Towards understanding the energetics in polymorphs through charge density studies

Sovago, Ioana (2013) Towards understanding the energetics in polymorphs through charge density studies. PhD thesis, University of Glasgow.

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Abstract

The detailed study of the structure and electron density distributions of polymorphic and phase transition materials is presented. Understanding and predicting the appearance of polymorphism and phase transitions in organic and organometallic materials is of considerable interest in fields such as pharmaceutical science, solid-state chemistry, and materials science. However, the small lattice energy difference between the different molecular conformations and packing between these materials are often particularly challenging in this area. Consequently, obtaining the most accurate description of the atomic positions and the electronic distributions plays an extremely important role in obtaining the best estimation of the lattice energies. In the present work,high-resolution X-ray diffraction as well as neutron diffraction tehniques have been used in reaching these aims. For minimizing the data collection times, synchrotron sources were also used for obtaining X-ray diffraction data, including Diamond, I19 beam line and Soleil, CRYSTAL beam line. Molecular complexes of lutidine isomers and chloranilic acid are also studied, in both 1:1 and 2:1 ratios, in order to investigate their relative stabilities through hydrogen bond contributions towards stabilising stoichiometrically different ‘compositional polymorphs’.

The energy stability rankings in small organic molecules and transition metal complexes which exhibit polymorphism or displacive phase transitions are calculated using
experimental charge density and fully theoretical approaches. The effect of the hydrogen bonds in the rank stabilities is also investigated. The pharmaceutical sulfathiazole and piracetam compounds are identified to have very small lattice energy differences between the polymorphs studied and the ranking stability orders are not maintained in the approaches used. Studies of the coordination complex [Ni(en)3]2+(NO3-)2 show that, contrary to expectation, the higher temperature phase is calculated to be the most stable one, showing the strongest intermolecular interaction energies. Overall, the presented
studies show that current methodologies for estimating solid state lattice energies, even using high quality diffraction data and complex modeling of the electron density, are not sufficiently accurate to allow reliable estimation of polymorph energy differences. The results obtained for all studied polymorphic and phase transition materials using the experimental charge density approach show a high dependence of the lattice energies on the multipole model used.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: charge density, lattice energy, polymorphism, crystallography, high-resolution X-ray diffraction
Subjects: Q Science > QD Chemistry
Colleges/Schools: College of Science and Engineering > School of Chemistry
Supervisor's Name: Farrugia, Dr. Louis
Date of Award: 2013
Depositing User: Miss Ioana Sovago
Unique ID: glathesis:2013-3885
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 24 Jan 2013 11:51
Last Modified: 24 Jan 2013 11:54
URI: http://theses.gla.ac.uk/id/eprint/3885

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