Nino Wili

To enable dipolar recoupling in presence of large chemical shielding anisotropies, we present quaternion-based effective Hamiltonian optimal control procedures and experiments for ¹⁹ F to ¹³ C polarization transfer.

Pulsed dynamic nuclear polarization (DNP) is currently receiving substantial interest as a means to enhance the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging by orders of magnitude. It has also received much attention as a central ingredient in many modalities of electron–spin–involved quantum sensing. Relative to spin engineering associated with NMR, the design of efficient pulsed DNP experiments with a broad experimental scope is challenged by large electron-nuclear spin systems, large electron–spin–involved interactions, and instrumental nonidealities and limitations. All of this may challenge traditional NMR-like theoretical and numerical pulse sequence engineering. Exploiting state-of-the-art instrumentation and taking advantage of the high sensitivity of DNP relative to NMR, we here demonstrate the use of combinations of Bayesian machine learning methods and constrained random walk procedures to design pulse sequences in situ, by experiments, directly on the spin systems responding to spectrometer instructions. For trityl and nitroxide samples, it is demonstrated that efficient broadband DNP pulse sequences can be designed in situ with experimental protocols benchmarked against in silico analogs.

Reverse DNP combined with a band-selective inversion pulse enabled electron-detected proton spectroscopy. A 5.4–6.8 Å spin-diffusion barrier was determined and hole burning on the proton line revealed a three-spin electron diffusion mechanism.

Aimed at the simulation, design, and interpretation of advanced pulse experiments crossing the boundaries between nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), including the rapidly emerging, hybrid discipline of pulsed dynamic nuclear polarisation (DNP), we present a host of novel features in the widely used SIMPSON software package addressing these aspects. Along with this come new features for advanced pulse sequence evaluation in terms of propagator splitting, high-order spin operator cross terms, and pulse phase transients. These fundamental new tools are introduced in a C++-based next generation of the SIMPSON software, which improves calculations speed in some aspects, is better prepared for further developments, and facilitates easier community contributions to the open-source software package.

Pulsed dynamic nuclear polarization (DNP) is gradually emerging to enhance the performance of prevailing continuous-wave DNP which boosts NMR sensitivity by transferring the much higher unpaired electron polarization to nuclear spins. In pulsed operation, taking inspiration from NMR spectroscopy, periodic irradiation schemes have recently shown remarkable performance. Most efforts have focused on solid-state NMR sequences within the conventional paradigm of transverse spin-locked Hartmann–Hahn matching, whose performance in DNP applications may be compromised by the initial excitation pulse. Leveraging the flexibility of optimal control theory combined with underlying insights from effective Hamiltonian theory, we present a new family of broadband DNP pulse sequences, termed LOOP (Longitudinally Optimized with Overarching Periodicity), that alleviate the excitation-pulse challenge by accomplishing longitudinal polarization transfer. These sequences define robust single-spin effective z rotations, with impressive compensation toward microwave field inhomogeneity, and are capable of delivering DNP transfer with bandwidths exceeding 100 MHz, while employing a peak microwave field amplitude of only 32 MHz, at an external magnetic field of 0.35 T.

Aimed at the simulation, design, and interpretation of advanced pulse experiments crossing the boundaries between nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), including the rapidly emerging, hybrid discipline of pulsed dynamic nuclear polarisation (DNP), we present a host of novel features in the widely used SIMPSON software package addressing these aspects. Along with this come new features for advanced pulse sequence evaluation in terms of propagator splitting, high-order spin operator cross terms, and pulse phase transients. These fundamental new tools are introduced in a C++-based next generation of the SIMPSON software, which improves calculations speed in some aspects, is better prepared for further developments, and facilitates easier community contributions to the open-source software package.

CHEESY ENDOR: fast 2D chirp echo EPR technique aides in resolving spectral overlap and reveals hyperfine and quadrupole interactions of nuclei coupled to paramagnetic centers.

We present constrained random walk (cRW) and figure of merit (FOM) based non-linear optimization procedures for systematic design and fundamental understanding of magnetic resonance experiments, addressing bilinear and linear effective Hamiltonians to provide broadband polarization transfer. cRW can be used directly for fast random experiment design or in combination with non-linear optimization or optimal control, leveraging the optimization of a FOM function for efficient control of linear and bilinear terms derived by exact effective Hamiltonian theory. The efficacy of the combined cRW and FOM-based optimization approach is demonstrated by the design of broadband dynamic nuclear polarization pulse sequences for static solids with an electron spin excitation bandwidth reaching 100 MHz.

High performance epoxy composites are key materials for wind energy, aeronautical engineering, and other structurally demanding applications. The crosslinked epoxy polymers are highly durable with enclosed carbon or glass fibres defining the mechanical properties of the composite. Being designed to last, no viable and sustainable end-of-life recycling solutions have been implemented for these materials. Here, we report the rhodium-catalysed selective disconnection of C–O bonds in epoxy polymers using water as a benign solvent. This depolymerisation allows the recovery of bisphenol building blocks and carbon fibres from commercial epoxy composites, without addition of an external reductant or oxidant. Comprehensive experimental, theoretical and spectroscopical studies reveal that the rhodium catalysis operates in an unexplored mechanistic space with an open shell rhodium(0) species at its heart. The targeted C–O bond is disconnected via electron transfer from two metalloradical intermediates. Our results demonstrate a proof-of-concept for catalytically recovering value from end-of-life composites using the most sustainable solvent, while also expanding the known reaction modes known for rhodium catalysts. We anticipate this work to contribute to the ongoing development of depolymerisation methods for thermosets. Furthermore, the mechanistic insights present a starting point for exploring unprecedented applications in organic synthesis using rhodium.