Category Archives: Thermal Mixing

Dynamic nuclear polarization via the cross effect and thermal mixing: B. Energy transport #DNPNMR

Wenckebach, W.Th. “Dynamic Nuclear Polarization via the Cross Effect and Thermal Mixing: B. Energy Transport.” Journal of Magnetic Resonance 299 (February 2019): 151–67.

The fundamental process of dynamic nuclear polarization (DNP) via the cross effect (CE) and thermal mixing (TM) is a triple spin flip, in which two interacting electron spins and a nuclear spin interacting with one of these electron spins flip together. In the previous article (Wenckebach, 2018) these triple spin flips were treated by first determining the eigenstates of the two interacting electron spins exactly and next investigating transitions involving these exact eigenstates and the nuclear spin states. It was found that two previously developed approaches—the scrambled states approach and the fluctuating field approach—are just two distinct limiting cases of this more general approach. It was also shown that triple spin flips constitute a single process causing two flows of energy: a flow originating in the electron Zeeman energy and a flow originating in the mutual interactions between the electron spins. In order to render their definitions more precise, the former flow was denoted as the CE and the latter as TM.

Spectral diffusion and dynamic nuclear polarization: Beyond the high temperature approximation #DNPNMR

Wenckebach, W.T., Spectral diffusion and dynamic nuclear polarization: Beyond the high temperature approximation. J. Magn. Reson., 2017. 284(Supplement C): p. 104-114.

Dynamic Nuclear Polarization (DNP) has proven itself most powerful for the orientation of nuclear spins in polarized targets and for hyperpolarization in magnetic resonance imaging (MRI). Unfortunately, the theoretical description of some of the processes involved in DNP invokes the high temperature approximation, in which Boltzmann factors are expanded up to first order, while the high electron and nuclear spin polarization required for many applications do not justify such an approximation. A previous article extended the description of one of the mechanisms of DNP—thermal mixing—beyond the high temperature approximation (Wenckebach, 2017). But that extension is still limited: it assumes that fast spectral diffusion creates a local equilibrium in the electron spin system. Provotorov’s theory of cross-relaxation enables a consistent further extension to slower spectral diffusion, but also invokes the high temperature approximation. The present article extends the theory of cross-relaxation to low temperature and applies it to spectral diffusion in glasses doped with paramagnetic centres with anisotropic g-tensors. The formalism is used to describe DNP via the mechanism of the cross effect. In the limit of fast spectral diffusion the results converge to those obtained in Wenckebach (2017) for thermal mixing. In the limit of slow spectral diffusion a hole is burnt in the electron spin resonance (ESR) signal, just as predicted by more simple models. The theory is applied to DNP of proton and 13C spins in samples doped with the radical TEMPO.

Characterizing Thermal Mixing Dynamic Nuclear Polarization via Cross-Talk between Spin Reservoirs #DNPNMR

Guarin, D., et al., Characterizing Thermal Mixing Dynamic Nuclear Polarization via Cross-Talk between Spin Reservoirs. The Journal of Physical Chemistry Letters, 2017. 8(22): p. 5531-5536.

Dynamic nuclear polarization (DNP) embraces a family of methods to increase signal intensities in nuclear magnetic resonance (NMR) spectroscopy. Despite extensive theoretical work that allows one to distinguish at least five distinct mechanisms, it remains challenging to determine the relative weights of the processes that are responsible for DNP in state-of-the-art experiments operating with stable organic radicals like nitroxides at high magnetic fields and low temperatures. Specifically, determining experimental conditions where DNP involves thermal mixing, which denotes a spontaneous heat exchange between different spin reservoirs, remains challenging. We propose an experimental approach to ascertain the prevalence of the thermal mixing regime by monitoring characteristic signature properties of the time evolution of the hyperpolarization. We find that thermal mixing is the dominant DNP mechanism at high nitroxide radical concentrations, while a mixture of different mechanisms prevails at lower concentrations.

Dynamic nuclear polarisation by thermal mixing: quantum theory and macroscopic simulations #DNPNMR

Karabanov, A., et al., Dynamic nuclear polarisation by thermal mixing: quantum theory and macroscopic simulations. Phys. Chem. Chem. Phys., 2016. 18(43): p. 30093-30104.

A theory of dynamic nuclear polarisation (DNP) by thermal mixing is suggested based on purely quantum considerations. A minimal 6-level microscopic model is developed to test the theory and link it to the well-known thermodynamic model. Optimal conditions for the nuclear polarization enhancement and effects of inhomogeneous broadening of the electron resonance are discussed. Macroscopic simulations of nuclear polarization spectra displaying good agreement with experiments, involving BDPA and trityl free radicals, are presented.

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