Simultaneous DNP enhancements of 1H and 13C nuclei: theory and experiments

DNP on heteronuclear spin systems often results in interesting phenomena such as the polarization enhancement of one nucleus during MW irradiation at the “forbidden” transition frequencies of another nucleus or the polarization transfer between the nuclei without MW irradiation. In this work we discuss the spin dynamics in a four-spin model system of the form {ea–eb–(1H,13C)}, with the Larmor frequencies ωa, ωb, ωH and ωC, by performing Liouville space simulations. This spin system exhibits the common 1H solid effect (SE), 13C cross effect (CE) and in addition high order CE-DNP enhancements. Here we show, in particular, the “proton shifted 13C-CE” mechanism that results in 13C polarization when the model system, at one of its 13C-CE conditions, is excited by a MW field at the zero quantum or double quantum electron–proton transitions ωMW = ωa ± ωH and ωMW = ωb ± ωH. Furthermore, we introduce the “heteronuclear” CE mechanism that becomes efficient when the system is at one of its combined CE conditions |ωa − ωb| = |ωH ± ωC|. At these conditions, simulations of the four-spin system show polarization transfer processes between the nuclei, during and without MW irradiation, resembling the polarization exchange effects often discussed in the literature. To link the “microscopic” four-spin simulations to the experimental results we use DNP lineshape simulations based on “macroscopic” rate equations describing the electron and nuclear polarization dynamics in large spin systems. This approach is applied based on electron–electron double resonance (ELDOR) measurements that show strong 1H-SE features outside the EPR frequency range. Simulated ELDOR spectra combined with the indirect 13C-CE (iCE) mechanism, result in additional “proton shifted 13C-CE” features that are similar to the experimental ones. These features are also observed experimentally in 13C-DNP spectra of a sample containing 15 mM of trityl in a glass forming solution of 13C-glycerol/H2O and are analyzed by calculating the basic 13C-SE and 13C-iCE shapes using simulated ELDOR spectra that were fitted to the experimental ones.