Normal-mode vibrational frequencies and thermodynamic properties of uracil and 4-hydroxyuracil

1980 ◽  
Vol 18 (1) ◽  
pp. 323-329 ◽  
Author(s):  
Masayuki Shibata ◽  
Theresa Julia Zielinski ◽  
Robert Rein
Keyword(s):  
Thermodynamic Properties ◽  
Normal Mode ◽  
Vibrational Frequencies

scholarly journals X-ray diffraction data as a source of the vibrational free-energy contribution in polymorphic systems

IUCrJ ◽  
2019 ◽  
Vol 6 (4) ◽  
pp. 558-571 ◽  
Author(s):  
Phillip Miguel Kofoed ◽  
Anna A. Hoser ◽  
Frederik Diness ◽  
Silvia C. Capelli ◽  
Anders Østergaard Madsen
Keyword(s):  
Thermodynamic Properties ◽  
Normal Mode ◽  
Diffraction Data ◽  
High Frequency ◽  
Low Frequency ◽  
General Question ◽  
Vibrational Entropy ◽  
X Ray Diffraction ◽  
X Ray ◽  
High Frequency Modes

In this contribution we attempt to answer a general question: can X-ray diffraction data combined with theoretical computations be a source of information about the thermodynamic properties of a given system? Newly collected sets of high-quality multi-temperature single-crystal X-ray diffraction data and complementary periodic DFT calculations of vibrational frequencies and normal mode vectors at the Γ point on the yellow and white polymorphs of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate are combined using two different approaches, aiming to obtain thermodynamic properties for the two compounds. The first approach uses low-frequency normal modes extracted from multi-temperature X-ray diffraction data (normal coordinate analysis), while the other uses DFT-calculated low-frequency normal mode in the refinement of the same data (normal mode refinement). Thermodynamic data from the literature [Yang et al. (1989), Acta Cryst. B45, 312–323] and new periodic ab initio DFT supercell calculations are used as a reference point. Both approaches tested in this work capture the most essential features of the systems: the polymorphs are enantiotropically related, with the yellow form being the thermodynamically stable system at low temperature, and the white form at higher temperatures. However, the inferred phase transition temperature varies between different approaches. Thanks to the application of unconventional methods of X-ray data refinement and analysis, it was additionally found that, in the case of the yellow polymorph, anharmonicity is an important issue. By discussing contributions from low- and high-frequency modes to the vibrational entropy and enthalpy, the importance of high-frequency modes is highlighted. The analysis shows that larger anisotropic displacement parameters are not always related to the polymorph with the higher vibrational entropy contribution.

Vibrational frequencies, vapour pressure and an assessment of the thermodynamic properties of dimethyl telluride

1996 ◽  
Vol 92 (6) ◽  
pp. 973 ◽  
Author(s):  
Martin G. M. van der Vis ◽  
Erik H. P. Cordfunke ◽  
Rudy J. M. Konings ◽  
Ad Oskam
Keyword(s):  
Thermodynamic Properties ◽  
Vapour Pressure ◽  
Vibrational Frequencies
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