Thermochemical study of two anhydrous polymorphs of caffeine

Abstract

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 Δ c u ∘ (C 8H 10O 2N 4, I) = −(21823.27 ± 0.68) J · g −1 and Δ c u ∘ (C 8H 10O 2N 4, 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, Δ f H m ∘ ( C 8 H 10 O 2 N 4 , cr, I ) = - ( 340.6 ± 2.3 ) kJ · mol - 1 and Δ f H m ∘ ( C 8 H 10 O 2 N 4 , 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: Δ f H m ∘ ( C 8 H 10 O 2 N 4 , 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.