Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: VaporizedMango on March 28, 2023, 10:18:45 AM
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Hi there, I've been struggling to process my raw FID of a given proton spectrum, specifically assignment of multiplicity of a multiplet of which would be in the Phenyl region of my spectrum. I'm not sure whether it's down to my signal processing, or just analysis. I will post a snap of the two multiplets: the one with δ=7.27ppm, I think are two roofing doublets, but the further downfield multiplet I just can't figure; I thought it could be a doublet of triplets, but there are other peaks within those peaks which could make it a complex multiplet, or just mean it has been poorly processed by myself. Any help is greatly appreciated!
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I am not sure how much NMR theory that you know, so my comments may or may not be helpful. When two nuclei are coupled and similar in chemical shift, they are said to be "close coupled" and first-order analysis does not apply. In some instances in which nuclei are close coupled, they still resemble doublets or triplets or whatnot, but in others, they take on very different appearances.
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I will begin by saying that I'm not much of a NMR person, so I normally simulate a spectrum using nmrdb.org before I run, so that I know what to expect. It's normally fairly accurate although the chemical shifts do tend to differ a little from reality...
According to the predicted spectrum for what I believe is your compound, you should have a triplet of triplets at about 7.25 PPM (about 7.27 on your example), but also a doublet of doublet of doublet of doublets at 7.317 and also 7.32 PPM (probably what you're seeing at 7.35ish)...
In other words a complete nightmare when it comes to interpretation! From what I can see you have a Bruker 400MHz instrument. Is there anything you can do processing wise to improve resolution - I'm not familiar with Bruker software?