Theoretical and spectroscopic investigation of the oxidation and degradation of protocatechuic acid

Abstract

In this work, we report a combined experimental and theoretical study on molecular structure and spectroscopic properties of the most stable conformers of PCA. 1 H, 13 C NMR and 2D COSY NMR, ESR, IR and electronic spectroscopies were coupled with DFT theoretical calculations performed at the B3LYP/6-31G ∗∗ level. The calculated geometrical parameters for the neutral protocatechuic acid PCA-H 3 , its anions, its oxidized forms and the peroxo-derivative [PCA-H-O 2 ] 2− are in line with the experimental data. The neutral catecholate is the most stable form of PCA-H 3 whilst the dianion [PCA-H] 2− presents higher energy. This anion is (experimentally) stable only under argon, reacting with dioxygen, in the presence of air. The semiquinone [PCA-H-sq(3)] − is very close in energy from [PCA-H-sq(4)] − form and an equilibrium between these two oxidized radical forms might be expected. The energetically advantageous pathway for preparation of the symmetrically delocalized [PCA-sq] 2− is to oxidize the [PCA] 3− . The occurrence of this radical dianion form was justified experimentally by ESR, IR, UV–vis and NMR spectra. The structural calculations for [PCA-H-O 2 ] 2− indicate that C 3 (and to a lesser extent C1) may undergo a nucleophilic attack from the “co-ordinated” peroxo-group. The conditions for the non-enzymatic degradation of PCA have been established and some new products are observed: ionization of PCA-H 3 , the presence of O 2 and aprotic solvents provide the semiquinone-superoxo adduct which is then degraded to lactones, while in protic solvents, addition of H 2 O 2 and the presence of air, are essential, providing aliphatic degradation products.