Competitive Threshold Collision-Induced Dissociation:  Gas-Phase Acidity and O−H Bond Dissociation Enthalpy of Phenol

Abstract

Energy-resolved competitive collision-induced dissociation methods are used to measure the gas-phase acidity of phenol relative to hydrogen cyanide. The competitive dissociation of the [C6H5O·H·CN]- complex into C6H5OH + CN- and C6H5O- + HCN is studied using a guided ion beam tandem mass spectrometer. The reaction cross sections and product branching fractions are measured as a function of collision energy. The enthalpy difference between the two reaction channels is found by modeling the reaction cross sections near threshold using RRKM theory to account for the energy-dependent product branching ratio and kinetic shift. From the enthalpy difference, the phenol gas-phase acidity, Δacid H 0(C6H5OH) = 1448 ± 8 kJ/mol, is determined relative to the established literature value of hydrogen cyanide, Δacid H 0(HCN) = 1462.3 ± 0.9 kJ/mol. We then derive Δacid H 298(C6H5OH) = 1454 ± 8 kJ/mol and the bond dissociation energy of D 298(C6H5O−H) = 359 ± 8 kJ/mol.

Ab initio study of the formation and degradation reactions of chlorinated phenols

“…However, if one is interested in the differences in these parameters between similar compounds, then DFT calculations are accurate [20,22,23,27,33,34]. In terms of absolute values of a thermodynamic parameter, the bond dissociation energy of phenol has been measured experimentally and found to be between 84.0 and 88.8 kcal/mol [8,16,20,22,23,27,33,35–38], in excellent agreement with our calculated value of 89.4 kcal/mol (QClSD(T)/6-31G(d,p)//BHandHLYP/6-31G(d,p)) and in good agreement with our DFT values of ~80–81 kcal/mol [39]. Δ E rxn for three other routes to the formation of the phenoxyl radical can be estimated from heats of reaction available from experiments [40].…”

Benzoylnitrene radical anion: A new reagent for the generation of M-2H anions

“…HAT in these reactions is exothermic, as the hydrogen atom affinity of 1 Ϫ is 96 kcal/mol (Table 1), whereas the O-H bond dissociation energies (BDEs) are ca. 90 kcal/mol in phenols [11,12]. Although the C-H and N-H BDEs in toluene and aniline, respectively, are similar to that in phenol [13][14][15][16], no reaction is observed between 1 Ϫ and toluene or aniline, indicating that there may be energy barriers to slow these reactions.…”

DFT and ab initio calculations of ionization potentials, proton affinities and bond dissociation enthalpies of aromatic compounds

“…Using the mass-analyzed threshold ionization technique and literature data, Kim et al (2000) experimentally determined O-H BDE to be 381 kJ mol -1 . Later, Angel and Ervin (2004) used mass spectrometry (threshold collision-induced dissociation method) determining the value of 359  8 kJ mol -1 . Mulder et al (2005) recommended the experimental gasphase value of 362  3 kJ mol -1 using two gas-phase and three solution-phase values.…”