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Molecular rod rotors and their solid state internal dynamics


Kasumaj, F. Molecular rod rotors and their solid state internal dynamics. 2011, University of Zurich, Faculty of Science.

Abstract

This dissertation describes the synthesis and characterization of a new class of Brownian molecular rod rotors and is divided into three parts: 1.) A short historical review is given, starting from the observation of the Brownian motion and followed by a "gedankenexperiment " of Einstein and Smoluchowski, who dreamed about being able to harvest the Brownian motion in order to build molecular machines. This overview is followed by a discussion of Brownian molecular rotors in nature and in chemistry, including examples and their rotation barriers. 2.) In the second part physical methods for dynamic studies are presented and discussed, the focus lies on solid state dynamic studies. In the last part variable temperature NMR studies are explained. 3.) The third main part of this thesis focuses on molecular rod rotors. It starts with a history of trioxatricornan syntheses and a description of its physical properties. This introduction is then followed by the description of my own work: First the synthesis of different molecular rod rotors with trioxatricornan caps is described and discussed, then physical properties of a selected system are described and finally dynamic studies are presented. Molecular rod rotors consist of three distinct parts: The stator, the axle and the rotator. Our design is based on a new class of stators, namely trioxatricornan, a rigid, tripod-shaped molecule with a large surface area, which was first synthesized by Smith and Martin in 1964. Due to its structure, the momentum of inertia of trioxatricornan is much bigger than it is for the rotator part of the rod rotors discussed here and this might lead to a fast rotation of the the rotator. In order to probe this hypothesis a series of superstructures were synthesized and the solidstate properties and dynamics of one such structure was investigated. Our studies began with a cubic crystal structure of a phenyl-linked rod rotor which showed a three-fold disorder of the phenyl ring. This led to the assumption, that a dynamic process could lead to the disorder. Therefore, variable temperature X-Ray studies were performed to test this hypothesis. DFT calculations estimated a rotation barrier of 0.3 kcal mol−1 for the rotation of the phenyl ring. In order to confirm the calculation atomic displacement parameters (ADPs) from three data sets were studied and found their conclusions to agree with the results of the DFT calculation. To further confirm the very low rotation barrier of the phenyl ring rotation solid state NMR studies were performed. For these experiments a deuterated version of the phenyl-linked rod
rotor had to be synthesized in 500 mg scale. The NMR studies showed a surprising result: mixed dynamics- fast rotating and slower rotating components could be identified. From XRay powder diffraction patterns of the sample measured, three pseudo-polymorphs could be identified, explaining the mixed dynamics.

This dissertation describes the synthesis and characterization of a new class of Brownian molecular rod rotors and is divided into three parts: 1.) A short historical review is given, starting from the observation of the Brownian motion and followed by a "gedankenexperiment " of Einstein and Smoluchowski, who dreamed about being able to harvest the Brownian motion in order to build molecular machines. This overview is followed by a discussion of Brownian molecular rotors in nature and in chemistry, including examples and their rotation barriers. 2.) In the second part physical methods for dynamic studies are presented and discussed, the focus lies on solid state dynamic studies. In the last part variable temperature NMR studies are explained. 3.) The third main part of this thesis focuses on molecular rod rotors. It starts with a history of trioxatricornan syntheses and a description of its physical properties. This introduction is then followed by the description of my own work: First the synthesis of different molecular rod rotors with trioxatricornan caps is described and discussed, then physical properties of a selected system are described and finally dynamic studies are presented. Molecular rod rotors consist of three distinct parts: The stator, the axle and the rotator. Our design is based on a new class of stators, namely trioxatricornan, a rigid, tripod-shaped molecule with a large surface area, which was first synthesized by Smith and Martin in 1964. Due to its structure, the momentum of inertia of trioxatricornan is much bigger than it is for the rotator part of the rod rotors discussed here and this might lead to a fast rotation of the the rotator. In order to probe this hypothesis a series of superstructures were synthesized and the solidstate properties and dynamics of one such structure was investigated. Our studies began with a cubic crystal structure of a phenyl-linked rod rotor which showed a three-fold disorder of the phenyl ring. This led to the assumption, that a dynamic process could lead to the disorder. Therefore, variable temperature X-Ray studies were performed to test this hypothesis. DFT calculations estimated a rotation barrier of 0.3 kcal mol−1 for the rotation of the phenyl ring. In order to confirm the calculation atomic displacement parameters (ADPs) from three data sets were studied and found their conclusions to agree with the results of the DFT calculation. To further confirm the very low rotation barrier of the phenyl ring rotation solid state NMR studies were performed. For these experiments a deuterated version of the phenyl-linked rod
rotor had to be synthesized in 500 mg scale. The NMR studies showed a surprising result: mixed dynamics- fast rotating and slower rotating components could be identified. From XRay powder diffraction patterns of the sample measured, three pseudo-polymorphs could be identified, explaining the mixed dynamics.

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Additional indexing

Item Type:Dissertation
Referees:Siegel J S
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Language:English
Date:23 May 2011
Deposited On:05 Mar 2012 15:20
Last Modified:05 Apr 2016 15:30
Related URLs:http://opac.nebis.ch/F/?local_base=NEBIS&CON_LNG=GER&func=find-b&find_code=SYS&request=006678168
Permanent URL: http://doi.org/10.5167/uzh-57435

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