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首页> 外文期刊>Physical chemistry chemical physics: PCCP >Pulsed electron spin nutation spectroscopy of weakly exchange-coupled biradicals: a general theoretical approach and determination of the spin dipolar interaction
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Pulsed electron spin nutation spectroscopy of weakly exchange-coupled biradicals: a general theoretical approach and determination of the spin dipolar interaction

机译：弱交换耦合双基的脉冲电子自旋章动光谱：一般理论方法和自旋偶极相互作用的确定

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摘要

Weakly exchange-coupled biradicals have attracted much attention in terms of their DNP application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits. Pulse-ESR based electron spin nutation (ESN) spectroscopy applied to biradicals is generally treated as transition moment spectroscopy from the theoretical side, illustrating that it is a powerful and facile tool to determine relatively short distances between weakly exchange-coupled electron spins. The nutation frequency as a function of the microwave irradiation strength ω1 (angular frequency) for any cases of weakly exchange-coupled systems can be classified into three categories; D_(12) (spin dipolar interaction)-driven, Δg-driven and ω1-driven nutation behaviour with the increasing strength of ω1. For hetero-spin biradicals, Δg effects can be a dominating characteristic in the biradical nutation spectroscopy. Two-dimensional pulse-based electron spin nutation (2D-ESN) spectroscopy operating at the X-band can afford to determine small values of D_(12) in weakly exchange-coupled biradicals in rigid glasses. The analytical expressions derived here for ω1-dependent nutation frequencies are based on only four electronic spin states relevant to the biradicals, while real biradical systems often have sizable hyperfine interactions. Thus, we have evaluated nuclear hyperfme effects on the nutation frequencies to check the validity of the present theoretical treatment. The experimental spin dipolar coupling of a typical TEMPO-based biradical 1, (2,2,6,6-tetra[(~2H3)methyl]-[3,3-~2H2,4-~2H1,5,5-~2H2]piperidin-N-oxyl-4-yl)(2,2,6,6-tetra[(~2H3)methylH3,3-~2H2,4-~2H1, 5, 5-~2H2,~(15)N]piperidin-~(15)N-oxyl-4yl) terephthalate in a toluene glass, with a distance of 1.69 nm between the two spin sites is D_(12) = -32 MHz (the effect of the exchange coupling J_(12) is vanishing due to the homo-spin sites of 1, i.e. Ag = 0), while 0 < |J_(12)| ≤ 1.0 MHz as determined by simulating the random-orientation CW ESR spectra of 1. In addition, we have carried out Q-band pulsed ELDOR (ELectron-electron DOuble Resonance) experiments to confirm whether the obtained values for D_(12) and J_(12) are accurate. The distance is in a fuzzy region for the distance-measurements capability of the conventional, powerful ELDOR spectroscopy. The strong and weak points of the ESN spectroscopy with a single microwave frequency applicable to weakly exchange-coupled multi-electron systems are discussed in comparison with conventional ELDOR spectroscopy. The theoretical spin dipolar tensor and exchange interaction of the TEMPO biradical, as obtained by sophisticated quantum chemical calculations, agree with the experimental ones.
机译：弱交换耦合双基化合物在DNP在生物系统NMR光谱中的DNP应用或合成电子自旋量子位的使用方面引起了广泛关注。从理论上讲，应用于双自由基的基于脉冲ESR的电子自旋章动（ESN）光谱通常被视为过渡矩光谱，这说明它是确定弱交换耦合电子自旋之间相对较短距离的强大且便捷的工具。在任何弱交换耦合系统中，随微波辐射强度ω1（角频率）变化的章动频率可以分为三类： D_（12）（自旋偶极相互作用）驱动，Δg驱动和ω1驱动的章动行为随ω1强度的增加而增加。对于异旋双自由基，在双自由基章鱼光谱中，Δg效应可能是主要特征。在X波段上运行的基于二维脉冲的电子自旋电子化（2D-ESN）光谱可以确定刚性玻璃中弱交换耦合双自由基中的D_（12）值。此处针对与ω1相关的章动频率得出的解析表达式仅基于与双自由基相关的四个电子自旋态，而实际的双自由基系统通常具有相当大的超精细相互作用。因此，我们已经评估了核肥大对章动频率的影响，以检查当前理论治疗的有效性。典型的基于TEMPO的双自由基1，（2,2,6,6-四[（〜2H3）甲基]-[3,3-〜2H2,4-〜2H1,5,5-〜 2H2]哌啶-N-氧基-4-基）（2,2,6,6-四[（〜2H3）甲基H3,3-〜2H2,4-〜2H1，5，5-〜2H2，〜（15）在两个自旋位点之间的距离为1.69 nm的甲苯玻璃中，N]哌啶-〜（15）N-氧基-4基）对苯二甲酸酯D_（12）= -32 MHz（交换耦合的作用J_（12 ）由于1的自旋位点消失，即Ag = 0），而0 <| J_（12）|通过模拟1的随机取向CW ESR光谱确定为≤1.0 MHz。此外，我们已经进行了Q波段脉冲ELDOR（电子双倍共振）实验，以确认获得的D_（12）和J_值是否（12）准确。对于常规的功能强大的ELDOR光谱仪，其距离测量能力处于模糊区域。与传统的ELDOR光谱相比，讨论了适用于弱交换耦合多电子系统的单微波频率ESN光谱的优缺点。通过复杂的量子化学计算获得的TEMPO双自由基的理论自旋偶极张量和交换相互作用与实验结果一致。

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