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Does anyone know the proper format to report an iso-butyl group, eg. $\ce{R-CH2-C\color{red}{H}(CH3)2}$, in $\ce{^1H}$ NMR tabulation? The proton in red should be a triplet of septets, so I've written:

δ 2.72 (d, J = 5.5 Hz, 2H), 1.80 (tsep, J = 5.5, 5.3 Hz, 1H), 1.02 (d, J = 5.3 Hz, 6 H).

Is the use of "tsep" acceptable here?

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I personally use a space, so "t sep" as long as both "t" and "sep" are defined. I think it's slightly clearer than "tsep", but I don't think there's any official adjudication on what's a good or bad acronym.

Also, of some interest to me is how you manage to distinguish 5.5 and 5.3 Hz. If it looks quite like an nonet, I would probably prefer reporting it as an apparent nonet, although different people may have different opinions... I know some groups will have "in-house" rules / guidelines for reporting NMR spectra, so it might be a good idea to check with your supervisor on this front.

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  • $\begingroup$ I used to call it heptet, but nowadays, those have changed. For example, now they call all pentets as quintets. :-) About 5.5 vs 5.3, I think OP has been using $\pu{400 MHz}$ or bigger machine so they can measure up to $\pu{0.01 Hz}$. $\endgroup$ – Mathew Mahindaratne Apr 7 '19 at 18:47
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    $\begingroup$ @MathewMahindaratne, it's not realistically possible to measure couplings to an accuracy of 0.01 Hz. If you take a 400 MHz machine, a spectral width of 20 ppm (= 8000 Hz), and 64k data points during acquisition (32k real + 32k complex), then the resolution is ~0.25 Hz. Even this is not enough to accurately distinguish a coupling of 5.5 and 5.3 Hz, you can see this for yourself at this website, or using e.g. MestReNova - it is indistinguishable from a perfect nonet. $\endgroup$ – orthocresol♦ Apr 7 '19 at 18:55
  • $\begingroup$ You would also need to take into consideration linewidth contributions from e.g. field inhomogeneity and relaxation. It's very, very difficult to distinguish two couplings differing by 0.2 Hz. $\endgroup$ – orthocresol♦ Apr 7 '19 at 18:58
  • $\begingroup$ Sure, but computer gives your peak picking to 4 decimal places, so I think OP might think it's okay to report that way without concerning the uncertainty. Nonetheless, I wouldn't report that way in a journal article. $\endgroup$ – Mathew Mahindaratne Apr 7 '19 at 19:02
  • $\begingroup$ @MathewMahindaratne, sorry, I think I might have misunderstood you, actually; I think it's fair to say that you can measure 5.5 and 5.3 for two different multiplets, i.e. the CH2 and the (CH3)2 signals, although the uncertainty is definitely still there. I was more concerned about the CH peak, where the shape of that multiplet (on its own) won't allow you to extract two different couplings of 0.2 Hz difference from that one multiplet. $\endgroup$ – orthocresol♦ Apr 7 '19 at 19:05

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