Thermochemical study of two anhydrous polymorphs of caffeine

The mean values of the standard massic energy of combustion of caffeine in phase I (or alpha) and in phase II (or beta) measured by static-bomb combustion calorimetry in oxygen, at T = 298.15 K, are Δcu∘ (C8H10O2N4, I) = −(21823.27 ± 0.68) J · g−1 and Δcu∘ (C8H10O2N4, II) = −(21799.96 ± 1.08) J · g−1, respectively.The standard (p∘ = 0.1 MPa) molar enthalpy of formation in condensed phase for each form was derived from the corresponding standard molar enthalpies of combustion as, ΔfHm∘(C8H10O2N4,cr,I)=-(340.6±2.3)kJ·mol-1 and ΔfHm∘(C8H10O2N4,cr,II)=-(345.1±2.3)kJ·mol-1.The difference between the standard enthalpy of formation of the two polymorphs in condensed phase was also evaluated by using reaction-solution calorimetry. The obtained result, 2.04 ± 0.25 kJ · mol−1, is in agreement, within the uncertainty, with the difference between the molar enthalpies of formation obtained from combustion experiments (4.5 ± 3.2) kJ · mol−1, which can be considered as an internal test for consistency of the results.A value for the standard enthalpy of formation of caffeine in the gaseous state was proposed: ΔfHm∘(C8H10O2N4,g)=-(229.7±6.1)kJ·mol-1, estimated from the values of the standard enthalpies of formation of both crystalline forms obtained in this work, and the data on standard enthalpies of sublimation collected from the literature.