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Unformatted text preview: Exam 1: CHEM/BCMB 4190/6190/8189 (120 points) Tuesday, 12 September, 2006 1 ). In the example (right), the effect of a 90 ( /2) pulse applied along the x axis (90 x) is shown. For a-d below, show the effect of the indicated pulses on the indicated magnetization vectors by drawing the resulting vectors on the coordinate axes. (8 points) a . c . b . d . 2 ). You prepare some anisole (right) in your lab and you want to record a 1 H NMR spectrum to confirm its identity. You decide to dissolve the compound in some deuterated chloroform (CDCl 3 ), and you also include some TMS as a chemical shift reference. Why do you not simply use the 1 H signal from the residual CHCl 3 in the CDCl 3 as the chemical shift reference in this case? ( 5 points ) We know that the signals from aromatic benzene derivatives are shifted downfield, and, like benzene, the chemical shifts of these signals are often between 7 and 8 ppm. Although you may not know what the exact 1 H chemical shifts of anisole are before you acquire the spectrum (unless you are wise and find a published spectrum first ! ), you might suspect, correctly, that one or more of the 1 H signals from the benzene ring could overlap with the 1 H signal from the residual CHCl 3 in CDCl 3 . In such a case, it may be very difficult to properly reference the chemical shifts using the small signal from CHCl 3 , which could be obscured by the signal(s) from anisole. Therefore, it is a good idea to add some TMS and reference the spectrum to the chemical shifts of TMS at 0.0 ppm. 90 x M y z x y z x 360 -y y z x y z x y z x y z x 90 y y z x y z x 90 x y z x y z x 270 -x Name _________________________ Exam 1, 2006: CHEM/BCMB 4190/6190/8189 2 3 ). What will the 1 H NMR signal for the indicated ( bolded ) 1 H nucleus in the molecule shown (right) look like? Below are shown six ( a- f ) signals. For each of these, state whether or not the indicated 1 H nucleus could produce such a signal, and, if so, describe how the observed splitting pattern arises. ( 12 points ) The indicated (bolded) nucleus could produce each of these signals, although some are more likely than others. The singlet ( c ) is possible, but somewhat unlikely because ordinarily the couplings with hydrogens on neighboring carbons are large enough to be observed and would result in splitting of the signal from the indicated 1 H. Likewise, the doublet ( e , resulting from splitting by the neighboring -CH) and the triplet ( a , resulting from splitting by the two neighboring CH 2 hydrogens) are possible, but somewhat less likely because the signal from the indicated 1 H would likely be split by the hydrogens on both of the neighboring carbons....
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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.
- Fall '08