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 at 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.

交叉效应（CE）动态核极化（DNP）依赖于两个电子自旋之间的偶极（D）和交换（J）耦合相互作用。直到最近，在量化DNP机理中仅明确包括电子自旋D耦合。最近的文献讨论了J耦合在DNP中的潜在作用，但没有提供DNP中常用双自由基的电子自旋J耦合的分布和来源。在这项研究中，我们通过在一系列溶剂中以及在温度可变的情况下，使用连续波（CW）X波段电子顺磁共振（EPR）线形分析的组合来量化AMUPol和TOTAPol双自由基中电子自旋J耦合的分布– DNP需玻璃化的状态。我们发现两个自由基都显示出温度依赖性的J偶合分布，并且这种分布的来源是构象动力学。为了验证两个分子中J耦合的这种构象依赖性，我们进行了破碎对称DFT计算，该计算表明双自由基旋转异构体分布可以解释J耦合的较大分布，而J耦合的大小直接取决于电子的相对方向。自旋对。我们证明了AMUPol和TOTAPol中的电子自旋J耦合比文献中所建议的分布宽得多。我们通过在Liouville空间和魔角旋转下DNP的密度矩阵模拟来确认DNP电子自旋J耦合的重要性，证明了与电子自旋对的随机取向相比，具有高J耦合和最佳相对g张量取向的旋转异构体可以显着提高DNP性能。我们得出的结论是，高于阈值的中等电子自旋J耦合可以促进DNP增强。