Finally, echo detected EPR spectra were recorded with a 140 GHz EPR spectrometer at 80K for an 80mM sample and compared to results from a lower concentration, 8mM sample, shown in Figure . We observe a small asymmetry in the lineshape from the g-anisotropy, which should contribute about 140GHz 40MHz line width (the g-tensor value is taken from ). The detected line width is 45G ∼ 120 MHz for both samples. The extra broadening from 8 to 80mM is ∼11 MHz. Thus, the electronelectron dipolar interaction should be below 100 MHz as predicted from the size of trityl radicals. Assuming a homogeneous distribution of radicals in the solvent, we estimate the distance between the electron spins for the 80mM sample to be 2.7 nm, corresponding to an electronelectron dipolar interaction of 8 MHz. This confirms the measured line broadening from 8 to 80mM. Therefore, our EPR study argues against the proposition of thermal mixing, as the EPR line width though broadened by the electronelectron dipolar interaction remains below the nuclear Larmor frequency. Thus, the fundamental triple spin flip processes for thermal mixing are attenuated. In going from 140 GHz to 395 GHz, the g-anisotropy of trityl will broaden the EPR line by another ∼60 MHz, while the electronelectron dipolar interaction is invariant, further attenuating TM. Additional EPR data at 9 GHz is included in the SI.