exam3-2008 - Name _ Exam 3: CHEM/BCMB 4190/6190/8189 (129...

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Name _________________________ Exam 3, 2008: CHEM/BCMB 4190/6190/8189 1 Exam 3: CHEM/BCMB 4190/6190/8189 (129 points) Thursday, 30 October, 2008 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’, ‘d’ and ‘e’ below, show the effects of the indicated pulses by drawing the missing (originating or resulting) vectors on the coordinate axes. For ‘a’, ‘b’ and ‘f’, fill in the blank with the correct pulse (angle and axis along which it is applied) that will promote the indicated movement of the bulk magnetization vector. In some cases there may be more than one correct answer. (12 points) a . d . b . e . c . f . 90 ° x M 0 y z x y z x 90 ° - y y z x y z x 90 ° - x y z x y z x 90° ² y , 90° x y z x y z x y z x y z x 270 ° y , 90 ° - x 180 ° x y z x y z x y z x y z x ± - y this vector is in the x-z plane this vector is in the x-z plane
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Name _________________________ Exam 3, 2008: CHEM/BCMB 4190/6190/8189 2 2 ). In the DEPT experiment, the pulse angle ( ± y , pulse width) of the third 1 H pulse (applied along the ‘ y ’ axis) can be set to any value in order to achieve the desired result. Shown in the diagram below (right) are the intensities of signals from –CH, -CH 2 , -CH 3 groups as a function of the phase angle y . For the molecule codeine (above, left), the 13 C chemical shifts for thirteen of the eighteen carbon atoms are shown (above, center). a. For the thirteen 13 C atoms assigned above, sketch the signals that will be observed in DEPT(90), DEPT(135), and DEPT(45)-DEPT(135) spectra below: ( 12 points ) CH CH 2 CH 3 3 5 10 9 15 16 14 11 12 2 8 7 6 1 4 17 13 18 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ± 13 C (ppm) 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ± 13 C (ppm) 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ± 13 C (ppm) DEPT(90) DEPT(135) DEPT(45)-DEPT(135)
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Name _________________________ Exam 3, 2008: CHEM/BCMB 4190/6190/8189 3 b. Explain your reasoning, in detail, for your answers to part ‘a’. ( 9 points ) For the codeine molecule, of the assigned 13 C chemical shifts shown in the table, eight of these are for methine (-CH) groups, three for methylene (-CH 2 ) groups, and two for methyl (-CH 3 ) groups. For the “DEPT(90)” experiment, as shown in the plot above, when the pulse angle ( ± y) is 90 degrees, no signals are observed from -CH 2 and -CH 3 groups, and positive signals are observed from -CH groups. Thus, eight signals are observed in this experiment, corresponding to the eight -CH groups, at the chemical shifts listed in the table. For the “DEPT(45)-DEPT(135)” experiment, the signal intensities from -CH and -CH 3 groups when y is 45 degrees are the same as when y is 135 degrees. Subtracting the signals observed when y is 135 degrees from the signals observed when y is 45 degrees leaves no signals for these groups. However, for -CH 2 groups, when y is 45 degrees the intensities are maximal with positive phase, and they are also maximal, but with negative phase, when y is 135 degrees. Thus, subtracting these gives very large signals for -CH 2 groups. Therefore, for codeine, three large signals will be observed in the “DEPT(45)-DEPT(135)” experiment from the three -CH 2 groups.
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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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exam3-2008 - Name _ Exam 3: CHEM/BCMB 4190/6190/8189 (129...

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