How many bound valence states does the C₆₀⁻ anion have?

Can anions possess bound doubly-excited electronic states?

Anions play an important role in many fields of chemistry. Many molecules possess stable anions, but these anions often do not have stable electronic excited states and the anion loses its excess electron once excited. All the known stable valence excited states of anions are singly-excited states, i.e., valence doubly-excited states have not been reported. As excited states are relevant for numerous applications, and constitute basic properties, we searched for valence doubly-excited states which are stable, i.e., exhibit energies below that of the ground state of the respective neutral molecule. We concentrated on two promising prototype candidates, the anions of the smallest endocircular carbon ring Li@C₁₂ and of the smallest endohedral fullerene Li@C₂₀. By employing accurate state-of-the-art many-electron quantum chemistry methods, we investigated the low-lying excited states of these anions and found that they possess several low-lying stable singly-excited states and, in particular, a stable doubly-excited state each. It is noteworthy that the found doubly-excited state of Li@C₁₂^- possesses a cumulenic carbon ring in sharp contrast to the ground and singly-excited states. The findings shed light on how to design anions with stable valence singly- and doubly-excited states. Possible applications are mentioned.

Endocircular Li Carbon Rings

By employing accurate state-of-the-art many-electron quantum-chemistry methods, we establish that monocyclic carbon rings can accommodate Li guest atoms. The low-lying electronic states of these endocircular systems are analyzed and found to include both charge-separated states where the guest Li atom appears as a cation and the ring as an anion and encircled-electron states where Li and the ring are neutral. The electron binding energies of the encircled-electron states increase drastically at their highly symmetric equilibrium geometries with increasing size of the ring, and in Li@C24 , this state becomes the ground state. Li is very weakly bound vertical to the rings in the low-lying encircled-electron states, hinting to van-der-Waals binding. Applcations are mentioned.

Caged-electron states and split-electron states in the endohedral alkali C60

The low-lying electronic states of neutral X@C60 (X = Li, Na, K, Rb) have been computed and analyzed by employing state-of-the-art high level many-electron methods. Apart from the common charge-separated states, well known to be present in endohedral fullerenes, one non-charge-separated state has been found in each of the investigated systems. In Li@C60 and Na@C60, the non-charge-separated state is a caged-electron state already discussed before for Li@C60. This indicates that the application of this low-lying state of Li@C60 discussed before is also applicable for Na@C60. In K@C60 and Rb@C60, the electronic radial distribution analysis shows that this hitherto unknown non-charge-separated state possesses a different nature from that of a caged-electron state.

Bound states and symmetry breaking of the ring C20 - anion

Determining the geometry of carbon rings is an ongoing challenge. Based on our calculations at a state-of-the-art level, we found that the C₂₀ ^- ring possesses five bound electronic states, including a superatomic state, which is the first superatomic state found for a ring. The nature of these electronic states is discussed. Our calculation reveals a symmetry breaking of the C₂₀ ^- ring anion ground electronic structure occurring upon attaching an electron to the neutral ring. The discussion of the possible symmetry breaking mechanisms indicates that the shrinking and distortion of the ring upon electron attachment, leading to the symmetry breaking, is a result of the interplay between the symmetry breaking and the totally symmetric modes. The discussion enriches the palette of possible symmetry breaking phenomena in carbon clusters.

Caged-Electron States in Endohedral Li Fullerenes

By employing large-scale high-level EA-EOM-CCSD calculations, we have computed and analyzed the low-lying states of neutral Li@C₆₀. Apart from one state, all states are found to be charge-separated states of the type Li⁺@C₆₀^-. The new state is the first reported non-charge-separated state in endohedral alkali fullerenes. This caged-electron state is analyzed in detail. Arguments are given that in larger highly symmetric endohedral fullerenes the caged-electron state can be the electronic ground state of the system. HF and DFT calculations on Li@C₁₈₀ indeed find that the caged-electron state is the ground state and that in its equilibrium geometry Li sits at the center of the cage. Applications are mentioned.

Charge separated states of endohedral fullerene Li@C20

We report on high-level coupled-cluster calculations of electronic states of the neutral endohedral fullerene Li@C₂₀. All computed states of neutral Li@C₂₀ are found to be the charge separated states of the Li⁺@C₂₀ ^- type. Using the state-of-the-art EA-EOM-CCSD method, we found that neutral Li@C₂₀ (D_3d) possesses several valence and superatomic charge separated states with considerable electron binding energies, the strongest bound state of Li⁺@C₂₀ ^- being the 1²E_u state (6.73 eV). The valence charge separated states correspond to two sets of states of C₂₀ ^-. The states 1²E_u, 1²A_2u, 2²E_u, and 2²A_2u correspond to the respective bound states of C₂₀ ^-, and the states 2²A_2g, 1²E_g, 1²A_1g, and 4²E_u correspond to the unbound states of C₂₀ ^-. There are eight superatomic states with electron binding energy higher than 1.0 eV, being much stronger bound than the single weakly bound superatomic state of the parent fullerene anion. The analysis of the radial density distribution of the excess electron on the carbon cage indicates the important role of the inner part of the superatomic states in forming the charge separated states.

Spin Signature of the C60 Fullerene Anion: A Combined X- and D-Band EPR and DFT Study

Fullerenes attract much attention in various scientific fields, but their electronic properties are still not completely understood. Here we report on a combined EPR and DFT study of the fullerene anion C60- in solid glassy environment. DFT calculations were used to characterize its electronic structure through spin density distribution and magnetic resonance parameters. The electron spin density is not uniformly distributed throughout the C60- cage but shows a pattern similar to PC61BM-. EPR spectroscopy reveals a rhombic g-tensor sensitive to the environment in the frozen glassy solutions, which can be rationalized by deformation of the fullerenes along low-frequency vibrational modes upon cooling. DFT modeling confirms that these deformations lead to variation in the C60- g values. The decrease in g-tensor anisotropy with sample annealing is related to the lessening of g-tensor strain upon temperature relaxation of the most distorted sites in the glassy state.

Bound electronic states of the smallest fullerene C20- anion

We report on high-level coupled-cluster calculations for the anion states of the smallest fullerene C20. Using the state-of-the-art EA-EOM-CCSD method we revealed that the C20- anion has five bound electronic states at the C20 neutral ground-state D3d equilibrium configuration. These are two pairs of 2Eu and 2A2u states and one 2A1g state. The binding energies vary from 2.05 eV for the most bound 2Eu state to <1 meV for the 2A1g state. An analysis in terms of radial and angular density distribution of the excess electron revealed that the two 2Eu/2A2u pairs are valence-like states while 2A1g corresponds to a super-atomic-like (SAMO) state. The valence states of the first 2Eu/2A2u pair were found to be of p-type whereas those of the second pair are of hybrid sp-type. We have also applied a simple model to understand the binding of the excess electron in C20-. The model confirms the existence of only one SAMO state of the s-type for C20-. At the same time, it overestimates the number of the bound valence states of C20-.