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Saturday, 22 November 2014

NMR STUDY EXAMPLE 2





In this spectrum, the non-equivalent alkene hydrogens, labeled Ha and Hb, have distinctly different chemical shifts. Hb is downfield, mixed in among the absorptions for the benzene hydrogens (b), (c), and (d). [Can you draw a resonance structure that explains why Hb is downfield from Ha?] Ha is coupled to both Hb and to the aldehyde H (a), which can be seen as a doublet near 10 ppm.
Since the two hydrogens to which Ha is coupled are different, we must apply the couplings separately in order to predict the observed splitting pattern. This is shown in the sketch below:




The coupling to Hbtrans- across a double bond, is quite large, and splits the Ha absorption into a doublet. Then the smaller coupling to (a) splits each half of the doublet again, giving a quartet. The kind of diagram shown here is called a "splitting tree", and can be constructed for any case in which a group of nuclei is coupled to two distinctly different neighbors.


Let's construct the splitting tree for this one:

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