Gas-Phase Acidities and O−H Bond Dissociation Enthalpies of Phenol, 3-Methylphenol, 2,4,6-Trimethylphenol, and Ethanoic Acid

Abstract

Energy-resolved, competitive threshold collision-induced dissociation (TCID) methods are used to measure the gas-phase acidities of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid relative to hydrogen cyanide, hydrogen sulfide, and the hydroperoxyl radical using guided ion beam tandem mass spectrometry. The gas-phase acidities of Delta(acid)H298(C6H5OH) = 1456 +/- 4 kJ/mol, Delta(acid)H298(3-CH3C6H4OH) = 1457 +/- 5 kJ/mol, Delta(acid)H298(2,4,6-(CH3)3C6H2OH) = 1456 +/- 4 kJ/mol, and Delta(acid)H298(CH3COOH) = 1457 +/- 6 kJ/mol are determined. The O-H bond dissociation enthalpy of D298(C6H5O-H) = 361 +/- 4 kJ/mol is derived using the previously published experimental electron affinity for C6H5O, and thermochemical values for the other species are reported. A comparison of the new TCID values with both experimental and theoretical values from the literature is presented.

Determination of gas-phase acidities of dimethylphenols: Combined experimental and theoretical study

“…These results indicate that the introduction of additional methyl groups does not lead to acidities that are significantly different from those observed for dimethylphenols (only 2,3,5,6-tetramethylphenol is predicted to be more acidic than 2,6-dimethylphenol and none is predicted to be less acidic than 3,4-dimethylphenol). To our knowledge, the only experimental value reported for a polymethylphenol is for 2,4,6-trimethylphenol [22]. The relative acidity value measured for this compound is ␦⌬ ac G 0 ϭ 0, in excellent agreement with the estimated value in Table 4 (Ϫ0.04 kJ mol Ϫ1 ).…”

Anion photoelectron spectroscopy of deprotonated ortho-, meta-, and para-methylphenol

“…The dominant source of uncertainty arises from the error associated with the H-A bond strength, as seen in Table I. We may further combine this 0 K result with the calculated constant pressure heat capacities of the species involved in the acid reaction in order to obtain the enthalpy at 298 K: 34 36 and Angel et al 27 ) examined m-methylphenol and determined the gas phase acidity at 298 K to be 347.3 and 348.2 ± 1.2 kcal/mol, respectively. Finally, p-methylphenol was studied by McMahon 36 and determined the gas phase acidity at 298 K to be 347.7 kcal/mol.…”

“…The current study presented here agrees well with the previous studies, and has greatly improved on these measurements with an error of ± 0.25 kcal/mol. In addition to a direct comparison, one could also compare the acidity of phenol at 298 K (348.0 ± 1.0 kcal/mol, reported by Angel and Ervin,27 or the 0 K acidity derived from the works of Kim et al 38 and…”

Bond dissociation enthalpies in chlorinated benzenes and phenols and enthalpies of formation of their free radicals: A Gaussian-4 prediction

“…Previous evaluations have reported D 298K of 371.3 ± 2.3 [11], 376.6 ± 12.6 [7], and 362.8 ± 0.7 kJ/mol [8]. The last value received supports from two afterward measurements as D 298K of 361 ± 4 kJ/mol from threshold collision-induced dissociation study by Angel and Ervin [9], and D 0 of 359.1 ± 0.5 kJ/mol (D 298K of 364.8 kJ/mol after thermal corrections) from high-resolution photofragment translational spectroscopy study of C 6 H 5 OH by Nix et al [10] of the available experimental values, the direct measurement by Nix et al will be taken as the reference for its high reliability and accuracy. High-level ab initio calculations have also predicted D 298K of 372.8 kJ/mol at the CCSD/cc-pV(D,T)Z-extrapolation level [19], 372.4 ± 4.2 kJ/mol at Gaussian-3 (G3), Gaussian-3-B3 (G3B3), and CBS-APNO levels with bond-isodesmic reactions [28], 366.9, 366.1, and 361.5 kJ/mol at G3, CBS-APNO, and CBS-QB3 levels with isodesmic corrections as well [8], and 371.3, 372.9, and 367.3 kJ/mol at G3X-QCI, G3X-CC, and G4 levels from direct dissociation reaction (Table II), favoring different evaluations.…”