Theoretical Investigation of the Jahn-Teller Effect and the Influence of the Jahn-Teller Distortion on the Properties of Chemical Systems

• Main Author: Anđelković, Ljubica D
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2015
• Subjects: Arsenic; Chemistry; Energy; Fullerenes; Geometry; Hydrocarbons; Low Temperature Physics; Phase transitions; Superconductivity; Symmetry
• ISBN: 9798383141182

Quantum mechanical description of the changes in electronic structure due to distortions in molecular shape and vice versa is given in the form of the vibronic coupling theory. Probably, the most famous concept based on this theory is the Jahn−Teller (JT) effect. The JT theorem states that a molecule with a degenerate electronic state spontaneously distorts along a non-totally symmetric vibrational coordinates. This removes the degeneracy and lowers the energy. In fact, the vibronic coupling, correlation between electronic states and vibrational motion of nuclei, describes all spontaneous symmetry breaking distortions, attributed to the JT, Renner-Teller and pseudo JT effects. The consequences of the JT effect are far-reaching. JT effect affects the high magneto-resistance in manganites, superconductivity in fullerides, aromaticity, molecular stereochemistry, reactivity, magnetic properties of molecules, as well as many other properties. It should be emphasized that the JT effect has inspired very significant scientific discoveries, e.g. the concept of high-temperature superconductivity. The significance of the JT effect is increasingly recognized, hence, quantifying the distortion and getting insight into the mechanism lies at the heart of modern investigations. In this thesis, the JT effect and its consequences on the structure and properties of organic and inorganic molecules, aromaticity, excitonic coupling and excitation energy transfer are studied.The JT effect was analyzed and the JT parameters were determined for the JT active cyclobutadienyl radical cation (C4H4 +•), cyclopentadienyl radical (C5H5•), benzene cation (C6H6+), benzene anion (C6H6-), tropyl radical (C7H7•), anions and cations of corannulene and coronene (C20H10- , C20H10+, C24H12- and C24H12+), small metal and metalloid clusters (Na3, Ag3, As4− , Sb4−), hexaflurocuprat(II) ion ([CuF6]4-), manganese chelate complex ([Mn(acac)3]) and the organometallic compound cobaltocene (CoCp2) by the means of multideterminantal Density Functional Theory (DFT) approach. The validation of multideterminantal DFT method was performed by systematic investigation of the influence of the exchange-correlation functional (LDA, BP86, BLYP, OPBE and B3LYP) on the determination of the JT parameters and comparison with available data from the literature. JT distortion is a consequence of the electronic factors, but also essentially depends on the geometry of the investigated molecule. The choice of functional is directly associated with a specific chemical problem, however it is concluded that in the analysis of the JT distortion all used functionals give qualitatively satisfactory results.The analysis of the multimode JT problem in As4 − and Sb4−, C5H5• , C6H6+, C6H6-,C7H7•, C20H10-, C20H10+, C24H12and C24H12wasperformed by the means of the Intrinsic Distortion Path (IDP) model. All the required information, to calculate the vibronic coupling coefficients is contained in the minimum energy lower-symmetry structure. This structure is the real minimum on the potential energy surface, allowing normal mode analysis in the straightforward manner. The correlation of the normal modes in the minimum energy structure with the high symmetry normal modes according to the epikernel principle, has been described. With this model, it is possible to directly separate the contributions of the different normal modes to the IT distortion, their energy contributions to the IT stabilization energy along a relevant particular path of distortion and the forces responsible for the distortion, giving further insight into the origin and mechanism of the vibronic coupling. The distortion is always dominated by the modes that fulfill symmetry requirements, according to the JT theory.