Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
2485573 | Journal of Pharmaceutical Sciences | 2011 | 10 Pages |
Abstract
Combustion calorimetry, solution calorimetry, and differential scanning calorimetry (DSC) were used to determine the standard (p° = 0.1 MPa) molar enthalpies of formation of Forms I, II, and III piracetam at 298.15 K, namely, ÎfHm° (C6H10O2N2, cr I) = â520.6 ± 1.6 kJ·molâ1, ÎfHm° (C6H10O2N2, cr II) = â523.8 ± 1.6 kJ·molâ1, and ÎfHm° (C6H10O2N2, cr III) = â524.1 ± 1.6 kJ·molâ1. The enthalpy of formation of gaseous piracetam at 298.15 K was also derived as ÎfHm° (C6H10O2N2, g) = â401.3 ± 2.1 kJ·molâ1, by combining the standard molar enthalpy of formation of Form II piracetam with the corresponding enthalpy of sublimation, ÎsubHm° (C6H10O2N2, cr II) = 122.5 ± 1.4 kJ·molâ1, obtained by drop-sublimation Calvet microcalorimetry and the Knudsen effusion method. The ÎfHm° (C6H10O2N2, g) value was used to assess the corresponding predictions by the B3LYP/cc-pVTZ (â335.3 kJ·molâ1), G3MP2 (â388.7 kJ·molâ1), and CBS-QB3 (â402.8 kJ·molâ1) 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
Keywords
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Authors
Ricardo Picciochi, HermÃnio P. Diogo, Manuel E. Minas da Piedade,