Experimental and theoretical studies on corals. I. Toward understanding the origin of color in precious red corals from Raman and IR spectroscopies and DFT calculations
✍ Scribed by Teobald Kupka; Hong Ming Lin; Leszek Stobiński; Cheng-Han Chen; Wei-Jen Liou; Roman Wrzalik; Zygmunt Flisak
- Publisher
- John Wiley and Sons
- Year
- 2009
- Tongue
- English
- Weight
- 527 KB
- Volume
- 41
- Category
- Article
- ISSN
- 0377-0486
- DOI
- 10.1002/jrs.2502
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✦ Synopsis
Abstract
An attempt to explain the origin of the vivid red color in precious pink and red corals was undertaken. Raman and IR spectroscopies were applied to characterize white, pink and red corals. The position of the Raman signal near 1500 cm^−1^ of some corals and pearls was associated by several authors with the presence of the mixture of all‐trans‐polyenic pigments, containing 6–16 conjugated CC bonds or β‐carotenoids. This hypothesis was examined theoretically by performing extensive B3LYP‐DFT calculations of vibrational spectra of the model polyenic compounds. The B3LYP/6‐311++G** predicted positions of the dominating Raman mode depend on the number of CC units (C__n__ parameter) and can be accurately predicted for larger systems from a simple nonlinear fit. The DFT‐predicted Raman activities of these modes are extremely sensitive to C__n__, and sharply increase with the number of double bonds. This implies a presence of only—two to three polyenes differing slightly in the number of CC units as the source of color in pink and red corals. Copyright © 2009 John Wiley & Sons, Ltd.