Abstract
The Cross-Effect (CE) Dynamic Nuclear Polarization (DNP) mechanism under Magic Angle Spinning (MAS) induces depletion or "depolarization" of the NMR signal, in the absence of microwave irradiation. In this study, the role of T(1e) on nuclear depolarization under MAS was tested experimentally by systematically varying the local and global electron spin concentration using mono-, bi- and tri-radicals. These spin systems show different depolarization effects that systematically tracked with their different T(1e) rates, consistent with theoretical predictions. In order to test whether the effect of T(1e) is directly or indirectly convoluted with other spin parameters, the tri-radical system was doped with different concentrations of GdCl(3), only tuning the T(1e) rates, while keeping other parameters unchanged. Gratifyingly, the changes in the depolarization factor tracked the changes in the T(1e) rates. The experimental results are corroborated by quantum mechanics based numerical simulations which recapitulated the critical role of T(1e). Simulations showed that the relative orientation of the two g-tensors and e-e dipolar interaction tensors of the CE fulfilling spin pair also plays a major role in determining the extent of depolarization, besides the enhancement. This is expected as orientations influence the efficiency of the various level anti-crossings or the "rotor events" under MAS. However, experimental evaluation of the empirical spectral diffusion parameter at static condition showed that the local vs. global e-e dipolar interaction network is not a significant variable in the commonly used nitroxide radical system studied here, leaving T(1e) rates as the major modulator of depolarization.