Geometric and electronic structures of linear SMnS, cyclic η2-MnS2, and linear η1-MnS2 isomers of MnS2– clusters have been investigated with B3LYP, CCSD(T), and NEVPT2 methods. The ground state of the anionic cluster is determined as 5Πg of the linear SMnS– isomer, while the ground state of the neutral cluster may be either the 4Σg– of the same isomer or the 6A1 of the η2-MnS2 cluster. The experimental photoelectron spectrum of the MnS2– cluster is interpreted by contributions of these two isomers. The high-intensity band at a binding energy of 2.94 eV is attributed to the 5Πg → 4Σg– transition between the linear SMnS–/0 clusters. The lower energy feature in the spectrum at binding energies between 1.9 and 2.8 eV and exhibiting a low intensity, is ascribed to electron detachments within the less stable η2-MnS2–/0 clusters. Ionizations from the lowest energy 7A1 state of this isomer to the neutral 6A1, 6A2, 8A2, and 6B2 states are responsible for this part of the spectrum. The extreme low intensity part between 1.3 and 1.9 eV can be due to excited states of either SMnS– or η2-MnS2–.

Electronic structures of VO2 and its anion were investigated with density functional theory (DFT), complete active space second-order perturbation theory (CASPT2), and restricted coupled-cluster with single, double, and perturbative triple excitations (RCCSD(T)) computational quantum chemical methods. The results show that there is a near-degeneracy of the lowest 3B1, 3A1, and 1A1 states of the anion. Therefore, the 532 and 193 nm photoelectron spectra of VO2– are interpreted by exploring these states as possible initial states. The anionic ground state was identified at the highest computational level, that is, RCCSD(T), as 3B1 allowing the X band to be assigned to the 3B1 → 2B1 transition, while the lower intensity and lower binding energy X′ and X″ features are ascribed to the 1A1 → 2A1 and 3A1 → 2A1 ionizations, respectively. The latter assignment is different from the recently proposed assignment of the corresponding slow electron velocity-map imaging (SEVI) spectra. Further, the A band is suggested to be mainly the result of an ionization from 3B1 to 22A1. For all these ionizations an electron is removed from a predominant metal orbital. The higher energy bands B and C on the contrary can be ascribed as electron detachments out of molecular orbitals largely located on the oxygen centers. More precisely, the B band is attributed to the ionizations from 3B1 to 4A2 and 2A2, while the C band is proposed to originate from the 3B1 → 4B1 and 3B1 → 22B1 ionizations. The proposed novel assignment is further corroborated by calculating the Franck–Condon factors, which largely agree with the experimental vibrational progressions of the SEVI spectra.

•B3LYP, CASPT2 and RCCSD(T) geometry optimizations for MnC2-/0 carried out.•Ground states and spectroscopic relevant excited states determined.•CASPT2 and RCCSD(T) adiabatic and vertical detachment energies calculated.•Assignment for anion photoelectron spectra proposed.•Multidimensional Franck–Condon simulations given.Cyclic and linear MnC2-/0 clusters have been theoretically studied. The 7A1and6A1 states of the cyclic isomer are calculated to be the ground states of the anionic and neutral cluster, respectively. All observed bands in the experimental photoelectron spectra of MnC2- could be assigned to the cyclic isomer. The X band is ascribed to an ionization during which an electron is detached from a nonbonding 4s4p hybrid orbital. The higher bands in the spectra are the result of ionization processes in which an electron from the nonbonding HOMO of the C22- ligand is removed.

In this work, the computational quantum chemical DFT, CASPT2, and RCCSD(T) methods have been utilized to investigate the geometric and electronic structures of cyclic and linear CrC2–/0 clusters. The neutral ground state is firmly identified as the cyclic 5A1 state. For the anionic cluster, two nearly degenerate isomers were recognized, namely a cyclic 6A1 state and a linear 6Σ+ state. Therefore, assignments of the observed bands in the photoelectron spectra of CrC2– have been made based on both of these isomers. With the exception of the B band all other experimental observed bands could be ascribed to the cyclic isomer. The computed detachment energies show that the former band must be exclusively assigned to the ionization of 6Σ+ of the linear structure, which can possibly also contribute to some higher energy bands. Additional support for the proposed assignments is provided by multidimensional Franck–Condon factor simulations for the 6A1→5A1 and 6A1→5B1 transitions that show a nearly perfect match with the observed vibrational progressions of the X and A bands in the 532 nm spectra.

The photoelectron spectrum of FeO2– has been assigned by performing geometry optimizations at the CASPT2 and RCCSD(T) levels of computation. All relevant states are found to possess floppy C2v geometrical structures as the Renner-Teller splittings of the linear states are extremely small and the corresponding energy barriers for the OFeO bond angle inversions are calculated in the range of a few hundred wavenumbers. In this sense, the description of the electronic structure in terms of the D∞h point group is acceptable, and the experimentally proposed linear structure for FeO2– is theoretically confirmed. High accuracy single-point multireference RASPT2 and single-reference RCCSD(T) calculations support a 2Δg as the ground state of the anion, even though the energy differences between the 4Πg and 6Σg+ states are smaller than 0.2 eV. After this identification of the doublet ground state, the photoelectron spectra of FeO2– could be assigned in all aspects. The 2Δg→3Δg ionization appears to be at the origin of the X band at 2.36 eV, while the A band at 3.31 eV should be ascribed to the 2Δg→3Σg+ ionization. This assignment is substantiated by Franck–Condon factors for which BP86 optimized geometries and harmonic vibrational frequencies were employed. Indeed, no pronounced vibrational progression should be observed since both bands involve electron detachments out of nonbonding mainly 3d iron molecular orbitals.

DFT and multireference methods were used to investigate the electronic structure of FeO3 and FeO3− clusters. Geometries of different spin multiplicities and conformations were optimized without any symmetry restrictions at the BP/QZVP level and further refined with the CASPT2 method. Although the latter type of calculations were performed by using the C2v point group, all low-lying states relevant to the photoelectron spectrum were found to correspond to or to resemble closely a planar D3h iron trioxide with no bonds between the oxygen atoms. Depending on the computational method used, the ground state of the FeO3− anion can be either 2E′′ or 4A1′. The two lowest binding energy bands of the photoelectron spectrum of FeO3− can only be ascribed to electron detachments from the 2E′′ state. The first band is the result of a transition to the 1A1′ ground state of FeO3, whereas the second band originates from the first excited 3E′′ state. A harmonic vibrational analysis of the symmetric stretch shows that the observed vibrational progressions of these two bands in the photoelectron spectrum of FeO3− are also in line with the assignment. A molecular orbital analysis led to the conclusion that the electronic structures of the anionic and neutral clusters can formally be described by an oxidation state of iron of +5 and +6, respectively. A population analysis, on the contrary, points to an ionization that takes place on the oxygen atoms.

The geometric structures of FeS3 and FeS3– with spin multiplicities ranging from singlet to octet were optimized at the B3LYP level, allowing two low-lying conformations for these clusters to be identified. The planar D3h conformation contains three S2– atomic ligands (S3Fe0/–), whereas the C2v structure contains, in addition to an atomic S2– ligand, also a S22– ligand that is side-on-bound to the iron cation: an η2-S2FeS conformation. Subsequently, energy differences between the various states of these conformations were estimated by carrying out geometry optimizations at the multireference CASPT2 level. Several competing structures for the ground state of the anionic cluster were recognized at this level. Relative stabilities were also estimated by performing single-point RCSSD(T) calculations on the B3LYP geometries. The ground state of the neutral complex was unambiguously found to be 5B2. The ground state of the anion is considerably less certain. The 14B2, 24B2, 4B1, and 6A1 states were all found as low-lying η2-S2FeS– states. Also, 4B2 of S3Fe– has a comparable CASPT2 energy. In contrast, B3LYP and RCCSD(T) mutually agree that the S3Fe– state is at a much higher energy. Energetically, the bands of the photoelectron spectra of FeS3– are reproduced at the CASPT2 level as ionizations from either the 4B2 or 6A1 state of η2-S2FeS. However, the Franck–Condon factors obtained from a harmonic vibrational analysis at the B3LYP level show that only the 4B2-to-5B2 ionization, which preserves the η2-S2Fe–S conformation, provides the best vibrational progression match with the X band of the experimental photoelectron spectra.

High-level multireference CASPT2 calculations are performed to determine the electronic structures of the FeO and FeO− clusters. Geometry optimizations of all possible low-lying states of the two unsaturated complexes are carried out at the mentioned theoretical level by calculating their potential energy curves. FeO− is proposed to possess a 6Σ+ ground state as opposed to the 4Δ state, which has been put forward as the ground state by previous computational studies. On this basis an alternative assignment of the photoelectron spectra of this anion is postulated. All features of these spectra are tentatively attributed to either septet or quintet states of the neutral FeO cluster. The lowest energy band in the spectra is then the result of electron detachments to the 5Δ ground state and the 5Σ+ lowest excited state. The second lowest band might be due to the 7Σ+. Two higher energy bands are thought to originate from two 5Π states and a 5Φ. Vibrational progressions that are observed for some bands could be explained in terms of the calculated potential energy profiles of the relevant states. Credibility for the proposed assignment is rendered by the good correspondence between experiment and our computational method concerning the bond distance and dipole moment for the ground state of FeO.