9.

(a) Predicted rotational correlation times in 2-MeTHF estimated using the Stokes–Einstein relation. Molecular volumes were estimated with and without an associated K­(THF)₆ ⁺ cation and are plotted with c _slip = 1 (SI Section 3.4.1). (b) Best-fit curves for the SR (dotted), SD (dashed), g anisotropy (dot-dashed), and local-mode (solid) mechanisms for [1]^•– (blue) and [3]^•– (red). The effective molecular volume c _slip V, the spin diffusion constant R _SD,max, the local mode energy Δ, and the spin–phonon proportionality constant C _local were floated in these fitting routines. Combined SR+SD relaxation also provided a close fit but it resulted in unexpectedly small c _slip and R _SD,max values (SI Section 3.4.2). (c) First and second derivatives of the g _iso value calculated for each local mode (Q) of D₂-symmetric [1]^•– are shown up to 500 cm^–1 with their associated irreducible representation (irrep); the asterisk labels a strongly mixed pair of A and B₂-symmetric modes. Only totally symmetric (A) modes are proficient at relaxing spin at first order, matching observation here. All D₂ irreps follow Γ² = A so the second derivatives can be nonzero regardless of irrep.