The electronic structure and deexcitation pathways of an isolated metalloporphyrin ion resolved by metal L-edge spectroscopy

Abstract

Near-edge X-ray absorption mass spectrometry (NEXAMS) and restricted active space (RAS) quantum mechanical calculations at the metal L-edge reveal the electronic structure and orbital-specific deexcitation pathways of gas-phase metalloporphyrins.

Ionization and Photofragmentation of Isolated Metalloporphyrin Cations Investigated by VUV Action Spectroscopy**

“…Note that water losses contribute comparatively weakly to the VUV photofragmentation in the singly charged region (Figure 3). However, we have observed such loss channels after soft X‐ray excitation of CoPPIX + [29] …”

“…In addition, we observe peaks at m/z 276.0 and 267.0, corresponding to losses of −64 u and −82 u, which are not present as singly charged species and do not match any combination of side‐chain cleavages. In our previous study upon metal inner‐shell excitation of CoPPIX + we assigned the −64 u and −82 u losses to the loss of a pentadiyne (−C 5 H 4 , −p) fragment and an additional water molecule (−p−w), respectively [29] …”

“…where R m ( z ) is the radius of the porphyrin ring, ϵ is the absolute permittivity of the medium and e the electrical charge. R m ( z ) is here the maximum distance of the metal center and a carbon atom of the porphyrin ring ( R m ( z )=4.27 Å) and is taken from the DFT‐optimized geometry of Co III PPIX + [29] . If an ionization energy for neutral FePPIX of 6.25 eV is assumed, the calculated ionization energy of 9.6 eV is within the uncertainty window of the experimentally determined value.…”

“…[ 16 , 20 , 24 ] Recently, we have studied the electronic structure of isolated CoPPIX + and its photofragmentation after cobalt inner‐shell excitation in the soft X‐ray regime. [29] The post‐ionization side‐chain cleavages observed were differing, to some extent, from optical excitation experiments, suggesting that different mechanisms are involved in the formation of these fragments. Furthermore, we observed that a specific deexcitation mechanism is involved upon inner‐shell excitation of the metal center and subsequent ionization leading to for example water losses from the photoionized precursor.…”

“…CH 2 COOH side‐chain losses [16,20,24] . Recently, we have studied the electronic structure of isolated CoPPIX + and its photofragmentation after cobalt inner‐shell excitation in the soft X‐ray regime [29] . The post‐ionization side‐chain cleavages observed were differing, to some extent, from optical excitation experiments, suggesting that different mechanisms are involved in the formation of these fragments.…”