Baba Keita, Ono Hajime, Itoh Eri, Itoh Sumitaka, Noda Kyoko, Usui Toshinori, Ishihara Koji, Inamo Masahiko, Takagi Hideo D, Asano Tsutomu
Research Center for Materials Science, Nagoya University, Nagoya 464-8602, Japan.
Chemistry. 2006 Jul 5;12(20):5328-33. doi: 10.1002/chem.200600081.
Thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4'-nitroazobenzene were examined in three ionic liquids of general formula 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (R = butyl, pentyl, and hexyl). The first-order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4-dimethylamino-4'-nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers-Grote-Hynes model or by the Agmon-Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 10(4)-10(7) s(-1).
在三种通式为1-R-3-甲基咪唑鎓双（三氟甲基磺酰）亚胺（R = 丁基、戊基和己基）的离子液体中，研究了偶氮苯和4-二甲基氨基-4'-硝基偶氮苯的热Z到E异构化反应。在这些离子液体中测得的偶氮苯反应的一级速率常数和活化能与在普通有机溶剂中测得的一致，这表明通过具有非极性过渡态的反转机制进行的缓慢异构化受离子液体的溶剂性质（如粘度和介电常数）影响很小。另一方面，与在具有相似介电性质的普通有机分子溶剂中的异构化相比，离子液体中4-二甲基氨基-4'-硝基偶氮苯异构化的速率常数和阿仑尼乌斯图相应的频率因子显著降低。尽管这些离子液体具有粘性，但速率常数对表观粘度的依赖性既不能用溶液反应的Kramers-Grote-Hynes模型也不能用Agmon-Hopfield模型来解释。有人提出，高极性旋转过渡态的正电荷中心和负电荷中心分别被周围的阴离子和阳离子稳定，并且离子必须重新排列，以便在过渡态反应物的电荷中心周围形成高度有序的溶剂化壳层。这种在过渡态对有序溶剂化的要求导致了异常小的频率因子，为10(4)-10(7) s(-1)。
