Investigation of L (+)-ascorbic acid with raman spectroscopy in visible and UV light

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 CC 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.