Enthalpy of Formation of Anisole: Implications for the Controversy on the O-H Bond Dissociation Enthalpy in Phenol

Significant discrepancies in the literature data for the enthalpy of formation of gaseous anisole, ?fHmo(PhOCH3, g), have fueled an ongoing controversy regarding the most reliable enthalpy of formation of the phenoxy radical and of the gas phase OH bond dissociation enthalpy, DHo(PhOH), in phenol. In the present work triangle H-f(m)o(PhOCH3, g) was reassessed using a combination of calorimetric determinations and high-level (W2-F12) ab initio calculations. Static-bomb combustion calorimetry led to the standard molar enthalpy of formation of liquid anisole at 298.15 K, triangle H-f(m)o(PhOCH3, l) = -(117.1 +/- 1.4) kJmol(-1). The corresponding enthalpy of vaporization was obtained as, triangle H-vap(m)o(PhOCH3) = 46.41 +/- 0.26 kJ mol(-1), by Calvet-drop microcalorimetry. These results give ?fHmo(PhOCH3, g) = -(70.7 +/- 1.4) kJ mol(-1), in excellent agreement with triangle H-f(m)o(PhOCH3, g) = -(70.8 +/- 3.2) kJ mol(-1), obtained from the W2-F12 calculations. The triangle H-f(m)o(PhOCH3, g) here recommended leads to triangle H-f(m)o(PhO, g) = 55.5 +/- 2.4 kJmol1 and DH degrees(PhOH) = 368.1 +/- 2.6 kJmol(-1).