Understanding H NMR Coupling Constants: Matching J-Values vs. Peak Alignment

[HappMatt]

background:

I have a HNMR spectra and I am trying to figure out what is coupled to what. There is a set of 4 doublets where it is know that there 2 sets of coupled doublets. the doublets have j-values of 8.8 @ 8.04ppm, 8.8@ 7.58ppm, 8.4@7.39ppm and 8.4@7.00ppm. While I may have assumed that the 8.04/7.58 and the 7.39/7.00 are the coupled sets due to j-values, the peaks do not lean towards each other. on the other hand, the 8.04/7.0 and the 7.58/7.39 set do lean towards each other. In addition appears as though the chemical shift of the 7.00ppm H's is near what i would expect for the H's next to the 8.04ppm H's

Question:

If trying to predict what's coupled to what, is it more important that the j-values match or that the peaks lean towards each other? In addition since the chemical shifts of the 7.0 h's seem's to be near the 8.04 H's on the molecule and the fact that they lean towards each other enough to assume they are coupled, or since their j-values are a little different would that negate them being coupled?

 

[chemisttree]

The j-couplings are the more important ones here.

 

[Blueshift5]

Both the j-values and peak alignment are important factors to consider when trying to determine coupling in an HNMR spectrum. The j-values represent the coupling constant, which is a measure of the strength of the coupling between two nuclei. Matching j-values can provide valuable information about which peaks are coupled to each other. However, peak alignment can also be a useful indicator of coupling. When two peaks are coupled, their intensity will be affected, causing them to lean towards each other. This is known as the "splitting" of the peaks and is a result of the spin-spin coupling between the nuclei. Therefore, it is important to consider both the j-values and peak alignment when trying to determine coupling in an HNMR spectrum.

In regards to the specific example provided, it is possible that the peaks with j-values of 8.04 and 7.00 are coupled, despite their slightly different j-values. It is also possible that the peaks with j-values of 8.8 and 7.58 are coupled, as the peak alignment suggests. The chemical shifts of the peaks can also provide useful information, as you have noted. However, it is important to keep in mind that the chemical shifts can be influenced by other factors such as neighboring functional groups or solvent effects. Therefore, it is best to use a combination of all of these factors to make a confident determination of coupling in an HNMR spectrum. Further analysis, such as COSY or NOESY experiments, may also be helpful in confirming coupling between specific peaks.