Vujosevi Cacute Danilo, Dilger Herbert, McKenzie Iain, Martyniak Aleksandra, Scheuermann Robert, Roduner Emil
Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
J Phys Chem B. 2007 Jan 11;111(1):199-208. doi: 10.1021/jp065488q.
Ordering of solvent molecules in the vicinity of a dipolar free radical affects its hyperfine coupling constants (hfcs). Specifically, it is demonstrated how the variation of the experimental methylene proton and muon hfcs of the muoniated cyclohexadienyl radical in several solvents and solvent mixtures of varying polarity can be accounted for by a dipole-dipole reaction field model that is based on the model of Reddoch and Konishi (J. Chem. Phys. 1979, 70, 2121) which was developed to explain the solvent dependence of the 14N hfc in the di-tert-butyl-nitroxide radical. Ab initio calculations were carried out with the cyclohexadienyl radical in an electric field to model the electric field arising from the electric dipole moments of the surrounding solvent molecules. An extension of the model that includes the dipole-quadrupole interaction can account for the larger hfc in benzene compared with that in octadecane, and it is predicted that the hfc will be proportional to the concentration of quadrupole moments to the 4/3 power. The influence of hydrogen bonding between the radicals' pi electrons and the OH groups of the solvent on the hfcs is also discussed. Comparison with gas-phase data permits a separation of vibrational effects and reveals that approximately 28% of the temperature dependence in water is due to increasing solvent disorder.
偶极自由基附近溶剂分子的排列会影响其超精细耦合常数（hfcs）。具体而言，本文展示了基于Reddoch和Konishi（《化学物理杂志》，1979年，70卷，2121页）模型的偶极 - 偶极反应场模型，如何解释在几种极性不同的溶剂和溶剂混合物中，μ化环己二烯基自由基的实验亚甲基质子和μ子hfcs的变化。该模型最初是为解释二叔丁基氮氧化物自由基中14N hfc的溶剂依赖性而开发的。利用环己二烯基自由基在电场中的从头算来模拟由周围溶剂分子的电偶极矩产生的电场。包含偶极 - 四极相互作用的模型扩展可以解释苯中hfc比十八烷中的hfc更大的现象，并且预测hfc将与四极矩浓度的4/3次方成正比。还讨论了自由基的π电子与溶剂的OH基团之间的氢键对hfcs的影响。与气相数据的比较允许分离振动效应，并揭示出水中约28%的温度依赖性是由于溶剂无序度增加所致。
