[Mg3@C20]+: an s-orbital electron spin system protected in a carbon ring framework.

Increasing the proportion of s-orbital character and employing rigid ligands with zero nuclear spin can effectively suppress spin-orbit coupling, electron spin-nuclear spin hyperfine coupling, and spin-lattice vibrational coupling. This is crucial for designing electron spin-based two-level states molecular qubits. In this study, we investigated the electronic structure and magnetic properties of a Mg3+ cluster embedded in a C20 ring ([Mg3@C20]+) using density functional theory (DFT) and the complete active space self-consistent field (CASSCF) method. An unpaired electron is located on the Mg3 cluster. The Mg-Mg metal-metal bonds exhibit substantial covalent character. The s-orbital of the Mg3 cluster contributes 67% to the spin density. Nearly isotropic g-factors, nearly isotropic hyperfine structure coupling parameters, and a relatively narrow single peak in the EPR spectrum reveal that the unpaired electron is of s-orbital nature. The s-orbital electron spin system, protected by the carbon ring, represents an interesting model system for electron spin-based two-level states.