy. Similar enhancement is seen for Im^- modes in [(Im)Fe(CN)₅]^3- upon excitation in a weak band at ~650 nm and a strong band at ~440 nm, which are suggested to be the two π CT transitions, the π orbitals being split by the bonding alterations accompanying deprotonation of the bound ImH. The Fe-imidazole stretching vibration is weak or absent in these spectra, consistent with the nonbonding character of the Fe terminal orbital. Detection of ring modes of ImH or Im^- bound to low-spin FeIII in heme proteins may require a concentration of ~25 mM if similar CT transitions can be located and excited. Detection limits might be lower if the CT dipole strengths are increased in the proteins via favorable orientations of the imidazole rings.