l15c

l15c - 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: Simple 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: Simple splitting The NMR spectrum of ethyl ethanoate is shown below. We are by now familiar enough with the factors that determine the chemical shift (i.e., the electro- negativity and ring current effect) that we can assign each signal. In this lecture we explain why the signals at ! 4.12 and ! 1.26 are split into components whereas the signal at ! 2.04 is not. When to expect signal splitting The first situation in which splitting is expected is called geminal or two- bond splitting: it is observed when hydrogen atoms that are not chemical- shift equivalent to each other are two bonds away from each other. H a C H b geminal splitting H a is not chemical-shift equivalent to H b A molecule that exhibits geminal splitting is 1,1-dichloro-2- bromocyclopropane: H 3 C O O CH 3 H H –O– CH 2 –CH 3 –O–CH 2 CH 3 CH 3 –CO–
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NMR spectroscopy: Simple splitting © Bruno I. Rubio 2 H b and H c are not chemical-shift equivalent because they are in different en- vironments: H b is cis to a bromine atom whereas H c is trans to that bromine atom. Moreover, H b and H c are two bonds away from each other. The signals of these hydrogens are split into components. Note that splitting is reciprocal: H b splits the signal of H c and H c splits the signal of H b . The second situation in which splitting is expected is called vicinal or three-bond splitting: it is observed when hydrogen atoms that are not chemi- cal-shift equivalent to each other are three bonds away from each other. H a C C H b vicinal splitting H a is not chemical-shift equivalent to H b A molecule that exhibits vicinal splitting is 1-bromo-3-methyl-2-butene: The H a s do not split each other because they are chemical-shift equivalent by a rapid mechanism, that is, there is no geminal splitting. The H a s and H b are Cl Cl H a H b H c Br geminal splitting H a H b H c vicinal splitting Br CH 3 c CH 3 d H b H a H a H a H b H c , H d
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NMR spectroscopy: Simple splitting © Bruno I. Rubio 3 not chemical-shift equivalent because they are in different environments. Moreover, the H a s and H b are three bonds away from each other. The signals of these hydrogens are split into components. Again, note that splitting is re- ciprocal: the H a s split the signal of H b and H b splits the signal of the H a s. Hydrogens separated by more than three bonds sometimes split each other’s signals; we will not consider such long-range splitting. The physical basis of splitting Recall that when a collection of protons is immersed in the magnetic field provided by a strong laboratory magnet, they partition themselves between their two spin states designated as the lower-energy +1/2 spin state (spin- up) and the higher energy –1/2 spin state (spin-down). The NMR signal arises when the protons in the lower-energy +1/2 spin state absorb radio-frequency (rf) electromagnetic radiation of energy E given by E = hB p = h ( B lab B e ) where h is Planck’s constant, B p
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l15c - CH 203 O R G A N I C C H E M I S T R Y I NMR...

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