The interplay between the solid effect and the cross effect mechanisms in solid state 13C DNP at 95 GHz using trityl radicals

Abstract

The 13C solid state Dynamic Nuclear Polarization (DNP) mechanism using trityl radicals (OX63) as polarizers was investigated in the temperature range of 10–60K. The solutions used were 6M 13C urea in DMSO/H2O (50% v/v) with 15mM and 30mM OX63. The measurements were carried out at ∼3.5T, which corresponds to Larmor frequencies of 95GHz and 36MHz for the OX63 and the 13C nuclei, respectively. Measurements of the 13C signal intensity as a function of the microwave (MW) irradiation frequency yielded 13C DNP spectra with temperature dependent lineshapes for both samples. The maximum enhancement for the 30mM sample was reached at 40K, while that of the 15mM sample at 20–30K. Furthermore, the lineshapes observed showed that both the cross effect (CE) and the solid effect (SE) DNP mechanisms are active in this temperature range and that their relative contribution is temperature dependent. Simulations of the spectra with the relative contributions of the CE and SE mechanisms as a fit parameter revealed that for both samples the CE contribution decreases with decreasing temperature while the SE contribution increases. In addition, for the 15mM sample the contributions of the two mechanisms are comparable from 20K to 60K while for the 30mM the CE dominates in this range, as expected from the higher concentration. The steep decrease of the CE contribution towards low temperatures is however unexpected. The temperature dependence of the OX63 longitudinal relaxation, DNP buildup times and 13C spin lattice relaxation times did not reveal any obvious correlation with the DNP temperature dependence. A similar behavior of the CE and SE mechanism was observed for 1H DNP with the nitroxide radical TEMPOL as a polarizer. This suggests that this effect is a general phenomenon involving a temperature dependent competition between the CE and SE mechanisms, the source of which is, however, still unknown.