Centro de Química Estrutural, Complexo Interdisciplinar, Instituto Superior Técnico, Lisboa, Portugal.
J Pharm Sci. 2011 Feb;100(2):594-603. doi: 10.1002/jps.22294. Epub 2010 Oct 1.
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, Δ(f) H(m)° (C(6)H(10)O(2)N(2), cr I) = -520.6 ± 1.6 kJ·mol(-1), Δ(f) H(m)° (C(6)H(10)O(2)N(2), cr II) = -523.8 ± 1.6 kJ·mol(-1), and Δ(f) H(m)° (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 Δ(f) H(m)° (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, Δ(sub) H(m)° (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 Δ(f) H(m)° (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.
利用燃烧量热法、溶液量热法和差示扫描量热法（DSC），在 298.15 K 下测定了 I 型、II 型和 III 型吡拉西坦的标准（p°=0.1 MPa）摩尔生成焓，即 Δ(f) H(m)°(C(6)H(10)O(2)N(2), cr I)=-520.6±1.6 kJ·mol(-1)、Δ(f) H(m)°(C(6)H(10)O(2)N(2), cr II)=-523.8±1.6 kJ·mol(-1)和 Δ(f) H(m)°(C(6)H(10)O(2)N(2), cr III)=-524.1±1.6 kJ·mol(-1)。通过将 II 型吡拉西坦的标准摩尔生成焓与相应的升华焓 Δ(sub) H(m)°(C(6)H(10)O(2)N(2), cr II)=122.5±1.4 kJ·mol(-1)结合，我们还推导出了 298.15 K 下气态吡拉西坦的生成焓 Δ(f) H(m)°(C(6)H(10)O(2)N(2), g)=-401.3±2.1 kJ·mol(-1)，该值是通过滴升华 Calvet 微量热法和 Knudsen 逸出法获得的。利用吡拉西坦的原子化焓计算，我们对 B3LYP/cc-pVTZ（-335.3 kJ·mol(-1)）、G3MP2（-388.7 kJ·mol(-1)）和 CBS-QB3（-402.8 kJ·mol(-1)）方法的相应预测进行了评估，结果表明，在 298.15 K 下，I 型、II 型和 III 型吡拉西坦的稳定性顺序为 III>II>I，而文献中对此存在争议。最后，溶液和 DSC 实验的结果表明，在 298.15 K 下，I 型、II 型和 III 型吡拉西坦的稳定性顺序为 III>II>I，这与文献中的证据相矛盾。
