A couple of workings are shown, and the theoretical expectation of these peaks is shown. You can work through predicting what you would expect based on known typical couplings. the 1,2 disubstituted and 1,3 disubstituted case.A triplet coupling implies coupling to 2 identical nuclei It is much more difficult to start from the appearance of a spectrum and work backwards to determine/justify structure. So a reported doublet of doublets with similar couplings may look like a triplet.Īlthough both methods are used in the literature, the strong preference is for the latter, as it provides structural information, and a means for justifying the appearance of the spectrum. expected splittings based on molecular structure.
observed splitting pattern so a doublet of doublets with similar couplings is likely to be reported as a triplet.Splittings in a spectrum can be reported as either 5J H-H para couplings are typically 4J H-H meta couplings are typically 2-3Hz.3J H-H ortho couplings are typically 7-8 Hz.Some standard couplings that you should commit to memory for aromatic rings: Without understanding this key concept, you would not be able to rationalise the appearance of the spectrum, which actually looks like: In the case above, the two protons shown in red are in different environments the same chemical environment, but different magnetic environments. Although they hgave the same chemical shift, their coupling patterns are very complicated, and usually involve coupling to each other. Nuclei that are chemically equivalent may not couple to other nuclei in an identical manner, and these nuclei are therefore magnetically non-equivalent. They will have the same chemical shift.Īn additional element of equivalence needs to be considered also, and this relates to magnetic equivalence. If you substitute, in turn, one of the 1H nuclei in the molecule below, you end up with identical molecules, therefore the protons are chemically equivalent. In all four molecules you have drawn, every single proton is in a separate environment.Įquivalence of the proton environments can be assessed on whether a particular nucleus is identical to another site, based on a substitution test. If you want to look at it from an NMR perspective (and you ARE asking as an NMR question), then you have to consider magnetic environments also. It might also be an appropriate assessment if you do not intend to make any interpretation of the splittings of peaks from coupling. This can be understood and determined by someone who has never heard of NMR. As an argument about chemical environments alone, you need only look at the symmetry of the molecule to make a judgement. 'Environments' is a vague term, often misused and misunderstood, unless you specify what type of environment: chemical or magnetic. Some background knowledge before tackling your molecules. And incorrect enough for me to have a rant about NMR teaching at the introductory level. In a word, no, your drawings are not correct.
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