This work is part of the effort on establishing reliable thermodynamic data for amino acids and, in a broader context, benchmarking first-principles calculations of thermodynamic properties of molecular crystals against reliable experimental data. In this work, crystal heat capacities of l-alanine (CAS RN: 56-41-7), l-valine (CAS RN: 72-18-4), l-isoleucine (CAS RN: 73-32-5), and l-leucine (CAS RN: 61-90-5) were newly measured in the temperature range 262–450 K by Tian–Calvet calorimetry and power-compensation differential scanning calorimetry (DSC) and combined with the critically assessed literature data to obtain the reference data from near 0 to 450 K. The heat capacity measurements were accompanied by thermogravimetric analysis to determine the decomposition temperatures of the studied amino acids and phase behavior studies by X-ray powder diffraction and heat-flux DSC to identify the initial crystal structures and their possible transformations. The crystal heat capacities calculated by combining the periodic density functional theory and the quasi-harmonic approximation showed an agreement with the developed reference experimental data within 10% which can be considered as success of the employed computational methodology. Quantum chemical calculations further helped interpret the differences in thermodynamic and structural properties of the studied crystalline amino acids.

中文翻译：

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l-丙氨酸，l-缬氨酸，l-异亮氨酸和l-亮氨酸的热容量：实验和计算研究

这项工作是建立氨基酸的可靠热力学数据的努力的一部分，并且在更广泛的背景下，将分子晶体的热力学性质的第一性原理计算与可靠的实验数据进行了比较。在这项工作中，l-丙氨酸（CAS RN：56-41-7），l-缬氨酸（CAS RN：72-18-4），l-异亮氨酸（CAS RN：73-32-5），和我通过结合周期性密度泛函理论和准谐波近似计算得出的晶体热容与已开发的参考实验数据相吻合在10％以内，这可以认为是所采用计算方法的成功。量子化学计算进一步帮助解释了所研究的结晶氨基酸的热力学和结构性质的差异。