The chemical nature of the red pigment of Corallium Rubrum (CR) has not yet been clearly identified. We have recorded the Raman spectra of CR, canthaxantin and parrot feather and we propose an interpretation with the help of quantum chemistry and of the Effective Conjugation Coordinate (ECC) theory which accounts for the main Raman lines (ν1, ν2, ν3) as originating from the polyene backbone. In this study two additional lines in the 1000 cm-1 range (ν4 and ν5) are considered as well as a series of overtones and combination in the second order Raman spectrum (2000-3000 cm-1). Density Functional Theory (DFT) calculations predict that moving with successive methylation from a simple unsubstituted polyene chain with 9 C=C bonds (as psyttacofulvins) to a 9, 9’, 13, 13’ tetra methylated chain (as carotenoids) the geometric structures of the molecules and their Raman spectra show a trend which can be rationalized if the ECC theory is extended in order to account for mechanical confinement. This turns out to be a new concept which helps the interpretation of the Raman spectra of partially methylated conjugated oligoenes. Evidence is found that the main component of the pigment of CR does not posess a fully demethylated polyene chain (psyttacofulfine) nor a tetra-methylated chain (carotenoid), thus indicating that the polyene chain is partially methylated. Since we consider resonance Raman spectra, the target of this study is the structure of the polyene backbone; no direct spectroscopic information can be obtained in this way on the chemical nature of the groups attached to both chain ends.

Raman Spectroscopy of Polyconjugated Molecules with Electronic and Mechanical Confinement: the spectrum of Corallium Rubrum

BRAMBILLA, LUIGI;TOMMASINI, MATTEO MARIA SAVERIO;ZERBI, GIUSEPPE;
2012-01-01

Abstract

The chemical nature of the red pigment of Corallium Rubrum (CR) has not yet been clearly identified. We have recorded the Raman spectra of CR, canthaxantin and parrot feather and we propose an interpretation with the help of quantum chemistry and of the Effective Conjugation Coordinate (ECC) theory which accounts for the main Raman lines (ν1, ν2, ν3) as originating from the polyene backbone. In this study two additional lines in the 1000 cm-1 range (ν4 and ν5) are considered as well as a series of overtones and combination in the second order Raman spectrum (2000-3000 cm-1). Density Functional Theory (DFT) calculations predict that moving with successive methylation from a simple unsubstituted polyene chain with 9 C=C bonds (as psyttacofulvins) to a 9, 9’, 13, 13’ tetra methylated chain (as carotenoids) the geometric structures of the molecules and their Raman spectra show a trend which can be rationalized if the ECC theory is extended in order to account for mechanical confinement. This turns out to be a new concept which helps the interpretation of the Raman spectra of partially methylated conjugated oligoenes. Evidence is found that the main component of the pigment of CR does not posess a fully demethylated polyene chain (psyttacofulfine) nor a tetra-methylated chain (carotenoid), thus indicating that the polyene chain is partially methylated. Since we consider resonance Raman spectra, the target of this study is the structure of the polyene backbone; no direct spectroscopic information can be obtained in this way on the chemical nature of the groups attached to both chain ends.
2012
Corallium Rubrum; Raman spectroscopy; quantum chemical calculations; effective conjugation coordinate theory; carotenoids
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11311/637128
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