Alfa and remote substituent effects on the homolytic dissociation energies of O–H bonds

Abstract

In the study we used a number of high level theoretical methods to calculate the O–H bond dissociation energies (BDEs) as well as α and remote para substituent effects on them. We found that only G3 and CBS-Q methods can be used to calculate the absolute O–H BDEs. Other methods including B3LYP, MP2, and CCSD(T), either open-shelled or close-shelled, significantly underestimate the O–H BDEs. To be even worse, except for G3 and CBS-Q, the other theoretical methods cannot accurately predict the α substituent effects on O–H BDEs either. Methods including UMP2, ROMP2, and UCCSD(T) may even provide qualitatively erratic α substituent effects. Using the G3 and CBS-Q results, we found that the α substituent effect on the O–H BDEs is usually much larger than that on the C–H or N–H BDEs. Both the π donors and acceptors reduce the O–H BDEs because of the conjugation and hyperconjugation between the substituent and the radical center. Polyfluorinated alkyl groups increase the O–H BDEs because of the inductive electron-withdrawing effect. In comparison, an electron-withdrawing para substituent increases the O–H BDE of phenol, whereas an electron-donating group reduces it. The calculated ρ + value for the O–H BDEs of phenols is about 4–5 kcal/mol. Compared to it, the experimentally determined ρ + value is significantly larger because of the solvent effect. Furthermore, the ρ + values for the O–Y BDEs of 4-X–C 6H 4–O–Y decrease in the order O–CH 3>O–H>O–OCH 3>O–OH>O–NO.