**Preface to "Hydrogen Bonds"**

The eponymous hydrogen bonds are generally considered to be the most important type of so-called non-covalent interactions. This position results from both their ubiquity and their enormous importance in practically all natural sciences. Undoubtedly, the main example is the presence of hydrogen bonds O-H...O in liquid and solid water, which gives it specific properties, e.g., an exceptionally high boiling point compared to its heavier counterpart, hydrogen sulfide, or the relatively low density of ice. Nevertheless, hydrogen bonds are just as important, if not more so, in macromolecules including amino acids and proteins. In this case, the template example is the N-H...O and N-H...N hydrogen bonds in the complementary nitrogen base pairs cytosine–guanine and adenine–thymine. Their presence allows the binding of two DNA strands, giving them a double-helical structure, which is of fundamental importance in the replication of genetic information. In addition to these instances of intermolecular hydrogen bonding, intramolecular hydrogen bonding is of similar importance. Particularly important in this case is the impact of their presence and strength on conformational preferences, which in turn may translate into a specific structure in the solid state determined by crystallographic measurements. Another area of the manifestation of hydrogen bonds is the movement and even transfer of the proton in X...H...Y hydrogen bridges.

Hydrogen bonds owe their extraordinary role and importance mainly to the fact that they are defined by a unique atom—the smallest hydrogen. On the one hand, its presence between the larger X and Y atoms in the X-H...Y bridge allows these atoms to approach a fairly short distance without significant steric effects. On the other hand, the partial positive charge on the hydrogen atom allows for relatively strong attractive interactions with a high-electron-density region, most often the electron lone pair on a strongly electronegative Y atom. All these specific properties make hydrogen bonds act as a glue in the rich world of various intermolecular interactions, binding single molecules into dimers and larger aggregates. At the same time, their relative weakness allows for the full dynamics of this process.

It might seem that after more than a hundred years of research into hydrogen bonding, this area has been completely exploited; there is nothing more to study on the subject of hydrogen bonds. However, this reprint is excellent proof that despite more than a hundred years of research into hydrogen bonds, they are still an interesting and fundamental topic in the natural sciences. The collected articles deal with various aspects of the existence of hydrogen bonds and show human ingenuity in the field of inventing and applying various techniques, both experimental and theoretical, in order to describe them more fully.

> **Mirosław Jabło ´nski** *Editor*
