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首页> 外文期刊>The journal of physical chemistry, B. Condensed matter, materials, surfaces, interfaces & biophysical >Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids
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Rotation of a Charged Spin Probe in Room-Temperature Ionic Liquids

机译：室温离子液体中带电旋转探针的旋转

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Y X-band electron paramagnetic resonance spectroscopy has been used to investigate the rotational diffusion of a stable, positively charged nitroxide 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (Cat-1) in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons. The rotation of Cat-1 is anisotropic with the preferential axis of rotation along the NO center dot moiety. The Stokes-Einstein-Debye law describes the mean rotational correlation time of Cat-1, assuming that the hydrodynamic radius is smaller than the van der Waals radius of the probe. This implies that the probe rotates freely, experiencing slip boundary condition, which is solvent-dependent. The rotational correlation time of Cat-1 in RTILs can very well be fitted to a power-law functionality with a singular temperature, which suggests that the apparent activation energy of rotation exhibits non-Arrhenius behavior. Compared to the rotation of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO), which is neutral, the rotation of Cat-1 is several times slower. The rotational anisotropy, the ratio of the rotational times of pDTO and Cat-1, and the apparent activation energy indicate the transition from a homogeneously globular structure to a spongelike structure when the alkyl chain has four carbons, which is also observed in molecular dynamics computational studies. For the first time, we have been able to show that the rotational correlation time of a solute molecule can be analyzed in terms of the Cohen-Turnbull free volume theory. The Cohen-Turnbull theory fully describes the rotation of Cat-1 in all ionic liquids in the measured temperature range.
机译：用X-带电子顺磁共振谱研究了一种稳定的、带正电的氮氧化物4-三甲基铵-2,2,6,6-四甲基哌啶-1-氧基碘（Cat-1）在一系列烷基-3-甲基咪唑四氟硼酸盐室温离子液体（RTIL）中的旋转扩散。Cat-1的旋转是各向异性的，优先旋转轴沿着无中心点部分。Stokes-Einstein-Debye定律描述了Cat-1的平均旋转相关时间，假设流体动力学半径小于探头的范德华半径。这意味着探针可以自由旋转，并经历滑动边界条件，这取决于溶剂。RTIL中Cat-1的旋转关联时间可以很好地拟合具有奇异温度的幂律函数，这表明旋转的表观活化能表现出非阿累尼乌斯行为。与中性的全氘化2,2,6,6-四甲基-4-氧哌啶-1-氧基（pDTO）的旋转相比，Cat-1的旋转速度慢几倍。旋转各向异性、pDTO和Cat-1的旋转时间之比以及表观活化能表明，当烷基链有四个碳时，从均匀球状结构过渡到海绵状结构，这也在分子动力学计算研究中观察到。我们第一次能够证明，溶质分子的旋转关联时间可以用科恩-特恩布尔自由体积理论来分析。Cohen-Turnbull理论充分描述了Cat-1在测量温度范围内所有离子液体中的旋转。

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