finalexam-2006-answers - Name _ Final Exam: CHEM/BCMB...

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Name _________________________ Final Exam, 2006: CHEM/BCMB 4190/6190/8189 1 Final Exam: CHEM/BCMB 4190/6190/8189 (247 points) Thursday, 14 December, 2006 1 ). Shown (below, right) are the chemical structure and 1D, 1 H NMR spectrum of ethylmethacrylate. The five individual signals in the spectrum are labeled A - E . a). Assign each of the five signals in the 1D, 1 H NMR spectrum of ethylmethacrylate to a hydrogen or hydrogens in the molecule. Justify (explain) each of your assignments. In your justifications make sure to include signal multiplicity. ( 5 points ) For the –CH 2 -CH 3 group, the coupling of the two methylene hydrogens splits the signal from the (equivalent) methyl hydrogens into a triplet, and the three methyl group hydrogens split the signal from the (equivalent) methylene hydrogens into a quartet. Therefore, signal E must result from the methyl hydrogens of the ethyl group, and signal C must result from the methylene hydrogens of the ethyl group. Also, due to the attachment of the methylene group to the oxygen, these hydrogens would be shifted downfield (deshielded) relative to the methyl group. Signal D must result from the remaining methyl group, as it is a singlet (hydrogens equivalent), and its intensity is much larger than the downfield shifted singlets, which correspond to the ethylenic hydrogens H E and H Z . One would expect that signal A would result from H E because H E is closer to the electron-withdrawing influence of the carbonyl group, leaving signal B to correspond to H Z . However, H Z and H E cannot be unambiguously assigned based on this information alone. A B C D E triplet quartet
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Name _________________________ Final Exam, 2006: CHEM/BCMB 4190/6190/8189 2 b). Shown (right) is the result of a selective difference NOE experiment on ethylmethacrylate. There are five observable signals in this spectrum. -Explain how a selective difference NOE experiment works. -Explain in detail the relative magnitudes and signs of these signals in the spectrum shown. -Explain in detail why the results of this experiment are consistent with the assignments that you made (part ‘a’), or why they are not. ( 10 points ) In a difference NOE experiment, a normal 1D spectrum is subtracted from a spectrum where a given signal (in this case, the signal from the methyl group at ~ 1.9 ppm) is saturated (irradiated). Signals that are then observed in the difference spectrum result from NOE enhancement from the hydrogen(s) that give rise to the irradiated signal. The exception is the signal in the difference spectrum that results from the irradiated hydrogen(s), which appears as a very large signal opposite in phase to the others. Selective irradiation of a signal removes it from the irradiated spectrum, which, when subtracted from the spectrum without irradiation, results in a large negative peak. In the spectrum above, very small positive peaks for three of the signals indicate very small NOE enhancements when the signal at D is saturated, which is consistent with the assignments above and the distances between the respective hydrogens. The larger signal at 5.5 ppm is consistent with the assignments in ‘a’, as the distance between the
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This note was uploaded on 11/13/2011 for the course CHEM 4190 taught by Professor Staff during the Fall '08 term at University of Georgia Athens.

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finalexam-2006-answers - Name _ Final Exam: CHEM/BCMB...

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