The discovery of a truncated cross-effect (CE) in dynamic nuclear polarization (DNP) NMR that has the features of an Overhauser-effect DNP (OE-DNP) is reported here. The apparent OE-DNP, where minimal μw power achieved optimum enhancement, was observed when doping Trityl-OX063 with a pyrroline nitroxide radical that possesses electron-withdrawing tetracarboxylate substituents (tetracarboxylate-ester-pyrroline or TCP) in vitrified water/glycerol at 6.9 T and at 3.3 to 85 K, in apparent contradiction to expectations. While the observations are fully consistent with OE-DNP, we discover that a truncated cross-effect ( tCE) is the underlying mechanism, owing to TCP's shortened T. We take this observation as a guideline and demonstrate that a crossover from CE to tCE can be replicated by simulating the CE of a narrow-line (Trityl-OX063) and a broad-line (TCP) radical pair, with a significantly shortened T of the broad-line radical.

Trityl-OX063 is a narrow-line, water-soluble, and biocompatible polarizing agent, widely used for dynamic nuclear polarization (DNP) amplified NMR of 13C, but not of the abundant 1H nuclear spin, for which the ineffective solid effect (SE) is expected to be operational. Surprisingly, we observed a crossover from SE to thermal mixing (TM) DNP of 1H with increasing Trityl-OX063 concentration at 7 T. We experimentally ascertained diagnostic signatures of TM-DNP that have only been theoretically predicted: (i) an electron paramagnetic resonance (EPR) spectrum that maintains an asymmetrically broadened EPR line from strong e–e couplings and (ii) hyperpolarization, i.e., cooling of select electron-spin populations, manifested in a characteristic pump–probe electron double-resonance spectrum under DNP conditions. Low microwave power requirements, high polarization transfer rates, and efficient DNP at high magnetic fields are the key benefits of TM-DNP.

Photoexcited organic chromophores appended to stable radicals can serve as qubit and/or qudit candidates for quantum information applications. 1,6,7,12-Tetra-(4-tert-butylphenoxy)-perylene-3,4 : 9,10bis(dicarboximide) (tpPDI) linked to a partially deuterated α,γ-bisdiphenylene-β-phenylallyl radical (BDPAd 16 ) was synthesized and characterized by time-resolved optical and electron paramagnetic resonance (EPR) spectroscopies. Photoexcitation of tpPDI-BDPA-d 16 results in ultrafast radical-enhanced intersystem crossing to produce a quartet state (Q) followed by formation of a spin-polarized doublet ground state (D 0 ). Pulse-EPR experiments confirmed the spin multiplicity of Q and yielded coherence times of T m = 2.1 � 0.1 μs and 2.8 � 0.2 μs for Q and D 0 , respectively. BDPA-d 16 eliminates the dominant 1 H hyperfine couplings, resulting in a single narrow line for both the Q and D 0 states, which enhances the spectral resolution needed for good qubit addressability.

The scope of this Perspective is to analytically describe NMR hyperpolarization by the three-spin cross effect (CE) dynamic nuclear polarization (DNP) using an effective Hamiltonian concept. We apply, for the first time, the bimodal operator-based Floquet theory in the Zeeman-interaction frame for two and three coupled spins to derive the known interaction Hamiltonian for CE-DNP. With a unified understanding of CE-DNP, and supported by empirical observation of the state of electron spin polarization under the given experimental conditions, we explain diverse manifestations of CE from oversaturation, enhanced hyperpolarization by broad-band saturation, to nuclear spin depolarization under magic-angle spinning.

Major advances have recently been made in the field of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance (NMR). These developments have improved the resolution and sensitivity of the NMR spectrum of spins coupled to protons. One such new scheme, denoted as rCW, has proven to be robust with practically no need for parameter optimization [A. Equbal et al. Chem. Phys. Lett., 635, 339 (2015)]. Most of the experiments with rCW have been carried out in the regimes of slow to moderate magic-angle spinning while simultaneously applying high decoupling radio-frequency amplitudes. Here, we explore the performance of the rCW sequence and its predecessor rCW in the regime of low-power radio-frequency irradiation and fast magic-angle spinning. The robustness of the refocused continuous-wave (rCW) schemes to experimental parameters such as pulse lengths and offset irradiation is demonstrated. Numerical simulations and analytical theory have been used to understand the effects of various nuclear spin interactions on the decoupling performance of the low-power rCW decoupling scheme relative to other decoupling methods. This has lead to the design of an "optimum low-power decoupling sequence" that can be used without parameter optimization. This result is particularly important in the context of samples with low signal to noise.

Balancing dipolar and exchange coupling is essential for efficient Cross Effect DNP. This explains the complex performance of standard radicals (AMUPOL and HyTek) at high magnetic field and fast spinning.

No abstract

Dynamic Nuclear Polarization (DNP) is a sensitivity enhancing technique for Nuclear Magnetic Resonance. A recent discovery of Overhauser Effect (OE) DNP in insulating systems under cryogenic conditions using 1,3-bisdiphenylene-2-phenylallyl (BDPA) as the polarizing agent (PA) has caught attention due to its promising DNP performance at a high magnetic field and under fast magic angle spinning conditions. However, the mechanism of OE in insulating-solids/BDPA is unclear. We present an alternative explanation that the dominant underlying DNP mechanism of BDPA is Thermal Mixing (TM). This is ascertained with the discovery that TM effect is enhanced by multi-electron spin coupling, which is corroborated by an asymmetric electron paramagnetic resonance line shape signifying the coexistence of clustered and isolated BDPA species, and by hyperpolarized electron spin populations giving rise to an electron spin polarization gradient which are characteristic signatures of TM DNP. Finally, quantum mechanical simulations using spatially asymmetrically coupled three electron spins and a nuclear spin demonstrate that triple-flip DNP, with hyperfine fluctuations turned off, can yield the 1 H DNP profile as observed with BDPA. Clarifying the DNP mechanism is critical to develop design principles for optimizing the PA for achieving optimal DNP efficiency.