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Showing posts with label NMR INSIGHT. Show all posts
Showing posts with label NMR INSIGHT. Show all posts

Thursday 30 January 2014

Determination of Stereochemistry......Cis Trans Organic Compounds, NMR INSIGHT








Cyclopropanes.
Dihedral angles in cyclopropanes are rigidly fixed by the geometry of the ring system. We therefore find that Jcis (7-10 Hz) is always larger than Jtrans (2-6 Hz), and this can be reliably used for structure assignment. The same relationship holds for the 3-membered ring heterocycles, although the range of observed couplings is wider.
  




Cyclobutanes. 
Cyclobutanes are even flatter than cyclopentanes, so that cis couplings are almost always larger (6-9 Hz) than trans (2-8). However, if structural features which promote strong puckering of the ring such as a trans ring fusion, large or electronegative substituents are present, then trans couplings can become larger than cis, as shown for 1,3-dibromocyclobutane and cyclobutanol below.
  





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