High-Spin-State-Induced Aromaticity Reversal in Cyclo[6]carbon

Abstract Cyclo[6]carbon (C6), the smallest even-membered cyclocarbon, provides a stringent platform for examining how a highly strained sp-hybridized carbon ring responds to changes in electronic state. Here, we investigate the structural, electronic, magnetic-response, excited-state, and stability/reactivity features of C6 using density functional theory, time-dependent density functional theory, ab initio molecular dynamics, and a CASSCF(12,12) occupation-number analysis. The optimized S0 structure is strictly planar and shows pronounced bond-angle alternation together with nearly uniform C–C bond lengths, indicating that its bonding cannot be described by a simple localized alternating-bond picture. Real-space descriptors, including IRI-π and ELF-π, outline a continuous dual-π delocalized framework in S0 and provide the electronic reference for analyzing state-dependent magnetic responses. Magnetic-response analyses based on ACID, ICSSzz, and NICSzz further show that the selected high-spin quintet Q1 state undergoes a pronounced reversal from the diatropic response of S0 to a paratropic response. A CASSCF(12,12) occupation-number analysis supports a predominantly closed-shell description of S0 C6, with no indication of pronounced open-shell multireference character, thereby supporting the qualitative interpretation of the DFT-based descriptors. Density-of-states and representative excited-state analyses further indicate that changes in electronic-state occupation redistribute electron density within the πin and πout manifolds, providing additional electronic-structure context for state-dependent changes within the dual-π framework. AIMD simulations indicate that the S0 ring framework is kinetically stable at low temperature but becomes increasingly flexible at elevated temperature, while ESP and ALIE analyses identify the C–C framework as the main reactive region. These results reveal a pronounced aromaticity reversal in the high-spin Q1 state and highlight the spin-state-dependent magnetic response of the dual-π framework in C6.