The vibrational bands of carbon monoxide bound to hemes or metal surfaces

Abstract

Infrared spectra of imidazole carbonyl complexes of 2,4-substituted hemes are presented. An increased CO stretch frequency is accompanied by a lowered FeC vibrational energy. Hartree-Fock-Slater electron structure calculations discern π and σ contributions to the observed shifts of vibrational energies. We conclude that an enhanced electron availability manifests itself as (i) a lowered ferric/ferrous reduction potential, (ii) increased filling of the 2π orbital of liganded CO which in turn reduces ν CO and increases ν Fec, and (iii) increased basicity of the liganded CO. Analogies between CO liganded to heme and CO adsorbed onto metal surfaces are discussed.