**1. Introduction**

Hydrogen bonding, both inter- and intramolecular, has profound effects on the properties of small molecules, reactivity, pKa values, polarity, solubility and with that, e.g., penetration of membranes and biological effects in general. Most drugs and bioactive molecules are small molecules and often depend on binding to receptors partly via hydrogen bonds [1,2]. Related to intramolecular hydrogen bonding is tautomerism. One of the effective tools in the study of hydrogen bonding is isotope effects on NMR chemical shifts. Both primary and secondary isotope effects may be used. The most common isotope is 2H, deuterium (D), but also 13C, 15N and 18O as well as more rare isotopes are used. For observation of secondary isotope effects, the most common nuclei are 1H, 13C, 15N, 19F and 18O. In case of primary isotope effects, the pair 1H, 2H is mostly used. The intrinsic isotope effects are defined as:

Primary: <sup>P</sup>ΔH(D) = <sup>δ</sup>(H) − <sup>δ</sup>(D);

Secondary: <sup>n</sup>ΔX(h) = <sup>δ</sup>X(l) − <sup>δ</sup>X(h).

l = light, h = heavy and X is the nucleus under investigation, n is the number of covalent bonds between the isotope and the investigated nucleus. For intramolecular hydrogen bonded cases, the isotope effects may be transmitted via the hydrogen bond. In the cases in which to opposite sign convention has been used, the sign is changed and the number is marked with an asterisk.

Deuterium isotope effects can be related to hydrogen bond strength and to hydrogen bond energies. Isotope effects are very useful in the equilibria of symmetric systems (lifting of degeneracy). They respond to nearby charges.

Most of the studies were performed in the liquid state, but isotope effects on chemical shifts can also in favorable cases be studied in the solid state.

The present review will primarily cover the last ten years. Other reviews that cover the subject are [3–6].

**Citation:** Hansen, P.E. Isotope Effects on Chemical Shifts in the Study of Hydrogen Bonds in Small Molecules. *Molecules* **2022**, *27*, 2405. https:// doi.org/10.3390/molecules27082405

Academic Editor: Miroslaw Jablonski

Received: 11 March 2022 Accepted: 5 April 2022 Published: 8 April 2022

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