On Mechanisms of Dynamic Nuclear Polarization in Solids

Abstract

Dynamic nuclear polarization (DNP) can dramatically increase the signal in nuclear magnetic resonance (NMR) experiments by transferring spin polarization from electrons to nuclei. We discuss quantum mechanical descriptions of the "solid effect" and "cross effect" mechanisms of DNP that typically occur in solid state NMR at high magnetic field, both with and without magic angle spinning (MAS). We present several extensions of recent theoretical descriptions, including (i) treatments of solid effect DNP and cross effect DNP with MAS, in which two nuclei change spin states simultaneously and (ii) a treatment of solid effect DNP in a two-spin system without MAS, in which the ratio of microwave field strength to hyperfine coupling strength can be arbitrary. We also briefly discuss factors that influence DNP efficiency in experiments, beyond the basic principles contained in simple quantum mechanical models.Keywords: adiabatic rapid passage · cross effect · dynamic nuclear polarization · Landau-Zener · magic angle spinning · NMR spectroscopy · radicals · solid effect Isr.

Theoretical aspects of Magic Angle Spinning - Dynamic Nuclear Polarization

“…Recent theoretical work performed by Thurber and Tycko [34][35][36] as well as in our group [37,33] has revealed the differences between the static DNP and MAS-DNP mechanism and its underlying complexity. They emphasized that the polarization transfer between the electrons and the nuclei in rotating samples takes place during crossings of the energy levels of the rotating samples.…”

“…In this section we follow references [34,35] using the LandauZener (LZ) formula [55] to describe the time dependence of the populations and polarizations of the eight eigenstates of the three-spin system at the rotor-events. We briefly summarize here how we use the LZ formula for quantifying the population changes during a two-level anticrossing event.…”

Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves

“…S1 of the supplementary material. 10 In 1 H and directly excited 13 C NMR measurements, the nuclear spin polarization was first destroyed with several (3)(4)(5) 90 • pulses, separated by 10 ms delays before the recycle delay (see Table II). The NMR signal was then excited with a single pulse, either a 90 • pulse for 13 C NMR or a 20 • pulse for 1 H NMR.…”

“…As a result, the combination of DNP with MAS has recently become prevalent in applications of solid-state NMR to a variety of chemical and biochemical systems, 1,2 making it important to understand how DNP mechanisms are affected by MAS. 3,4 This paper reports experiments and simulations showing that MAS alone can perturb nuclear spin polarizations in frozen solutions that are paramagnetically doped with nitroxide-based compounds, even without microwave irradiation. Such samples are commonly used in solid-state NMR experiments in which DNP occurs through the cross-effect mechanism.…”

A quasi-optical and corrugated waveguide microwave transmission system for simultaneous dynamic nuclear polarization NMR on two separate 14.1 T spectrometers

“…However, the mechanism of polarization transfer largely dictates experimentally achievable DNP efficiencies, which becomes of particular importance at high magnetic fields (≥ 5 T) [2]. In this work, the cross effect [3–6] mechanism is exploited for magic-angle-spinning (MAS) DNP experiments, while the Overhauser effect is exploited for DNP enhancement in solution NMR experiments [7,8]. …”