Resonance raman characterization of different forms of ground-state 8-bromo-7-hydroxyquinoline caged acetate in aqueous solutions

Hui Ying An, Ma Chensheng, Jameil L. Nganga, Yue Zhu, Timothy M. Dore, David Lee Phillips

Research output: Contribution to journalArticlepeer-review

Abstract

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), H20/MeCN (60:40, v/v, pH 6-7), and NaOH-H 20/MeCN (60:40, v/v, pH 11-12) 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.

Original languageEnglish (US)
Pages (from-to)2831-2837
Number of pages7
JournalJournal of Physical Chemistry A
Volume113
Issue number12
DOIs
StatePublished - Mar 26 2009

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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