Lopes Jesus A J, Tomé Luciana I N, Eusébio M Ermelinda, Redinha J S
Department of Chemistry, University of Coimbra, 3004-535, Coimbra, Portugal.
J Phys Chem B. 2005 Sep 29;109(38):18055-60. doi: 10.1021/jp051621n.
The enthalpies of sublimation of erythritol and L-threitol have been determined at 298.15 K by calorimetry. The values obtained for the two diastereomers differ from one another by 17 kJ mol(-1). An interpretation of these results is based on the decomposition of this thermodynamic property in a term coming from the intermolecular interactions of the molecules in the crystal (delta(int)H degrees) and another one related with the conformational change of the molecules on going from the crystal lattice to the most stable forms in the gas phase (delta(conf)H degrees). This last term was calculated from the values of the enthalpy of the molecules in the gas state and of the enthalpy of the isolated molecules with the crystal conformation. Both quantities were obtained by density functional theory (DFT) calculations at the B3LYP/6-311G++(d,p) level of theory. The results obtained in this study show that the most important contribution to the differences observed in the enthalpy of sublimation are the differences in the enthalpy of conformational change (13 kJ mol(-1)) rather than different intermolecular forces exhibited in the solid phase. This is explained by the lower enthalpy of threitol in the gas phase relative to erythritol, which is attributed to the higher strength of the intramolecular hydrogen bonds in the former. The comparison of the calculated infrared spectra obtained for the two compounds in the gas phase supports this interpretation.
通过量热法在298.15 K下测定了赤藓糖醇和L-苏糖醇的升华焓。两种非对映异构体得到的值彼此相差17 kJ·mol⁻¹。对这些结果的解释基于将这种热力学性质分解为来自晶体中分子间相互作用的一项（Δ(int)H°）和另一项与分子从晶格转变为气相中最稳定形式时的构象变化相关的项（Δ(conf)H°）。后一项是根据气态分子的焓值和具有晶体构象的孤立分子的焓值计算得出的。这两个量均通过密度泛函理论（DFT）在B3LYP/6 - 311G++(d,p)理论水平下计算得到。本研究获得的结果表明，升华焓差异的最重要贡献在于构象变化焓的差异（13 kJ·mol⁻¹），而非固相表现出的不同分子间作用力。这是因为相对于赤藓糖醇，苏糖醇在气相中的焓较低，这归因于前者分子内氢键更强。对两种化合物在气相中获得的计算红外光谱的比较支持了这一解释。
