Gas Phase NMR Studies of Conformational Kinetics

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In this project in the Physical Chemistry Program of the Chemistry Division, Prof. Nancy S. True of the Chemistry Department of the University of California at Davis will pursue a research program to determine if the kinetics of simple conformational processes in the gas phase is significantly influenced by restricted intramolecular vibrational energy redistribution (IVR). To this end the fall-off curves (i.e. unimolecular rate constants vs. pressure) will be determined under experimental conditions that will help address this question. Calculation of the energies, symmetries, and anharmonic coupling constants for vibrational states above the threshold energy will be used to develop models for the IVR process in the critically energized molecules. These models will be used in fall-off calculations employing a relaxation matrix approach which explicitly includes competition between IVR and decay. The studies are a first attempt to relate the experimentally determined shape of the fall-off for conformational processes to a kinetic model containing a description of the IVR process. Experimental fall-off curves will also be compared to those calculated using standard RRKM theory in order to test its realm of validity in transition state chemistry. The experimental techniques to be used in this work include high pressure gas phase NMR. Methyl nitrite, cyclohexane, and sulfur tetrafluoride, as well as processes involving the internal rotation dynamics in amides and substituted ethanes, trimethylsilyl group rotation in tetramethylsilylcyclopentadiene, pseudorotation in fluxional inorganic molecules and ring inversion in unsaturated six-membered rings will be investigated. The intramolecular redistribution of vibrational energy is at the heart of theories of chemical reaction dynamics and equilibrium. Several theories have ben proposed to account for such redistribution processes, the most prominent of which is that proposed by Rice, Rams perger, Kassel, and Marcus, which is known under the acronym RRKM theory. This theory is based on a statistical model of energy flow and recent spectroscopic experiments have indicated that not all intramolecular energy redistribution can be accounted for by it. The research performed in this project, using different experimental techniques, will provide additional data which will help delineate the range of applicability of the RRKM theory.