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

Abstract

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-((1)H,(13)C)}, with the Larmor frequencies ωa, ωb, ωH and ωC, by performing Liouville space simulations. This spin system exhibits the common (1)H solid effect (SE), (13)C cross effect (CE) and in addition high order CE-DNP enhancements. Here we show, in particular, the "proton shifted (13)C-CE" mechanism that results in (13)C polarization when the model system, at one of its (13)C-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 (1)H-SE features outside the EPR frequency range. Simulated ELDOR spectra combined with the indirect (13)C-CE (iCE) mechanism, result in additional "proton shifted (13)C-CE" features that are similar to the experimental ones. These features are also observed experimentally in (13)C-DNP spectra of a sample containing 15 mM of trityl in a glass forming solution of (13)C-glycerol/H2O and are analyzed by calculating the basic (13)C-SE and (13)C-iCE shapes using simulated ELDOR spectra that were fitted to the experimental ones.

The effect of Gd on trityl-based dynamic nuclear polarisation in solids

“…These spectral features are due to contributions from 1 H, and are discussed in detail in Ref. 24 were found to both be ~1.2 for the 5 mM and 10 mM samples, meaning that ~20% of the increase in the signal when adding Gd-DOTA can be attributed solely to the narrowing of the ELDOR spectra (less electron saturation). This is close to the ~30% experimental increase in the enhancement that was measured.…”

“…. The spectral features at ±100 MHz originating from radical clustering, will not be analyzed here 24. spectra of the 5 mM and 10 mM samples without Gd-DOTA were normalized to one by choosing proper iCE N and SE N factors.…”

Maximizing NMR signal per unit time by facilitating the e–e–n cross effect DNP rate

“…It is only with ≥ 3 coupled electron spins per radical that consistently large eSD effects were evident in the ELDOR profiles at all ν detect . Simulations of these profiles that account for differences in electron and nuclear relaxation rates can extract a phenomenological eSD parameter ( Λ eSD ), following the theory and simulation method described by Shimon et al 21,71 These extracted Λ eSD are listed in table 2. A comparison of the simulations and experimental ELDOR profiles are presented in figure S15, where good agreement was generally observed, with the relevant simulation parameters discussed in the ESI and reported in table S2.…”

Electron spin dynamics and spin–lattice relaxation of trityl radicals in frozen solutions

“…13–20 Unfortunately, experimentally-determined data needed for this quantitative modelling are very sparse. Relaxation of electron spin polarization to the lattice, diffusion of polarization through the EPR spectrum and multiple-spin flip-flops are important determinants of the entire hyperpolarization process; 14,19–21 and experimental data for conditions relevant to DNP are sparse or inconsistent.…”