Energetics of the O-H Bond and of Intramolecular Hydrogen Bonding in HOC6H4C(O)Y Compounds

Abstract

The energetics of the phenolic O−H bond in a series of 2- and 4-HOC6H4C(O)Y (Y = H, CH3, CH2CH═CH2, C≡CH, CH2F, NH2, NHCH3, NO2, OH, OCH3, OCN, CN, F, Cl, SH, and SCH3) compounds and of the intramolecular O···H hydrogen bond in 2-HOC6H4C(O)Y, was investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of 2-hydroxybenzaldehyde (2HBA), 4-hydroxybenzaldehyde (4HBA), 2′-hydroxyacetophenone (2HAP), 2-hydroxybenzamide (2HBM), and 4-hydroxybenzamide (4HBM), at 298.15 K, were determined by micro- or macrocombustion calorimetry. The corresponding enthalpies of vaporization or sublimation were also measured by Calvet drop-calorimetry and Knudsen effusion measurements. The combination of the obtained experimental data led to Δ_fH_m^o(2HBA, g) = −238.3 ± 2.5 kJ/mol, Δ_fH_m^o(4HBA, g) = −220.3 ± 2.0 kJ/mol, Δ_fH_m^o(2HAP, g) = −291.8 ± 2.1 kJ/mol, Δ_fH_m^o(2HBM, g) = −304.8 ± 1.5 kJ/mol, and Δ_fH_m^o (4HBM, g) = −278.4 ± 2.4 kJ/mol. These values, were used to assess the predictions of the B3LYP/6-31G(d,p), B3LYP/6-311+G(d,p), B3LYP/aug-cc-pVDZ, B3P86/6-31G(d,p), B3P86/6-311+G(d,p), B3P86/aug-cc-pVDZ, and CBS-QB3 methods, for the enthalpies of a series of isodesmic gas phase reactions. In general, the CBS-QB3 method was able to reproduce the experimental enthalpies of reaction within their uncertainties. The B3LYP/6-311+G(d,p) method, with a slightly poorer accuracy than the CBS-QB3 approach, achieved the best performance of the tested DFT models. It was further used to analyze the trends of the intramolecular O···H hydrogen bond in 2-HOC6H4C(O)Y evaluated by the ortho−para method and to compare the energetics of the phenolic O−H bond in 2- and 4-HOC6H4C(O)Y compounds. It was concluded that the O−H bond “strength” is systematically larger for 2-hydroxybenzoyl than for the corresponding 4-hydroxybenzoyl isomers mainly due to the presence of the intramolecular O···H hydrogen bond in the 2-isomers. The observed differences are, however, significantly dependent on the nature of the substituent Y, in particular, when an intramolecular H-bond can be present in the radical obtained upon cleavage of the O−H bond.

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