1H NMR patterns are more complex than predicted by the N+1 coupling rule when coupling of one proton or set of equivalent protons occurs to two different sets of protons with different size coupling constants or when coupling occurs between protons with similar but not identical chemical shifts. The former situation can still be analyzed in terms of overlapping N+1 patterns using stick diagrams. This is shown for the spectrum of phenyloxirane which has three oxirane protons of different chemical shift all coupled to each other. The protons are labeled HA, HM, and HX to reflect that they are not close to each other in chemical shift. Each resonance appears as a doublet of doublets, and the overall pattern of three doublets of doublets is called an AMX pattern.
The situation of protons with close chemical shifts coupled to each other is more complex. If only two protons are coupled to each other, the pattern still appears as two doublets but the intensities are no longer 1:1 and the chemical shifts are not the centers of the doublets; the separation between the lines of each doublet is still the coupling constant J. The chemical shifts are closer to the larger peaks of each doublet and can be calculated using a simple equation as shown below.
|The Molecular Expressions web site has an animation of AB patterns.|
If more than two protons of close chemical shift are coupled to each other, more complex patterns, often described as complex multiplets, are observed. Multiplets still provide useful structural information because they indicate the presence of coupled protons of similar chemical shift. The AB pattern and complex multiplet patterns result from what is called second order effects. Second order effects occur when the ratio of the chemical shift separation in Hz to the coupling constant is less than approximately 10 or /J < 10. Even when this ratio is greater than 10, slight intensity perturbation is evident in first order patterns as shown by the spectrum for 2-butanone. In fact, if we draw an arrow over the pattern showing the slight tilt (blue arrows in Figure 25), the arrows point toward each other. So we say the patterns for coupled protons point towards each other.
next section: coupling in 13C NMR
Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.