Interpretation of the ionization spectra of the cyanomethanes: CH3CN,CH2(CN)2 and C(CN)4

The valence ionization spectra of the mono-, di- and tetra-cyanomethane molecules are computed by Green function methods. As previously experienced on other cyano-derivatives of organic molecules, a hole-mixing effect is found. A partial reassignment of the spectrum is possible in the case of CH₂(CN)₂, whereas, due to the compactness of the spectrum and to the limitation of the basis set, an unambiguous assignment is not possible for C(CN)₄. The Koopmans ordering of the IP's is modified in two cases by the many-body perturbation.