Resonance Raman Characterization of Different Forms of Ground-State 8-Bromo-7-hydroxyquinoline Caged Acetate in Aqueous Solutions
The 8-bromo-7-hydroxyquinolinyl group (BHQ) is a deriv. of 7-hydroxyquinoline (7-HQ) and BHQ mols. coexisting as different forms in aq. soln. Absorption and resonance Raman spectroscopic methods were used to examine 8-bromo-7-hydroxyquinoline protected acetate (BHQ-OAc) in acetonitrile (MeCN), H2O/MeCN (60:40, vol./vol., pH 6.apprx.7), and NaOH-H2O/MeCN (60:40, vol./vol., pH 11.apprx.12) to obtain a better characterization of the forms of the ground-state species of BHQ-OAc in aq. solns. 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 obsd. for the tautomeric form in 7-HQ aq. soln. The resonance Raman spectra in conjunction with Raman spectra predicted from d. functional theory (DFT) calcns. 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 aq. soln. There is a modest increase in the amt. of the anionic form and a big decrease in the amt. of the tautomeric form of the mols. for BHQ-OAc compared to 7-HQ in neutral aq. soln. 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.