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We would expect the NMR spectrum of two spin-coupled protons, A & B, to display a pair of doublets. However, if the ratio of Δν to J (both in Hz) decreases to less than 10 a significant distortion of this expected pattern will take place, as shown in the following diagram. By repeated clicking on the diagram four examples of this behavior will be presented, starting with a compound in which Δν/J = 7.9 and followed by cases having smaller ratios. The fourth example, 2-methyl-2-phenyl-1,3-propanediol is especially interesting. Here, the two identical methylene groups have a pair of diastereotopic protons (Ha & Hb). Because these hydrogens have similar but different chemical shifts, the expected doublet pair experiences a second order distortion.
Second order splitting distortions are not limited to single proton pairs. The following spectrum of 2-dimethylaminoethyl acetate shows what appears to be a well behaved pair of triplets coming from adjacent methylene groups. Such cases are often deceptively simple, and become very complex when Δν/J is less than 10. Clicking on the diagram will display a simple example.
For additional examples of Second Order splitting patterns Click Here.
- William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry