On the interpretation of the Raman spectra of Maya Blue: a review on the literature data

Abstract

The Raman spectroscopy of Maya Blue (MB), a nanostructured organic-inorganic hybrid material, has received considerable attention. A re-evaluation of the Raman literature data for indigo dye, genuine MB samples and model specimens obtained by the binding of indigo to phyllosilicate clays, such as palygorskite and sepiolite, using chemometric analysis of normalised spectra is reported. Available data present features in support of the following ideas: (1) dehydroindigo accompanies indigo in MB; (2) different topological isomers, i.e. dye molecules attached to different coordinative sites of the palygorskite framework, are involved in MB; and (3) different procedures were probably used for preparing MB in ancient Mesoamerican civilisations.

Indigo adsorption on a silicate surface: a theoretical density functional study

“…At 1,668 cm −1 we find a symmetric ν(C=O) mode involving the two carbonyl groups, which is coupled with the ν(C=C) of the central C=C group. This is consistent with the assignment of the experimental band found at 1,701 cm −1 [47,49,50], though other authors [48] found an additional coupling with the δ(CC) of the five-membered ring. An asymmetric ν(C=O) mode of both carbonyl groups that is coupled with δ(NH) is found at 1,623 cm −1 in our calculations.…”

“…A similar frequency shift was observed in the experimental spectrum of indigo sorbed in palygorskite [46][47][48]. However, other authors have assigned the band at 1,623-1,625 cm −1 to C-C and C-H vibrations [49]. Our calculations allow us to assign the band at 884 cm −1 to in-plane bending of the carbonyl group δ(CO).…”

“…An independent δ(NH) s mode was detected at 1,341 cm −1 and assigned to the symmetric coupling of the δ(NH) modes of both NH groups. This mode was not assigned in previous experimental work [49], probably due to the large number of bands appearing in this zone, and they assigned bands in the range 1,362-1,365 cm −1 to δ(NH) + δ(CH) coupled modes. We find two bands at 1,300 and 1,155 cm −1 that are associated with the asymmetric δ(NH) modes of both NH groups, in agreement with the experimental bands at 1,389 and 1,123 cm −1 , assigned to δ(NH) + ν(CN) coupled modes [48].…”

“…The out-ofplane bending mode, γ(NH), of the NH group oriented out from the surface at 409 cm −1 occurs at a frequency that is similar to that seen in the gas phase (410 cm −1 ), whereas the γ(NH) of the surface-oriented NH group occurs at a higher frequency (439 cm −1 ) owing to the interactions of this group with the O atoms of the silicate surface. Very few experimental data for this mode have been reported [49], but this mode is interesting because it is very sensitive to intermolecular interactions and can be useful for monitoring the adsorption of indigo on silicates as well as the indigo-to-dehydroindigo transformation. No significant differences were observed between the results obtained using surface cluster models with one or two silanol groups.…”

The first direct evidence of pre-columbian sources of palygorskite for Maya Blue

“…That same year Shepard (1962) and Shepard and Gottlieb (1962) suggested that the pigment was a clayeorganic complex, and indeed, five years later, the remaining component was identified as the organic colorant, indigo (Kleber et al, 1967). Since then, research on Maya Blue has focused on several interrelated themes, and at least two authors have provided lengthy summaries of the Maya Blue research from a chemical and molecular perspective (Doménech et al, 2011;Giustetto et al, 2011).…”