Electron spin relaxation of $N@C60$ in $CS2$

We examine the temperature dependence of the electron spin relaxation times of the molecules $N@C60$ and $N@C70$ (which comprise atomic nitrogen trapped within a carbon cage) in liquid $CS2$ solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for $S⩾1$ spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for $N@C60$⁠, indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric $N@C70$ molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time $(T2)$ ever observed for a molecular electron spin, being $0.25ms$ at $170K$⁠.