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

Abstract

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(O 9–H 10) > ν(O 19–H 20) > ν(O 7–H 8) > ν(O 14–H 15) which could be due to complexity of hydrogen bonding in the lactone ring and the side chain. The C O stretching wavenumber ( ν 46) 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.