期刊
SOLID STATE NUCLEAR MAGNETIC RESONANCE
卷 101, 期 -, 页码 12-20出版社
ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.ssnmr.2019.04.002
关键词
AMUPol; DNP; MAS; Cross effect
资金
- National Science Foundation [1505038]
- Binational Science Foundation [2014149]
- Human Frontier Science Foundation
- Sam Tokuyama fellowship
- Direct For Mathematical & Physical Scien [1505038] Funding Source: National Science Foundation
- Division Of Chemistry [1505038] Funding Source: National Science Foundation
Cross Effect (CE) Dynamic Nuclear Polarization (DNP) relies on the dipolar (D) and exchange (J) coupling interaction between two electron spins. Until recently only the electron spin D coupling was explicitly included in quantifying the DNP mechanism. Recent literature discusses the potential role of J coupling in DNP, but does not provide an account of the distribution and source of electron spin J coupling of commonly used biradicals in DNP. In this study, we quantified the distribution of electron spin J coupling in AMUPol and TOTAPol biradicals using a combination of continuous wave (CW) X-band electron paramagnetic resonance (EPR) lineshape analysis in a series of solvents and a variable temperatures in solution - a state to be vitrified for DNP. We found that both radicals show a temperature dependent distribution of J couplings, and the source of this distribution to be conformational dynamics. To qualify this conformational dependence of J coupling in both molecules we carry out Broken Symmetry DFT calculations which show that the biradical rotamer distribution can account for a large distribution of J couplings, with the magnitude of J coupling directly depending on the relative orientation of the electron spin pair. We demonstrate that the electron spin J couplings in both AMUPol and TOTAPol span a much wider distribution than suggested in the literature. We affirm the importance of electron spin J coupling for DNP with density matrix simulations of DNP in Liouville space and under magic angle spinning, showcasing that a rotamer with high J coupling and optimum relative g-tensor orientation can significantly boost the DNP performance compared to random orientations of the electron spin pair. We conclude that moderate electron spin J coupling above a threshold value can facilitate DNP enhancements.
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