Hindered Rotation and Diastereotopicity This post illustrates how hindered rotation (due to steric constraints) of different parts of a molecule can make protons within seemingly identical groups chemically non-equivalent. In this case, because of the bulky tert-butyl substituent, rotation of the two CH₂-tBu groups around the Ar–CH₂ bond is hindered at low temperatures. This renders the methylene protons diastereotopic and gives rise to a characteristic AB spin system in the spectrum. From my own experience, I would also note that similar effects can occur in any molecule where rotation is h

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When preparing samples for NMR studies, the importance of thorough mixing can hardly be overstated. Minor concentration gradients of the compound in the solvent can "spoil" the appearance of signal splitting. Significant gradients (as in the photo) can make the spectrum completely uninformative. The reason: chemical shift is a concentration dependent value. Therefore, in the presence of a concentration gradient, the spectrum we observe is a superposition of an infinite number of spectra that differ slightly from one another and together form a "distorted" resulting spectrum. This is especially