The energies of homolytic cleavage of the O-H bonds in 36 phenols with various structures were calculated within the scope of density functional theory using a series of exchange-correlation functionals and basis sets. The best agreement between the calculated and experimental data is given by the M06-2X/6-311++G(2d,2p)Phenols are widely used in technology and medicine as antioxidants. Their arsenal in this respect is quite extensive and continues to grow at a fast pace both on account of artificially synthesized raw materials and of materials isolated from various natural sources. The effectiveness of their inhibiting action in oxidative processes is determined by a number of factors relating to the structure, medium, and conditions. One of the most important is the bond dissociation energy (BDE) of the phenolic hydroxyl bond. Its experimental determination is not always possible and justified, particularly in prognostic investigations, when the synthesis or isolation of the phenolic compounds or their mixtures may be superfluous on account of their poorly defined antioxidant characteristics [1]. Here it is appropriate to seek and develop reliable methods of quantum-chemical assessments of the BDE values.Unfortunately in papers aimed at validation of the new functionals of density functional theory (DFT), calculations of the BDE for phenolic compounds are poorly represented [2,3]. In the papers in which the applicability of the DFT methods for calculations of the BDE was investigated [4, 5] the homolysis of the O-H bonds was hardly discussed at all. For high-level ab initio methods (CBS-Q, G3) substituted phenols are excessively resource-intensive. Thus, in [6] only molecules with less than eight nonhydrogen atoms were investigated.In the earlier papers, devoted to investigation of the BDE of the hydroxyl bond in phenols, the theoretical data either were not compared with experimental data [7,8], or the authors concentrated attention solely on monosubstituted phenols [1,9,7] or only used the exchange-correlation functional B3LYP [7,[9][10][11]. In the opinion of the authors in [12], this method is characterized by increased error in the enthalpies of formation with increase in the size of the molecules. This restricts its applicability for calculations of large phenol molecules.The aim of the present work was to determine the exchange-correlation functional and basis set with which the calculated BDE values of the O-H bond correlate best with the experimental values. For this purpose the energies of the O-H bonds of 36 mono-and polysubstituted phenols were calculated and compared with the experimental data (the arithmetical mean of those presented in the reference book [15]) (see Table 1).

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