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2000
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In the Laboratory
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Quantitative Determination of Methylcyclohexanone Mixtures Using 13C NMR Spectroscopy: A Project for an Advanced Chemistry Laboratory
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Joseph W. LeFevre and Augustine Silveira Jr.
Chemistry Department, SUNY at Oswego, Oswego, NY 13126
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January 2000
Vol. 77 No. 1
p. 83
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| Abstract |
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The percentage composition of mixtures of four methylcyclohexanones was determined using 13C NMR spectroscopy as a quantitative analytical method. The data were acquired using standard broadband proton decoupling and inverse-gated decoupling, the latter done both with and without the paramagnetic relaxation reagent chromium(III) acetylacetonate [Cr(acac)3]. The standard broadband decoupled spectrum resulted in percentages far from the actual values owing to the varying nuclear Overhauser enhancements (NOEs) and spin-lattice relaxation times (T1's) of the various carbon atoms. These effects were eliminated in the inverse-gated experiments, and the results were very close to the actual percentages. Before examining the mixtures, the students studied a pure sample of 2-methylcyclohexanone. They assigned the 13C spectrum and determined the T1 of the carbonyl group both with and without Cr(acac)3 using the inversion-recovery method. Then a five-times-T1 delay was inserted between pulses in all subsequent inverse-gated decoupling experiments. This project provides students with valuable experience with modern NMR techniques. These include COrrelated SpectroscopY (COSY), Distortionless Enhancement by Polarization Transfer (DEPT) spectroscopy, HETeronuclear CORrelated (HETCOR) spectroscopy, T1 determination, standard broadband versus inverse-gated decoupling, and the addition of a paramagnetic relaxation reagent to dramatically shorten both the T's and the data acquisition time.
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| Supplement |
Detailed student procedures, questions, and instructorÕs notes are provided. The instructorÕs notes contain a schedule of the experiments, 13C-NMR acquisition parameters, T1 determinations, percentage composition calculations, data on additional mixtures, answer to questions, mixture preparation procedures, and a reagent list.
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| More Information |
Citation |
LeFevre, Joseph W. ; Silveira, Augustine, Jr. J. Chem. Educ. 2000 77 83.
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Keywords |
Laboratory Instruction; Organic Chemistry; NMR Spectrometry; Quantitative Analysis
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History |
Created:
Last Updated: |
December 9, 1999
August 31, 2005
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January
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