Nature of transients produced on hydrogen atom transfer from capsaicin

Structure and energies of capsaicin and its probable transients formed in oxidation processes (single electron transfer and hydrogen atom transfer) have been investigated using theoretical calculations. Molecular geometries and energies of truncated and complete capsaicin structures have been optimized using density functional theory (DFT) with Becke three-parameter Lee-Yang-Parr (B3LYP) functional and 6-31++G(d) basis set. The stable geometries have been confirmed by vibrational analysis. The calculations suggest that single-electron transfer takes place at phenolic 0-atom in the first step followed by delocalization of positive charge over the whole molecule. Further, the first step of hydrogen atom abstraction should take place at phenolic group due to lowest dissociation energy but post-optimization bond dissociation energy is least for benzylic group in the side chain as compared to other transients. Effect of water as a solvent on the energies has also been studied using self-consistent reaction field calculation. Similar results are obtained for truncated and complete capsaicin structures. The present study also includes Mulliken spin, charge, vibrational frequencies and assignments of frequencies of the transients. The present study provides explanation for the observation of phenoxyl radical in fast kinetic studies using pulse radiolysis study and, formation of breakdown and dimeric products in other studies.