Experimental IR and Raman spectra and quantum chemical studies of molecular structures, conformers and vibrational characteristics of l-ascorbic acid and its anion and cation

IR and spectra of the l-ascorbic acid (l-AA) also known as vitamin C have been recorded in the region 4000–50 cm−1. In order to make vibrational assignments of the observed IR and Raman bands computations were carried out by employing the RHF and DFT methods to calculate the molecular geometries and harmonic vibrational frequencies along with other related parameters for the neutral l-AA and its singly charged anionic (l-AA−) and cationic (l-AA+) species. Significant changes have been found for different characteristics of a number of vibrational modes. The four ν(O–H) modes of the l-AA molecule are found in the order ν(O9–H10) > ν(O19–H20) > ν(O7–H8) > ν(O14–H15) which could be due to complexity of hydrogen bonding in the lactone ring and the side chain. The CO stretching wavenumber (ν46) decreases by 151 cm−1 in going from the neutral to the anionic species whereas it increases by 151 cm−1 in going from the anionic to the cationic species. The anionic radicals have less kinetic stabilities and high chemical reactivity as compared to the neutral molecule. It is found that the cationic radical of l-AA is kinetically least stable and chemically most reactive as compared to its neutral and anionic species.

The compound l-ascorbic acid was used as such without further purification for recording the FT-IR and laser Raman spectra. The IR spectra have been recorded in KBr pellets and Nujol mull using Varian FTIR-3100 spectrometer in the spectral range 50–4000 cm−1 with the following experimental parameters: Varian FTIR-3100: scans – 200; resolution – 2 cm−1; gain – 50. The recorded IR and Far IR for the neutral l-AA molecule are reproduced in Fig. 1.Figure optionsDownload as PowerPoint slideHighlights
► l-Ascorbic acid, an antioxidant and electron donor plays a vital role in the chemical, biological, physiological reactions in the human body.
► All the 54 normal modes of the l-AA molecule have been assigned and discussed in details.
► It could be possible presently to correlate 47 normal modes to the experimentally observed IR/Raman frequencies.
► l-AA is a six-carbon keto-lactone, a strong reducing agent and serves as an antioxidant.
► The hydrogen donation from l-AA is considered to be responsible for the antioxidant properties attributed to this molecule.