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Effects of intra-unit hydrogen bonds into stability of your own radicals

Effects of intra-unit hydrogen bonds into stability of your own radicals

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. This supported hydrogen bond(s) could help to stabilize these radicals and be beneficial to the radical scavenging capacity of the parent molecules. Thus, in comparison to vanillic acid (02), an O-H···O hydrogen bond between the O • center and the meta OH in protocatechuic acid (10) was formed, thereby to stabilize the corresponding phenoxy radical, which could partly explain why the former has a low radical scavenging activity (pICfifty = 2.44) relative to the latter (pIC50 = 4.25).