Bridging Vibrations and Spins: Mode-Resolved Spin–Phonon Coupling Revealed through THz EPR/Magnetic IR Simulation

Molecular electron spin qubits show great potential for quantum information storage and processing but require paramagnetic systems with slow relaxation, where coherence is limited by spin-phonon coupling. Understanding this coupling remains challenging due to scarce observables and conflicting models, making direct experimental insights crucial. Vibrational spectroscopy (IR and Raman) under magnetic fields provides a promising approach. Here, we present the first comprehensive simulation protocol to extract spin-phonon coupling parameters from THz EPR/Magnetic IR spectra. Different spectral features are illustrated using a one-phonon model, emphasizing cases where the coupling is weaker than the line width. Using a tetrahedral high-spin Co(II) complex, we validate the method and benchmark it against quantum-chemical calculations. The dominant coupling is attributed to a twisting mode of the first coordination sphere. Excellent agreement among experiment, simulation, and computation demonstrates the power of THz EPR/Magnetic IR spectroscopy as a direct probe of spin-phonon coupling.