Abstract

Since the success in synthesizing and characterizing the first carbon ring C₁₈ in 2019, they have attracted great attention both experimentally and computationally. In this work we investigate the electronic and geometric structures of the C4n carbon rings C₈ and C₁₂ by employing state-of-the-art coupled cluster and multi-reference configuration interaction methods. Known is that C₁₂ has been synthesized and characterized and an experiment on C₈ has been performed. Computationally, only the ground states of these rings have been reported in the literature. We report on the ground and numerous excited electronic states. In the ground state the C4n rings are closed-shell systems possessing polyynic structures and can be classified as double anti-aromatic molecules. In their energetically lowest lying triplet state the rings exhibit aromatic cumulenic structures. The overall change in the electronic structures is rather dramatic upon the found moderate geometric changes from polyynic to cumulenic structure. Among others, Hund’s rule is violated in both C₈ and C₁₂ in their cumulenic structures. The reasons for the violation are analyzed. Much effort has been invested to understand the relaxation pathways of the low-lying states leading the C₈ from polyynic to cumulenic geometry and vice versa. On its minimum energy path, the first singlet excited state changes from open-shell character in the polyynic structure to a closed-shell state in the cumulenic structure. The cumulenic state lowest in energy is an open-shell singlet which relaxes to the closed-shell polyynic ground state. The lowest triplet polyynic state relaxes to a stable cumulenic structure energetically placed between these two singlets. The found states and interconnecting pathways allow us to construct a scheme to arrive at the cumulenic structure starting from the polyynic ground state of the ring.

Abstract

Since the success in synthesizing and characterizing the first carbon ring C₁₈ in 2019, they have attracted great attention both experimentally and computationally. In this work we investigate the electronic and geometric structures of the C4n carbon rings C₈ and C₁₂ by employing state-of-the-art coupled cluster and multi-reference configuration interaction methods. Known is that C₁₂ has been synthesized and characterized and an experiment on C₈ has been performed. Computationally, only the ground states of these rings have been reported in the literature. We report on the ground and numerous excited electronic states. In the ground state the C4n rings are closed-shell systems possessing polyynic structures and can be classified as double anti-aromatic molecules. In their energetically lowest lying triplet state the rings exhibit aromatic cumulenic structures. The overall change in the electronic structures is rather dramatic upon the found moderate geometric changes from polyynic to cumulenic structure. Among others, Hund’s rule is violated in both C₈ and C₁₂ in their cumulenic structures. The reasons for the violation are analyzed. Much effort has been invested to understand the relaxation pathways of the low-lying states leading the C₈ from polyynic to cumulenic geometry and vice versa. On its minimum energy path, the first singlet excited state changes from open-shell character in the polyynic structure to a closed-shell state in the cumulenic structure. The cumulenic state lowest in energy is an open-shell singlet which relaxes to the closed-shell polyynic ground state. The lowest triplet polyynic state relaxes to a stable cumulenic structure energetically placed between these two singlets. The found states and interconnecting pathways allow us to construct a scheme to arrive at the cumulenic structure starting from the polyynic ground state of the ring.

Abstract

Since the success in synthesizing and characterizing the first carbon ring C₁₈ in 2019, they have attracted great attention both experimentally and computationally. In this work we investigate the electronic and geometric structures of the C4n carbon rings C₈ and C₁₂ by employing state-of-the-art coupled cluster and multi-reference configuration interaction methods. Known is that C₁₂ has been synthesized and characterized and an experiment on C₈ has been performed. Computationally, only the ground states of these rings have been reported in the literature. We report on the ground and numerous excited electronic states. In the ground state the C4n rings are closed-shell systems possessing polyynic structures and can be classified as double anti-aromatic molecules. In their energetically lowest lying triplet state the rings exhibit aromatic cumulenic structures. The overall change in the electronic structures is rather dramatic upon the found moderate geometric changes from polyynic to cumulenic structure. Among others, Hund’s rule is violated in both C₈ and C₁₂ in their cumulenic structures. The reasons for the violation are analyzed. Much effort has been invested to understand the relaxation pathways of the low-lying states leading the C₈ from polyynic to cumulenic geometry and vice versa. On its minimum energy path, the first singlet excited state changes from open-shell character in the polyynic structure to a closed-shell state in the cumulenic structure. The cumulenic state lowest in energy is an open-shell singlet which relaxes to the closed-shell polyynic ground state. The lowest triplet polyynic state relaxes to a stable cumulenic structure energetically placed between these two singlets. The found states and interconnecting pathways allow us to construct a scheme to arrive at the cumulenic structure starting from the polyynic ground state of the ring.

Abstract

Since the success in synthesizing and characterizing the first carbon ring C₁₈ in 2019, they have attracted great attention both experimentally and computationally. In this work we investigate the electronic and geometric structures of the C4n carbon rings C₈ and C₁₂ by employing state-of-the-art coupled cluster and multi-reference configuration interaction methods. Known is that C₁₂ has been synthesized and characterized and an experiment on C₈ has been performed. Computationally, only the ground states of these rings have been reported in the literature. We report on the ground and numerous excited electronic states. In the ground state the C4n rings are closed-shell systems possessing polyynic structures and can be classified as double anti-aromatic molecules. In their energetically lowest lying triplet state the rings exhibit aromatic cumulenic structures. The overall change in the electronic structures is rather dramatic upon the found moderate geometric changes from polyynic to cumulenic structure. Among others, Hund’s rule is violated in both C₈ and C₁₂ in their cumulenic structures. The reasons for the violation are analyzed. Much effort has been invested to understand the relaxation pathways of the low-lying states leading the C₈ from polyynic to cumulenic geometry and vice versa. On its minimum energy path, the first singlet excited state changes from open-shell character in the polyynic structure to a closed-shell state in the cumulenic structure. The cumulenic state lowest in energy is an open-shell singlet which relaxes to the closed-shell polyynic ground state. The lowest triplet polyynic state relaxes to a stable cumulenic structure energetically placed between these two singlets. The found states and interconnecting pathways allow us to construct a scheme to arrive at the cumulenic structure starting from the polyynic ground state of the ring.