The Current State of Measuring Bimolecular Spin Exchange Rates by the EPR Spectral Manifestations of the Exchange and Dipole–Dipole Interactions in Dilute Solutions of Nitroxide Free Radicals with Proton Hyperfine Structure

The Dobryakov–Lebedev Relation Extended to Partially Resolved EPR Spectra

“…See Fig. 8 of [16] that nearly fulfills this expectation. With in hand, all of the correction procedures in Ref.…”

“…Figure 7 shows the effect of raising the concentration of Tempol from C = 0.1 to 0.5 mM. It is well-known that HSE increases the linewidths which is expected to reduce the resolution; however, concentration-dependent measurements are considerably more interesting than that as discussed in depth recently [16]. Briefly, the less intense proton lines are broadened the most; the proton lines move toward one another; and intra line HSE introduces HSE-induced signals with the form of dispersion signals [16].…”

“…Note that ΔH 0 pp increases by only 6% while ΔH L pp increases by 78%. Errors are always inevitable in using changes in ΔH 0 pp to estimate changes in ΔH L pp , even in deuterated nitroxides [16], but this illustrates that moving into the resolved regime exacerbates the problem.…”

“…Thus, at − 27 °C, V res ~ 0.65 V fit and V fit ~ 0.65 V spc ; at 32 °C, V res ~ 0.13 V fit and V fit ~ V spc . In the past [16], we have proposed that the ratio V res ∕V spc be less than 1% for a line to be considered a good V in the case of unresolved lines. Obviously, for resolved lines, this criterion cannot apply, and the question of whether an experimental line in Fig.…”

“…It is well-known that HSE increases the linewidths which is expected to reduce the resolution; however, concentration-dependent measurements are considerably more interesting than that as discussed in depth recently [16]. Briefly, the less intense proton lines are broadened the most; the proton lines move toward one another; and intra line HSE introduces HSE-induced signals with the form of dispersion signals [16]. This produces an interesting situation where the shifts tend to reduce ΔH G pp and therefore while the broadening increases ΔH L pp also reducing .…”