• Fulem, Michal
• Červinka, Ctirad
• Růžička, Květoslav
• Pokorný, Václav
• Havlín, Jakub
• Štejfa, Vojtěch

Abstract

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. Copyright © 2020 American Chemical Society.

Topics

• density
• impedance spectroscopy
• phase
• theory
• thermogravimetry
• density functional theory
• differential scanning calorimetry
• decomposition
• heat capacity