氮氧自由基的DFT研究。1.溶剂对4-氨基-2，2，5，5-四甲基-3-咪唑啉-N-氧自由基结构和电子特性的影响
DFT Study of Nitroxide Radicals. 1. Effects of solvent on structural and electronic characteristics of 4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl
摘要：
咪唑啉氮氧自由基常被用作不同体系中中等酸性和极性的自旋探针。本工作采用密度泛函理论（DFT）方法研究了pH敏感自旋标记物4-氨基-2，2，5，5-四甲基-3-咪唑啉-N-氧自由基（ATI）的物理化学性质（几何构型、原子电荷和电子自旋密度分布）与质子化和水环境的关系。我们的计算表明，ATI质子化应发生在氮原子的咪唑啉环，而不是在氨基。ATI的质子化导致氮氧片段>N-O的氮原子上的自旋密度降低。为了模拟ATI的水合作用，我们已经构建了一个水壳周围的自旋标记分子的方式逐渐（逐步）周围的ATI与水分子（n = 2-41）。计算的自旋密度的氮氧片段的氮原子上的ATI周围的水壳层的扩展而增加。质子化和水合ATI引起一定的变化，在计算的几何参数（键长和价角）。调查ATI-（H2O）n系统的结构和能量参数如何依赖于周围的水分子的数量，我们得出的结论是，氢键的n 41个水分子的集群可以被认为是一个合适的模型，用于模拟ATI的水合作用。
英文摘要：
Imidazoline-based nitroxide radicals are often used as spin probes for medium acidity and polarity in different systems. In this work, using the density functional theory (DFT) approach, we have studied how physico-chemical characteristics (geometry, atomic charges and electron spin density distribution) of pH-sensitive spin label 4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl (ATI) depend on protonation and aqueous surroundings. Our calculations demonstrate that ATI protonation should occur at the nitrogen atom of the imidazoline ring rather than at the amino group. Protonation of ATI leads to a decrease in a spin density on the nitrogen atom of the nitroxide fragment >N-O. For simulation of ATI hydration effects, we have constructed a water shell around a spin label molecule by means of gradual (step-by-step) surrounding of ATI with water molecules (n = 2-41). Calculated spin density on the nitrogen atom of the nitroxide fragment increased with an extension of a water shell around ATI. Both protonation and hydration of ATI caused certain changes in calculated geometric parameters (bond lengths and valence angles). Investigating how structural and energy parameters of a system ATI-(H2O)n depend on a number of surrounding water molecules, we came to the conclusion that a hydrogen-bonded cluster of n ≥ 41 water molecules could be considered as an appropriate model for simulation of ATI hydration effects.
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