| Abstract |
Combustion calorimetry, solution calorimetry, and differential scanning calorimetry (DSC) were used to determine the standard (p(o) = 0.1 MPa) molar enthalpies of formation of Forms I, II, and III piracetam at 298.15 K, namely, Delta(f)H(m)(o) (C(6)H(10)O(2)N(2), cr I) = -520.6 +/- 1.6 kJ.mol(-1), Delta(f)H(m)(o) (C(6)H(10)O(2)N(2), cr II) = -523.8 +/- 1.6 kJ.mol(-1), and Delta(f)H(m)(o) (C(6)H(10)O(2)N(2), Cr III) = -524.1 +/- 1.6 kJ.mol(-1). The enthalpy of formation of gaseous piracetam at 298.15 K was also derived as Delta(f)H(m)(o) (C(6)H(10)O(2)N(2), g) = -401.3 +/- 2.1 kJ.mol(-1), by combining the standard molar enthalpy of formation of Form II piracetam with the corresponding enthalpy of sublimation, Delta(sub)H(m)(o) (C(6)H(10)O(2)N(2), cr II) = 122.5 +/- 1.4 kJ.mol(-1), obtained by drop-sublimation Calvet microcalorimetry and the Knudsen effusion method. The Delta(f)H(m)(o) (C(6)H(10)O(2)N(2), g) value was used to assess the corresponding predictions by the B3LYP/cc-pVTZ (-335.3 kJ.mol(-1)), G3MP2 (-388.7 kJ.mol(-1)), and CBS-QB3 (-402.8 kJ.mol(-1)) methods, based on the calculation of the atomization enthalpy of piracetam. Finally, the results of the solution and DSC experiments indicate that the stability hierarchy of Forms I, II, and III piracetam at 298.15 K, for which there was conflicting evidence in the literature, is III > II > I. (C) 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 100:594-603, 2011 |
