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

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, ICSS zz , and NICS zz 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.