The nature and applications of secondary interactions in multi-center radical pi-dimers and conducting polymers
Thesis (Ph.D.)--Georgetown University, 2011.; Includes bibliographical references.; Text (Electronic thesis) in PDF format. Multi-center pi-radicals are a set of stable chemical species that are often used to construct new materials with promising magnetic and electric properties. In crystal structures, these radical species are often observed to dimerize by forming two-electron/multicenter (2e/mc) pi-bonding, which is unusual since the bonding distances are much shorter than typical van der Waals contacts but significantly longer than conventional covalent bonds. Trimers and other aggregates are also observed. For conducting materials the quasi one dimensional chain-like pi-stacked aggregates are especially important. One of the central questions of this dissertation is: what are the intermolecular forces that keep these aggregates together and how are these forces affecting the material properties?; With quantum mechanical calculations, we acquired new insight into the nature of the 2e/mc pi-bonding formed by these radicals. By using a variety of theory levels, we found that proper descriptions of the static and dynamic electron correlations are necessary to account for the physical nature of the 2e/mc pi-bonding. A significant charge shift bonding effect was found, which is attributed to the electron correlations between the nonbonding electron pairs and bonding electron pairs. By reducing the lone pair bond weakening effect (LPBWE) in phenalenyl dimers, we found that the 2e/mc pi-bonding strength can be considerably enhanced, which makes it promising to construct 2e/mc bonding with bonding strength approaching that of normal covalent bonds.; The importance of the 2e/mc bonding in pi-dimers of multi-center radicals was also identified in pericyclic reactions. In 2,5,8-tri-tert-butyl-1,3- diazaphenalenyl sigma-dimers, we computationally observed that the sigma-bonds are fluxional and the sigma-dimers can easily undergo a complete spectrum of sigmatropic rearrangement reactions via pi-dimer intermediates, which gives rise to thermochromism as observed by experiments. In hydrogen transfer reactions of phenalene molecules, the multi-center phenalenyl radical dimers are important intermediates accounting for the stepwise ene-like reaction mechanism.; The physical properties of organic conducting polymers are usually correlated with pi-conjugation. Our computational studies indicate that the intra-molecular noncovalent interactions also significantly affect the electronic structures of conducting polymers. Three types of noncovalent interactions were investigated in our studies, hydrogen bonding, noncovalent S***N interactions and pi-pi stacking. A series of low bandgap polymers were proposed based on intramolecular noncovalent interactions. In conjugated helicene polymers, we found electron communications arising from the transannular pi-pi interactions across the helical pitch. This property might be utilized for designing novel helicenes-based materials with unusual properties.
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