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exam2-2006 - Name Exam 2 CHEM/BCMB 4190/6190/8189(141...

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Name _________________________ Exam 2, 2006: CHEM/BCMB 4190/6190/8189 1 Exam 2: CHEM/BCMB 4190/6190/8189 (141 points) Tuesday, 3 October, 2006 1 ). In the example (right), the effect of a 90 ° ( ± /2) pulse applied along the “x” axis (90 ° x) is shown for a bulk magnetization vector ( M 0 ) at equilibrium (on the ‘ z ’ axis). For ‘c’ and ‘d’ below, show the effects of the indicated pulses on the indicated magnetization vectors by drawing the resulting vectors on the coordinate axes. For ‘a’, ‘b’, ‘e’, and ‘f’, fill in the blank with the correct pulse (for ‘f’, just a number) that will promote the indicated movement of the bulk magnetization vector (there may be many correct answers for each of these). (12 points) a. d. b. e. c. f. 180 ° y y z x y z x y z x y z x 450 ° y y z x y z x 90 ° x y z x y z x 180 ° -x, y z x y z x y z x y z x 90 ° x, 90 ° -y 90 ° x M 0 y z x y z x 90 ° -y ± -y 3/2
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Name _________________________ Exam 2, 2006: CHEM/BCMB 4190/6190/8189 2 2 ). Three schemes (A, B, and C) for heteronuclear decoupling – broadband 1 H decoupling of 13 C spectra – are shown below (note: in figures, bb ± broadband): a. Of those shown, which of the decoupling schemes removes 1 H- 13 C splittings? Provide an explanation for your answer. ( 4 points ) A and B. In order to remove the splittings and collapse the peaks into multiplets, broadband 1 H decoupling has occur during the FID acquisition period. In C, this is not the case. b. Of those shown, which of the decoupling schemes promotes signal-to-noise increases via the heteronuclear NOE? Provide an explanation for your answer. ( 4 points ) B and C. The heteronuclear NOE builds up during the 1 H decoupling. Because the interscan delay (delay preceding the 90° pulse) is long, the NOE builds up during this period. There will also be a small NOE enhancement in A, but it will not be as substantial as it is in B and C. 13 C 1 H 90 ° x bb decoupling FID 13 C 1 H 90 ° x bb decoupling FID 13 C 1 H 90 ° x bb decoupling FID A B C
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Name _________________________ Exam 2, 2006: CHEM/BCMB 4190/6190/8189 3 c. Compared to normal 13 C spectra acquired without decoupling, how does decoupling scheme B ’ affect signals from quaternary carbon atoms? Provide a justification for your answer. ( 4 points ) The 13 C spectra of quaternary carbon signals are not substantially affected by 1 H decoupling because there is no splitting to remove and no attached 1 H nuclei that promote a large signal-to-noise increase via the heteronuclear NOE. However, if the density of nearby 1 H nuclei is high, there will be some signal-to-noise enhancement via the NOE. d. If you needed to improve the signal-to-noise in your 13 C spectra but also needed to accurately integrate the signals, which of the schemes would you use? Provide a complete explanation. ( 6 points ) Scheme A would be best. The splittings would collapse, giving all singlets and increasing the signal-to-noise via the addition of the multiplet components, and because there is no NOE, there would be no selective NOE enhancement based on the number of attached hydrogens or density of neighboring hydrogens. So, the integrals would be based on the number of carbon atoms and not on selective NOE enhancements.
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