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Dissertation Defense |
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Candidate: Chad Eric Hadden Degree of: Doctor of Philosophy Committee: Dr. Mark Perkovic, Chair Abstract: Many of the two-dimensional NMR experiments employed for structural information are dependent upon an experimental delay that is indirectly proportional to the magnitude of the coupled nuclei. Unfortunately, many nuclei in different electronic environments will exhibit vastly different coupling magnitudes. If an experimental delay is to be based on a specific coupling then the signal response for all other couplings may be notably reduced. Most common, the couplings in question are unknown and an average value is chosen for the delay. If this provides insufficient data, the experiment can be re-acquired with a different optimization. This multiple experiment approach is generally unacceptable when spectrometer time is expensive, when sample is limited requiring extensive acquisition times, or when the sample may degrade rapidly. An alternative approach is accordion-optimized spectroscopy. This technique optimizes the necessary delay for a range of times based on a range of coupling magnitudes. If there are several different functionalities contained in a single molecule all exhibiting coupling magnitudes from say, A to Z, then the accordion-optimized experiment samples all delays based on couplings from A to Z. This differs tremendously from the previous method of static optimization of the average of M. The new approach allows for all species to be observed in a single experiment. Accordion-spectroscopy also has its drawbacks. The accordion delay is actually a variable delay that changes with each increment of the experiment. This creates an additional delay for unwanted nuclei to evolve creating a variety of artifacts. Some simple measures can be taken to remove these artifacts, such as the inclusion of a refocusing pulse to stop the unwanted evolution. Other techniques such as a randomization function aren't as simple. The accordion-optimization has been applied to both the direct and long-range proton-carbon experiments. This research centers on the development of these experiments, the artifacts observed, corrections when available, and the best applications for the new experiments. |
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