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exam3-2010 - Name Exam 3 CHEM/BCMB 4190/6190/8189(152...

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Name _________________________ Exam 3, 2010: CHEM/BCMB 4190/6190/8189 1 Exam 3: CHEM/BCMB 4190/6190/8189 (152 points) Thursday, 28 October, 2010 1 ). In the example (right), the effect of a 90 ° ( π /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 ‘b’, ‘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’ 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 (there may be more than one correct answer for each). (12 points) a. b. e. c. f. 90 ° x M 0 y z x y z x y z x y z x 90 ° - x y z x y z x 180° y y z x y z x y z x y z x 270 ° y y z x y z x y z x y z x π x 90 ° - y 90 ° - x
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Name _________________________ Exam 3, 2010: CHEM/BCMB 4190/6190/8189 2 2 ). In the DEPT experiment, the pulse angle ( Θ 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 (right) are the intensities of signals from –CH, -CH 2 , -CH 3 groups as a function of the phase angle Θ y . Broadband 1 H-decoupled 13 C spectra (spectra A, B, C, and D) in CDCl 3 solvent were acquired for samples of each of the molecules (1, 2, 3, and 4) shown. Expansions of these spectra around 0 ppm and 75-80 ppm are shown (below, right). Further below are shown DEPT spectra (E and F) for two of the molecules/samples. a. What does the acronym ‘DEPT’ stand for? ( 2 points ) D istortionless E nhancement by P olarization T ransfer CH CH 2 CH 3 1 2 3 4 E F
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Name _________________________ Exam 3, 2010: CHEM/BCMB 4190/6190/8189 3 b. Explain what gives rise to the small signals centered at 0 ppm and 77 ppm. Explain the multiplicities of the signals. You may need to provide a structural description to do so. Explain the multiplet structures and relative heights of the individual peaks in the signals. ( 8 points ). The signal at 0 ppm is most likely due to tetramethylsilane (TMS) added as a chemical shift standard. In TMS, the methyl groups are arranged at the four corners of a perfect tetrahedron, and therefore are chemically equivalent and give identical chemical shifts (singlets for both 13 C and 1 H). The 13 C and 1 H shifts typically define 0 ppm in 13 C and 1 H spectra, respectively. The signal at 77 ppm is most likely due to the 13 C in CDCl 3 . The 13 C signal is split by coupling to the attached (quadrupolar) 2 H (D) nucleus. When a 13 C nucleus is attached to a 1 H nucleus it is split into a doublet, where the individual peaks in the doublet are of equal intensity. This is because the attached 1 H nucleus (I= ½ ) can be in either the alpha ( α , low energy) or beta ( β , higher energy) state. Because the α and β states are nearly equally populated, the intensities of the doublet components in the 13 C spectrum are nearly identical. We know also that for a 2 H nucleus, where I=1, there will be three possible energy states, E = 1 γ ± B 0 , E = 0 γ ± B 0 , E = 1 γ ± B 0 , and that these will be very nearly equally populated. Thus, these will split the attached
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