l15d

l15d - CH 203 O R G A N I C C H E M I S T R Y I NMR...

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NMR spectroscopy: Complex splitting © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I NMR spectroscopy: Complex splitting Complex splitting In a previous lecture we introduced simple N + 1 rule splitting. The second type of splitting we will discuss is complex splitting, which, as implied by the name, is much more complicated than simple splitting. Let’s call the hydrogen whose splitting pattern we’re trying to predict the observed hydrogen H obs . If H obs is split by N other hydrogens and if those N other hydrogens are not all chemical-shift equivalent to each other, then complex splitting is observed and simple N + 1 rule splitting is not ob- served. Consider the NMR spectrum of 3,3-dimethyl-1-butene: The gigantic peak at ! 1.01 is due to the nine methyl hydrogens, but what really interests us now are the signals between ! 4.5 and ! 6.0 shown in the close-up below: H 3 a C CH 3 a H 3 a C H b H c H d a b c, d H 3 a C CH 3 a H 3 a C H b H c H d b c d
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NMR spectroscopy: Complex splitting © Bruno I. Rubio 2 The signal centered at ! 5.83 is due to H b . Although the signal consists of four components, it is not a quartet because Pascal’s triangle predicts that the components of a quartet exhibit a relative height ratio of 1:3:3:1, but the components of the signal at ! 5.83 are all approximately of the same height. So what is it? The signal at ! 5.83 is an example of a complex- splitting multiplet: H b undergoes vicinal splitting by H c and by H d , but H c and H d are not chemical-shift equivalent to each other. (The signals due to H c and H d also exhibit complex splitting; we will have more to say about them later.) A complex-splitting multiplet can be viewed as arising from sequential appli- cations of the N + 1 rule. In the present case, we first apply the J trans = 18 Hz doublet splitting of H b by H c . Next, we apply the J cis = 10 Hz doublet splitting of H b by H d : H 3 a C CH 3 a H 3 a C H b H c H d J trans = 18 Hz H 3 a C CH 3 a H 3 a C H b H c H d J cis = 10 Hz The appearance of a complex-splitting multiplet is predicted using a split- ting tree like the one shown below. Application of the 18-Hz doublet split- ting of H b by H c results in two components of equal height spaced 18 Hz apart. Application of the 10-Hz doublet splitting of H b by H d to each compo- nent in the previous splitting level results in the approximately 1:1:1:1 feature at ! 5.83 due to the H b in the spectrum of 3,3-dimethyl-1-butene.
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NMR spectroscopy: Complex splitting © Bruno I. Rubio 3 H b J = 18 Hz apply doublet splitting by H c J = 10 Hz J = 10 Hz apply doublet splitting by H d to each component in the previous splitting level midpoint of the multiplet +4 +8 +12 –4 –8 –12 Hz –16 +16 The signal is called a doublet of doublets (abbreviated “dd”) because it looks like the two 1:1 components of a doublet have each been replaced by a 1:1 doublet.
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l15d - CH 203 O R G A N I C C H E M I S T R Y I NMR...

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