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Characterization of Al(III) complexes with hematein in artistic alum logwood inks
Author(s) -
Centeno Silvia A.,
Ropret Polonca,
Federico Eleonora Del,
Shamir Jacob,
Itin Boris,
Jerschow Alexej
Publication year - 2010
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.2455
Subject(s) - raman spectroscopy , octahedron , chemistry , substrate (aquarium) , crystallography , nmr spectra database , alum , spectral line , organic chemistry , crystal structure , physics , oceanography , astronomy , optics , geology
The complexes between Al(III) and hematein, the main coloring matter in alum logwood inks, were characterized by Raman and 27 Al NMR (nuclear magnetic resonance) spectroscopies. Raman spectra of the crystallized complexes and of the compounds applied on a paper substrate are presented and assigned based on published data for the parent compounds. These Raman spectra show that the coordination of the hematein to the Al(III) ions takes place in both cases through the carbonyl and hydroxyl groups in the molecule, and that the aromatic rings are also involved in the interaction. The Raman spectra of the pure hematein–Al(III) complexes were found to be consistent with those recorded for a logwood ink prepared following a late 19th century recipe, using logwood chips instead of pure hematein, and applied on a paper substrate. These spectra can be used as references for the noninvasive identification of the compounds in works of art. 27 Al solid‐state NMR showed that the coordination of the Al(III) atoms in the crystallized powder is predominantly octahedral, while when applied on a paper substrate the colorant is present mainly as a tetrahedral complex, with an octahedral coordination also present in a smaller proportion. The fact that the predominant coordinations for the complexes in the crystallized material and for the ones present on the paper substrate are different is relevant for the study of the lightfastness and thermal stability of works of art bearing these media. Copyright © 2009 John Wiley & Sons, Ltd.