Cryo IR Spectroscopy of [Hemin]+ Complexes in Isolation.

We present cryo IR spectra of isolated [Hemin]+ adducts with CO, N2, and O2 ([Hemin(CO)1]+, [Hemin(CO)2]+, [Hemin(14N2)]+, [Hemin(15N2)]+, and [Hemin(O2)]+). Well resolved bands allow for the elucidation of structure and spin multiplicity in conjunction with density functional (DFT) calculations. There is a quartet spin state for the N2 and CO adducts and a sextet spin state for the O2 adduct, where the O2 retains its triplet state. The double CO adsorption induces significant changes in the vibrational patterns of the IR spectra, which we take as strong evidence for a spin quenching into a doublet. Our study characterizes [Hemin]+, which is the Fe3+ oxidation product of heme that is of ubiquitous presence in hemeproteins.