Considering that hydrogen bonds play an important role in the stability of the studied parent phenolic compounds and their corresponding hydrogen atom-abstracted radicals, we comparably calculated hydrogen bonds between the phenol-O • and the meta OH of both the radicals and their parent molecules. The energies of the optimized 7 parent molecules (Fig. A in S1 File) with hydrogen bond(s) between the phenol-OH and the meta OH are reduced by 3.1–8.7 kcal/mol relative to those of 7 parent molecules without hydrogen bond(s) between the phenol-OH and the meta OH. The corresponding radicals derived from 7 parent compounds display that a 4-O • ···H-O-3 hydrogen bond is formed between the 4-O • center and its neighboring OH group (Fig. 1), demonstrating an important contribution to the stability of the radicals. The energy difference (Fig. 2) of the radicals optimized at the B3LYP/6-311++G(d,p) level shows that among 4 different reaction conditions, the relative energies of the 4-O • centers forming a hydrogen bond with the meta OH are less than those of the 4-O • centers without any hydrogen bond.