Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan.
Phys Chem Chem Phys. 2012 Jul 7;14(25):9137-48. doi: 10.1039/c2cp40778g. Epub 2012 May 28.
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 ω(1) (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 ω(1)-driven nutation behaviour with the increasing strength of ω(1). 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 ω(1)-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 hyperfine 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[((2)H(3))methyl]-[3,3-(2)H(2),4-(2)H(1),5,5-(2)H(2)]piperidin-N-oxyl-4-yl)(2,2,6,6-tetra[((2)H(3))methyl]-[3,3-(2)H(2),4-(2)H(1),5,5-(2)H(2),(15)N]piperidin-(15)N-oxyl-4-yl) 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.Δg = 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.
弱交换耦合双自由基在 NMR 波谱学中用于生物系统的 DNP 应用或合成电子自旋量子位的应用方面引起了广泛关注。基于脉冲 ESR 的电子自旋旋进（ESN）光谱学应用于双自由基通常从理论上被视为跃迁矩光谱学，表明它是一种确定弱交换耦合电子自旋之间相对短距离的强大且简便的工具。对于任何弱交换耦合系统，作为微波辐照强度ω(1)（角频率）函数的旋进频率可以分为三类; D(12)（自旋偶极相互作用）驱动、Δg 驱动和ω(1)驱动的旋进行为，随着ω(1)的增加。对于异自旋双自由基，Δg 效应可能是双自由基旋进光谱学中的主要特征。在 X 波段工作的基于二维脉冲的电子自旋旋进（2D-ESN）光谱学可以在刚性玻璃中确定弱交换耦合双自由基中较小的 D(12)值。这里推导的与ω(1)相关的旋进频率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，我们评估了核超精细相互作用对旋进频率的影响，以检查当前理论处理的有效性。典型的 TEMPO 双自由基 1，（2,2,6,6-四[（2）H（3）甲基]-[3,3-（2）H（2），4-（2）H（1），5,5-（2）H（2）]哌啶-N-氧-4-基）（2,2,6,6-四[（2）H（3）甲基]-[3,3-（2）H（2），4-（2）H（1），5,5-（2）H（2），（15）N]哌啶-（15）N-氧-4-基）对苯二甲酸酯在甲苯玻璃中的自旋偶极耦合，两个自旋位点之间的距离为 1.69nm，D(12) = -32 MHz（由于 1 的同自旋位点，即Δg = 0，交换耦合 J(12)的影响为零），而 0 < |J(12)|≦ 1.0 MHz，这是通过模拟 1 的随机取向 CW ESR 光谱确定的。此外，我们进行了 Q 波段脉冲 ELDOR（电子-电子双共振）实验，以确认是否获得了准确的 D(12)和 J(12)值。对于传统的、强大的 ELDOR 光谱学的距离测量能力，该距离处于模糊区域。将适用于弱交换耦合多电子系统的单个微波频率的 ESN 光谱学与传统的 ELDOR 光谱学进行了比较，讨论了其优缺点。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。
