| Abstract |
Combustion calorimetry, Calvet-drop sublimation calorimetry, and the Knudsen effusion method were used to determine the standard (p(o) = 0.1 MPa) molar enthalpies of formation of monoclinic (form I) and gaseous paracetamol, at T = 298.15 K: Delta(f)H(m)(o)(C(8)H(9)O(2)N, cr I) = -(410.4 +/- 1.3) kJ mol(-1) and D(f)H(m)(o) (C(8)H(9)O(2)N, g) = -(280.5 +/- 1.9) kJ mol(-1). From the obtained Delta(f)H(m)(o) (C(8)H(9)O(2)N, cr I) value and published data, it was also possible to derive the standard molar enthalpies of formation of the two other known polymorphs of paracetamol (forms II and III), at 298.15 K: Delta(f)H(m)(o)(C(8)H(9)O(2)N, crII) = -(408.4 +/- 1.3) kJ mol(-1) and Delta(f)H(m)(o)(C(8)H(9)O(2)N, cr III) = -(407.4 +/- 1.3) kJ mol(-1) : The proposed Delta(f)H(m)(o)(C(8)H(9)O(2)N, g) value, together with the experimental enthalpies of formation of acetophenone and 4 -hydroxyacetophenone, taken from the literature, and a re-evaluated enthalpy of formation of acetanilide, Delta(f)H(m)(o)(C(8)H(9)ON, g) = -(109.2 +/- 2.2) kJmol(-1); were used to assess the predictions of the B3LYP/cc-pVTZ and CBS-QB3 methods for the enthalpy of a isodesmic and isogyric reaction involving those species. This test supported the reliability of the theoretical methods, and indicated a good thermodynamic consistency between the Delta(f)H(m)(o)(C(8)H(9)O(2)N, g) value obtained in this study and the remaining experimental data used in the Delta(r)H(m)(o) calculation. It also led to the conclusion that the presently recommended enthalpy of formation of gaseous acetanilide in Cox and Pilcher and Pedley s compilations should be corrected by similar to 20 kJ mol(-1). |
