**Abstract**

A theoretical evaluation of the antioxidant activity of natural pigment delphinidin (**1a**) and derivatives **1b**, **1c**, **1d** & **1e** was performed using the DFT-B3LYP/6–311 + G (d, p) level of theory. Three potential working mechanisms, hydrogen atom transfer (HAT), stepwise electron transfer proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET), have been investigated. The physiochemical parameters, including O–H bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), and electron transfer enthalpy (ETE), have been calculated in the gas phase and aqueous phase. The study found that the most suitable mechanism for explaining antioxidant activity is HAT in the gas phase and SPLET in the aqueous medium in this level of theory. Spin density calculation and delocalization index of studied molecules also support the radical scavenging activity. When incorporated into natural pigment delphinidin, the gallate moiety can enhance the activity and stability of the compounds.

**Keywords:** DFT, Multiwfn, Delphinidin, Radical scavenger, Gallic acid
