University of Warmia & Mazury Olsztyn, Faculty of Mathematics & Computer Science, Sloneczna 54, PL-10710 Olsztyn, Poland.
J Chem Phys. 2012 Jul 28;137(4):044512. doi: 10.1063/1.4736854.
(1)H relaxation dispersion of decalin and glycerol solutions of nitroxide radicals, 4-oxo-TEMPO-d(16)-(15)N and 4-oxo-TEMPO-d(16)-(14)N was measured in the frequency range of 10 kHz-20 MHz (for (1)H) using STELAR Field Cycling spectrometer. The purpose of the studies is to reveal how the spin dynamics of the free electron of the nitroxide radical affects the proton spin relaxation of the solvent molecules, depending on dynamical properties of the solvent. Combining the results for both solvents, the range of translational diffusion coefficients, 10(-9)-10(-11) m(2)/s, was covered (these values refer to the relative diffusion of the solvent and solute molecules). The data were analyzed in terms of relaxation formulas including the isotropic part of the electron spin - nitrogen spin hyperfine coupling (for the case of (14)N and (15)N) and therefore valid for an arbitrary magnetic field. The influence of the hyperfine coupling on (1)H relaxation of solvent molecules depending on frequency and time-scale of the translational dynamics was discussed in detail. Special attention was given to the effect of isotope substitution ((14)N/(15)N). In parallel, the influence of rotational dynamics on the inter-molecular (radical - solvent) electron spin - proton spin dipole-dipole coupling (which is the relaxation mechanism of solvent protons) was investigated. The rotational dynamics is of importance as the interacting spins are not placed in the molecular centers. It was demonstrated that the role of the isotropic hyperfine coupling increases for slower dynamics, but it is of importance already in the fast motion range (10(-9)m(2)/s). The isotope effects is small, however clearly visible; the (1)H relaxation rate for the case of (15)N is larger (in the range of lower frequencies) than for (14)N. It was shown that when the diffusion coefficient decreases below 5 × 10(-11) m(2)/s electron spin relaxation becomes of importance and its role becomes progressively more significant when the dynamics slows done. As far as the influence of the rotational dynamics is concerned, it was show that this process is of importance not only in the range of higher frequencies (like for diamagnetic solutions) but also at low and intermediate frequencies.
（1）使用 STELAR 场循环光谱仪在 10 kHz-20 MHz 的频率范围内测量了 4-氧代-TEMPO-d(16)-(15)N 和 4-氧代-TEMPO-d(16)-(14)N 氮氧化物自由基在葵烷和甘油溶液中的 H 弛豫弥散。研究的目的是揭示自由基的自由电子的自旋动力学如何影响溶剂分子的质子自旋弛豫，这取决于溶剂的动力学性质。将两种溶剂的结果结合起来，覆盖了 10(-9)-10(-11) m(2)/s 的平移扩散系数范围（这些值是指溶剂和溶质分子的相对扩散）。根据包括电子自旋-氮自旋超精细耦合各向同性部分（对于(14)N 和(15)N 的情况）的弛豫公式对数据进行了分析，因此适用于任意磁场。详细讨论了超精细耦合对溶剂分子（1）H 弛豫的影响，取决于平移动力学的频率和时间尺度。特别关注同位素取代（(14)N/(15)N）的影响。同时，研究了旋转动力学对分子间（自由基-溶剂）电子自旋-质子自旋偶极-偶极耦合的影响（这是溶剂质子的弛豫机制）。由于相互作用的自旋未置于分子中心，因此旋转动力学很重要。结果表明，对于较慢的动力学，各向同性超精细耦合的作用增大，但在快速运动范围内（10(-9)m(2)/s）也很重要。同位素效应很小，但很明显；对于(15)N 的情况，(1)H 弛豫率较大（在较低频率范围内）比(14)N。结果表明，当扩散系数降低到 5 × 10(-11) m(2)/s 以下时，电子自旋弛豫变得重要，并且当动力学减慢时，其作用变得越来越重要。就旋转动力学的影响而言，表明该过程不仅在较高频率范围内（如抗磁性溶液）重要，而且在低频和中频范围内也重要。
