Quantum chemistry and pharmacy: diagnostic check of the thermochemistry of ibuprofen

The thermodynamic data on ibuprofen available in the literature shows that the disarray of experimental results is unacceptable for this very important drug. The data on ibuprofens available in the literature were collected, combined with our complementary experimental results and evaluated. The enthalpies of combustion and formation of the crystalline RS-(±)- and S-(+)-ibuprofens were measured using high-precision combustion calorimetry. The temperature dependence of the vapour pressure of S-(+)-ibuprofen was measured using the transpiration method and the enthalpy of vaporisation was derived from this measurement. The enthalpies of fusion of both compounds were measured using DSC. The G4 calculations have been carried out to determine the enthalpy of formation in gaseous state of the most stable conformer. Thermochemical properties of studied compounds were evaluated and tested for consistency with the “centerpiece approach”. A set of reliable and consistent values of thermodynamic properties of ibuprofens at 298.15 К was recommended for thermochemical calculations of the pharmaceutic processes. The diagnostic protocol was developed to distinguish the “sick” or “healthy” thermodynamic data. This diagnostic is also applicable to other drugs with a different structure than ibuprofen.