Measuring Coupling Constants—Analysis of an Allylic System

8 MEASURING COUPLING CONSTANTS—ANALYSIS OF AN ALLYLIC SYSTEM 

The exact assigmnent of the multiplets in the allylic group depends not only on their chemical shift values, but also on the splitting patterns observed. Some initial analysis must be done before any assignments can be definitely made. 

Assigning the two terminal vinyl hydrogens relies on the difference in the magnitude of a cis-and a trani-coupling constant. Proton c will have a wider pattern than proton b because it will have a trans coupling /cd to proton d, while proton b will experience a smaller cis coupling. There- 

The preliminary assignments just given are tentative, and they must pass the test of a full tree analysis with coupling constants. This will require expansion of aU the multiplets so that the exact value (in Hertz) of each subpeak can be measured. Within reasonable error limits, all coupling constants must agree in magnitude wherever they appear. 

Tree-Based Analysis and Determination of Coupling Constants 

9 Measuring Coupling Constants—Analysis of an Allylic System 281 

282 Nuclear Magnetic Resonance Spectroscopy Part Three Spin-Spin Coupling 

FIGURE 5.50 Allyloxyanisole. (a) Expansion of Hj. (b) Peak positions (Hz) and selected frequency differences, (c) Splitting tree diagram showing the origin of the splitting pattern. 

Spectrum, but is shown in the expansions at 12.2 ppm. Also remember that Jtrans is qnite large in an alkene while the allylic couplings will be small. The multiplets may be described as a doublet of doublets (1.92 ppm), a doublet of quartets (5.86 ppm), and a doublet of quartets (7.10 ppm) with the peaks of the two quartets overlapping. 

The allylic OCH2 group (4.48 ppm) is labeled a on the spectrum and is the easiest multiplet to identify because it has an integral of 2H. It is also in the chemical shift range expected for a group 

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