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Investigation of L(+)-Ascorbic acid with Raman spectroscopy in visible and UV light
Authors:Rolf W Berg
Institution:1. Department of Chemistry, Technical University of Denmark, Lyngby, Denmarkrwb@kemi.dtu.dk
Abstract:Abstract: Raman spectroscopy investigations of l(+)-ascorbic acid and its mono- and di-deprotonated anions (AH? and A2?) are reviewed and new measurements reported with several wavelengths, 229, 244, 266, 488, and 532 nm. Results are interpreted, assisted by new DFT/B3LYP quantum chemical calculations with 6-311++G(d,p) basis sets for several conformations of ascorbic acid and the anions. Raman spectra were measured during titration with NaOH base in an oxygen-poor environment to avoid fluorescence when solutions were alkaline. The ultraviolet (UV) absorption band for ascorbic acid in aqueous solution at ~247 nm was found to cause strong resonance enhancement for the ring C?C stretching mode (called B) at ~1692 cm?1. The ascorbate mono-anion absorbs at ~264.8 nm giving Raman resonance enhancement for the same ring C–C bond stretching, downshifted to ~1591 cm?1. Finally, for the ascorbate di-anion, absorption was found at ~298.4 nm with molar absorptivity of ~7,000 L mol?1 cm?1 and below ~220 nm. With UV light (244 and 266 nm), strongly basic solutions gave pronounced Raman resonance enhancement at ~1556 cm?1. Relatively weak preresonance enhancement was seen for A2? when excitation was done with 229 nm UV light, allowing water bands to become observable as for normal visible light Raman spectra.
Keywords:Raman spectroscopy  ascorbic acid  ascorbate  X-ray structure  DFT calculation  conformation
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