An Hui-Ying, Ma Chensheng, Nganga Jameil L, Zhu Yue, Dore Timothy M, Phillips David Lee
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, PR China.
J Phys Chem A. 2009 Mar 26;113(12):2831-7. doi: 10.1021/jp809586h.
The 8-bromo-7-hydroxyquinolinyl group (BHQ) is a derivative of 7-hydroxyquinoline (7-HQ) and BHQ molecules coexisting as different forms in aqueous solution. Absorption and resonance Raman spectroscopic methods were used to examine 8-bromo-7-hydroxyquinoline protected acetate (BHQ-OAc) in acetonitrile (MeCN), H(2)O/MeCN (60:40, v/v, pH 6 approximately 7), and NaOH-H(2)O/MeCN (60:40, v/v, pH 11 approximately 12) to obtain a better characterization of the forms of the ground-state species of BHQ-OAc in aqueous solutions and to examine their properties. The absorption spectra of BHQ-OAc in water show no absorption bands of the tautomeric species unlike the strong band at about 400 nm observed for the tautomeric form in 7-HQ aqueous solution. The resonance Raman spectra in conjunction with Raman spectra predicted from density functional theory (DFT) calculations reveal the observation of a double Raman band system characteristic of the neutral form (the nominal C=C ring stretching, C-N stretching, and O-H bending modes at 1564 and 1607 cm(-1)) and a single Raman band diagnostic of the enol-deprotonated anionic form (the nominal C=C ring, C-N, and C-O(-) stretching modes in the 1593 cm(-1) region). These results suggest that the neutral form of BHQ-OAc is the major species in neutral aqueous solution. There is a modest increase in the amount of the anionic form and a big decrease in the amount of the tautomeric form of the molecules for BHQ-OAc compared to 7-HQ in neutral aqueous solution. The presence of the 8-bromo group and/or competitive hydrogen bonding that hinder the formation and transfer process of a BHQ-OAc-water cyclic complex may be responsible for this large substituent effect.
8-溴-7-羟基喹啉基（BHQ）是7-羟基喹啉（7-HQ）的衍生物，且BHQ分子在水溶液中以不同形式共存。采用吸收光谱法和共振拉曼光谱法对8-溴-7-羟基喹啉乙酸酯（BHQ-OAc）在乙腈（MeCN）、水/乙腈（60:40，v/v，pH 6左右至7）以及氢氧化钠-水/乙腈（60:40，v/v，pH 11左右至12）中的情况进行研究，以便更好地表征BHQ-OAc在水溶液中的基态物种形式并考察其性质。与7-HQ水溶液中互变异构体在约400 nm处观察到的强吸收带不同，BHQ-OAc在水中的吸收光谱未显示互变异构体的吸收带。结合密度泛函理论（DFT）计算预测的拉曼光谱的共振拉曼光谱揭示，观察到了中性形式特有的双拉曼带系统（名义上的C=C环伸缩振动、C-N伸缩振动和O-H弯曲振动模式在1564和1607 cm⁻¹处）以及烯醇去质子化阴离子形式的单一拉曼带诊断特征（名义上的C=C环、C-N和C-O⁻伸缩振动模式在1593 cm⁻¹区域）。这些结果表明，BHQ-OAc的中性形式是中性水溶液中的主要物种。与中性水溶液中的7-HQ相比，BHQ-OAc分子的阴离子形式数量有适度增加，互变异构体形式数量大幅减少。8-溴基团的存在和/或阻碍BHQ-OAc -水环状复合物形成和转移过程的竞争性氢键可能是造成这种大取代基效应的原因。
