Title

Organic charge transfer complexes and an open-shell organic radical studied by inelastic neutron scattering spectroscopy and DFT methods

Date of Award

2004

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Bruce S. Hudson

Keywords

Charge transfer, Open-shell, Organic radical, Inelastic neutron scattering spectroscopy

Subject Categories

Chemistry | Physical Sciences and Mathematics

Abstract

The existence of intermolecular forces stronger than predicted by van der Waals forces is widely known and studied. Charge transfer (CT) molecules are complexes in which an electron is transferred from a donor component to an acceptor component upon complex formation. These complexes range from 'weak' to 'strong' in terms of the extent of their charge transfer, which results in a coulomb attraction and a covalent interaction between the two components. Little vibrational and theoretical work has been performed on CT complexes. These systems are a challenge to theoretical descriptions because of their open-shell character and interactions that occur in their crystalline states. The practical use of density functional theory (DFT) depends on a good approximation to the unknown exchange correlation contribution to the total electronic energy. Reproduction of an inelastic neutron scattering (INS) vibrational spectrum provides an excellent method for testing the validity of a particular technique to reproduce both the vibrational dynamics and molecular structure. Two different types of computational methods were used in this work. One employs a conventional atom-centered basis using an isolated complex model with the B3LYP functional. The other method is based on the use of plane-wave basis set with periodic functions to simulate a molecular solid. In this method, we used the popular solid state functional, LDA, and the newly developed generalized gradient approximated functional, HCTH/120. It is observed in this work that the isolated complex model applied to weak charge transfer complexes provides an adequate description of their molecular geometry and vibrational spectra, but that the periodic solid does not. The reverse is observed in the case of strong charge transfer complexes. This work explores the idea that the reason for the observed differences in the ability of the isolated complex and periodic solid in treating the limiting types of charge transfer is due to the nature of the functionals used. Density functional theory was also test in the challenging case of an open-shell organic radical. It was found that an unSurface provides description only. Full text is available to ProQuest subscribers. Ask your Librarian for assistance. calculation in conjunction with a scaling factor could reproduce the INS spectrum.

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