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HERO ID
2877608
Reference Type
Journal Article
Title
Spectroscopic study of 2-, 4- and 5-substituents on pKa values of imidazole heterocycles prone to intramolecular proton-electrons transfer
Author(s)
Eseola, AO; Obi-Egbedi, NO
Year
2010
Is Peer Reviewed?
1
Journal
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
ISSN:
1386-1425
EISSN:
1873-3557
Volume
75
Issue
2
Page Numbers
693-701
Language
English
PMID
20018554
DOI
10.1016/j.saa.2009.11.041
Web of Science Id
WOS:000274591200031
Abstract
New 2-(1H-imidazol-2-yl)phenols (L1Et-L8tBuPt) bearing a phenolic proton in the vicinity of the imidazole base were prepared and characterized. Experimental studies of the dependence of their protonation/deprotonation equilibrium on substituent identities and intramolecular hydrogen bonding tendencies were carried out using electronic absorption spectroscopy at varying pH values. In order to make comparison, 2-(anthracen-10-yl)-4,5-diphenyl-1H-imidazole (L9Anthr) bearing no phenolic proton and 4,5-diphenyl-2-(4,5-diphenyl-1H-imidazol-2-yl)-1H-imidazole (L10BisIm) bearing two symmetrical imidazole base fragments were also prepared and experimentally investigated. DFT calculations were carried out to study frontier orbitals of the investigated molecules. While electron-releasing substituents produced increase in protonation-deprotonation pK(a)s for the hydroxyl group, values for the imidazole base were mainly affected by polarization of the imidazole ring aromaticity across the 2-imidazole carbon and the 4,5-imidazole carbons axis of the imidazole ring. It was concluded that electron-releasing substituents on the phenol ring and/or electron-withdrawing substituents on 4,5-imidazole carbons negatively affects donor strengths/coordination chemistries of 2-(1H-imidazol-2-yl)phenols, and vice versa. Change of substituents on the phenol ring significantly altered the donor strength of the imidazole base. The understanding of pK(a) variation on account of electronic effects of substituents in this work should aid the understanding of biochemical properties and substituent environments of imidazole-containing biomacromolecules.
Keywords
Ionization constants; UV spectroscopy; pK(a)s; Protonation-deprotonation; Substituent effects
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