DFT/B3LYP study of the substituent effects on OH bond dissociation enthalpies of chroman derivatives in the gas phase and solvent environment

In this paper, the study of various ortho and meta-substituted chroman-6-ol derivatives is presented. The OH bond dissociation enthalpies (BDE) related to hydrogen atom transfer (HAT) mechanism of the phenolic antioxidants action, for studied molecules were calculated using DFT/B3LYP method. Calculated BDEs for meta-substituted molecules were correlated with Hammett constants and calculated results indicate that dependence of BDE on Hammett constants of the substituents is linear. Comparison of BDEs of the studied molecules with non-substituted chroman-6-ol indicates that electron-withdrawing groups increase BDEs. On the contrary, electron-donating groups reduce BDE values. Furthermore, results for meta-substituted molecules show that calculated BDEs can be correlated with the lengths of phenolic CO bond of the studied molecules successfully. For meta-substituted molecules, the computed results also show that BDEs linearly depend on the partial charge on the phenoxy radical oxygen q(O). In comparison to the gas phase, the effect of studied substituents on the BDEs for meta position was increased in water phase, while in the two ortho positions it was decreased in water phase.