Designing broadband pulsed dynamic nuclear polarization sequences in static solids

Abstract

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons with large thermal polarization to NMR-active nuclei via microwave (mw) irradiation. The ability to generate arbitrarily shaped mw pulses using arbitrary waveform generators allows for remarkable improvement of the robustness and versatility of DNP. We present here novel design principles based on single-spin vector effective Hamiltonian theory to develop new broadband DNP pulse sequences, namely, an adiabatic version of XiX (X–inverse X)–DNP and a broadband excitation by amplitude modulation (BEAM)–DNP experiment. We demonstrate that the adiabatic BEAM-DNP pulse sequence may achieve a 1 H enhancement factor of ∼360, which is better than ramped-amplitude NOVEL (nuclear spin orientation via electron spin locking) at ∼0.35 T and 80 K in static solids doped with trityl radicals. In addition, the bandwidth of the BEAM-DNP experiments (~50 MHz) is about three times the 1 H Larmor frequency. The generality of our theoretical approach will be helpful in the development of new pulsed DNP sequences.

Reverse dynamic nuclear polarisation for indirect detection of nuclear spins close to unpaired electrons

“…Interestingly, with periodic in-version every 30 µs, there is no longer a difference between low and high concentrations. Together, these results suggest that electron-electron interactions influence nuclear spin diffusion away from the paramagnetic centre, as already discussed in Wolfe (1973). Note that a decoupling period of τ dec = 30 µs is not at all sufficient to completely suppress the hyperfine couplings, which can be in the megahertz (MHz) range.…”

“…The method has potential with respect to investigating the influence of spin diffusion away from a paramagnetic centre (Wolfe, 1973;Stern et al, 2021;Tan et al, 2019b;Jain et al, 2021). On the other hand, we envisage the use of reverse DNP in electron-nuclear double-resonance (ENDOR) experiments of nuclei with substantial hyperfine couplings (Harmer, 2016;Rizzato et al, 2013).…”

Efficient high-field MAS-DNP NMR reveals hidden side chains in amyloid fibrils

“…However, it is known that the main MAS-DNP mechanism, cross effect, has decreasing DNP performance (or lower ε) with increasing magnetic field. 37,43 Although pulsed DNP techniques with field-independent performances have been suggested to circumvent this issue, [44][45][46][47] their stringent microwave power requirement has limited experimental demonstrations to low fields (1.2 T). An alternate strategy is to design a new class of DNP polarizing agents (or hetero-biradicals) that maximally exploit the cross-effect matching conditions at high magnetic fields.…”

Coherent Dynamic Nuclear Polarization using Chirped Pulses

“…This approach, TOP-DNP, was first implemented with a train of pulses with constant phase , and yields an oscillatory nuclear Zeeman polarization frequency profile. More recently, the approach was extended by implementing analogs of the TPPM and XiX , 1 H decoupling sequences and (with much higher power) as BEAM-DNP, all of which seek to “recouple” electrons to 1 H.…”

“…The high DNP transfer ratio of chirped pulses is attributable to the adiabatic level crossing that is intrinsically more efficient than other transition pumping pulsed techniques. This is also the case for ramped-amplitude NOVEL and BEAM-DNP . These estimates are discussed in more detail in the Supporting Information.…”